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Why do protoplanetary disks appear not massive enough to form the known exoplanet population?
Abstract
When and how planets form in protoplanetary disks is still a topic of discussion. Exoplanet detection surveys and protoplanetary disk surveys are now providing results that are leading to new insights. We collect the masses of confirmed exoplanets and compare their dependence on stellar mass with the same dependence for protoplanetary disk masses measured in ∼1-3 Myr old star-forming regions. We recalculated the disk masses using the new estimates of their distances derived from Gaia DR2 parallaxes. We note that single and multiple exoplanetary systems form two different populations, probably pointing to a different formation mechanism for massive giant planets around very low-mass stars. While expecting that the mass in exoplanetary systems is much lower than the measured disk masses, we instead find that exoplanetary systems masses are comparable or higher than the most massive disks. This same result is found by converting the measured planet masses into heavy element content (core masses for the giant planets and full masses for the super-Earth systems) and by comparing this value with the disk dust masses. Unless disk dust masses are heavily underestimated, this is a big conundrum. An extremely efficient recycling of dust particles in the disk cannot solve this conundrum. This implies that either the cores of planets have formed very rapidly (<0.1-1 Myr) and a large amount of gas is expelled on the same timescales from the disk, or that disks are continuously replenished by fresh planet-forming material from the environment. These hypotheses can be tested by measuring disk masses in even younger targets and by better understanding if and how the disks are replenished by their surroundings.
... How did the Earth get its volatile materials? Several studies (e.g., Morbidelli et al. 2012;Pontoppidan et al. 2014;Hartogh et al. 2011;Altwegg et al. 2019) suggest that comets and other icy bodies provided the Earth with much of its complement of volatiles. It is thought that comets are the most pristine objects in the solar system, and their chemical composition has not been altered since the solar protoplanetary disk stage (Mumma & Charnley 2011;Pontoppidan et al. 2014). ...
... Most importantly, the earliest phases of star formation can provide insights into the chemical evolution of the seed material for the stars and planets. The protostellar phase, often referred to as the primary accretion phase (Fischer et al. 2017;Narang et al. 2023;Federman et al. 2023;Tobin & Sheehan 2024), plays a vital role in accumulating/transporting gas and icy dust from the molecular cloud onto the star and the protostellar/protoplanetary disk for planet formation (the birth-place of planets; see also Manara et al. 2018;Tychoniec et al. 2020). During this stage, most of the stellar mass is accreted (Fischer et al. 2017), and the initial conditions for planet formation are set. ...
The composition of protoplanetary disks, and hence the initial conditions of planet formation, may be strongly influenced by the infall and thermal processing of material during the protostellar phase. Composition of dust and ice in protostellar envelopes, shaped by energetic processes driven by the protostar, serves as the fundamental building material for planets and complex organic molecules. As part of the JWST GO program, "Investigating Protostellar Accretion" (IPA), we observed an intermediate-mass protostar HOPS 370 (OMC2-FIR3) using NIRSpec/IFU and MIRI/MRS. This study presents the gas and ice phase chemical inventory revealed with the JWST in the spectral range of 2.9 to 28 m and explores the spatial variation of volatile ice species in the protostellar envelope. We find evidence for thermal processing of ice species throughout the inner envelope. We present the first high-spatial resolution ( au) maps of key volatile ice species HO, CO, CO, CO, and OCN, which reveal a highly structured and inhomogeneous density distribution of the protostellar envelope, with a deficiency of ice column density that coincides with the jet/outflow shocked knots. Further, we observe high relative crystallinity of HO ice around the shocked knot seen in the H and OH wind/outflow, which can be explained by a lack of outer colder material in the envelope along the line of sight due to the irregular structure of the envelope. These observations show clear evidence of thermal processing of the ices in the inner envelope, close to the outflow cavity walls, heated by the luminous protostar.
... Besides these nearby Peter Pan disks, other long-lived accretion disks have also been observed outside the immediate solar neighbourhood (Currie et al. 2007;Currie & Kenyon 2009;Beccari et al. 2010;Spezzi et al. 2012;De Marchi et al. 2013a,b). In spite of the small number of Peter Pan disks, these "extremely old" PMS stars harboring primordial disks can greatly improve our understanding to the theory of planet formation (Greaves & Rice 2010;Najita & Kenyon 2014;Manara et al. 2018;Pfalzner & Bannister 2019) and provide upper limit on the lifetimes of gaseous disks. The existence of these old, low-mass accreting stars indicates that at least some low-mass stars can retain their gas reservoirs much longer than previously recognized. ...
While both observations and theories demonstrate that protoplanetary disks are not expected to live much longer than 10 Myr, several examples of prolonged disks have been observed in the past. In this work, we perform a systematic search for aged YSOs still surrounded by protoplanetary disks in the M star catalog from the LAMOST archive. We identify 14 sources older than 10 Myr, still surrounded by protoplanetary disks and with ongoing accretion activities, significantly improving the census of the category known as the Peter Pan disks. The stellar parameters, variability and accretion properties of these objects, as well as their spatial distribution, are investigated. Nearly all of these objects are distributed far away from nearby associations and star forming regions, but show evidence of being members of open clusters. Investigating the correlation between mass accretion rates and stellar masses, we find these long-lived disks accrete at systematically lower levels, compared to their younger counterparts with similar stellar masses. Studying the evolution of mass accretion rates with stellar ages, we find these aged disks follow similar trend as young ones.
... However, Herbig disks are generally quite old (a median age of 6 Myr for Herbig Ae stars, Vioque et al. 2018), many showing structures indicative of planet formation making these some of the most famous and best investigated disks (e.g., HD 100546, HD 163296, MWC 480;Fedele et al. 2017;Teague et al. 2018;Öberg et al. 2021;Booth et al. 2023). Planets may already form early on in the lifetime of the disk, as there is an apparent lack of solids in class II disks to form the observed family of giant exoplanets (Manara et al. 2018;Tychoniec et al. 2020), and structures are already visible in earlier stages (ALMA Partnership et al. Notes. ...
The precursors of Herbig stars are called Intermediate Mass T Tauri (IMTT) stars, which have spectral types later than F, but stellar masses between 1.5 and 5 M_\odot, and will eventually become Herbig stars with spectral types of A and B. ALMA Band 6 and 7 archival data are obtained for 34 IMTT disks with continuum observations, 32 of which have at least 12CO, 13CO, or C18O observations although most of them at quite shallow integrations. The disk integrated flux together with a stellar luminosity scaled disk temperature are used to obtain a total disk dust mass by assuming optically thin emission. Using thermochemical Dust And LInes (DALI) models from previous work, we additionally obtain gas masses of 10/35 of the IMTT disks based on the CO isotopologues. The IMTT disks in this study have the same dust mass and radius distributions as Herbig disks. The dust mass of the IMTT disks is higher compared to that of the T Tauri disks, as is also found for the Herbig disks. No differences in dust mass are found for group I versus group II disks, in contrast to Herbig disks. The disks for which a gas mass could be determined show similar high mass disks as for the Herbig disks. Comparing the disk dust and gas mass distributions to the mass distribution of exoplanets shows that there also is not enough dust mass in disks around intermediate mass stars to form the massive exoplanets. On the other hand there is more than enough gas to form the atmospheres of exoplanets. We conclude that the sampled IMTT disk population is almost indistinguishable compared to Herbig disks, as their disk masses are the same, even though these are younger objects. Based on this, we conclude that planet formation is already well on its way in these objects, and thus planet formation should start early on in the lifetime of Herbig disks.
... Pascucci et al. 2016), and the known exoplanet population (C. F. Manara et al. 2018). This implies either very rapid planet formation, late-stage infall onto disks around optically visible stars (A. ...
Millimeter emitting dust grains have sizes that make them susceptible to drift in protoplanetary disks due to the difference between their orbital speed and that of the gas. The characteristic drift timescale depends on the surface density of the gas. By comparing disk radius measurements from Atacama Large Millimeter/submillimeter Array CO and continuum observations at millimeter wavelengths, the gas surface density profile and dust drift time can be self-consistently determined. We find that profiles which match the measured dust mass have very short drift timescales, an order of magnitude or more shorter than the stellar age, whereas profiles for disks that are on the cusp of gravitational instability, defined via the minimum value of the Toomre parameter, Q min ∼ 1 − 2 , have drift timescales comparable to the stellar lifetime. This holds for disks with masses of dust ≳5 M ⊕ across a range of absolute ages from less than 1 Myr to over 10 Myr. The inferred disk masses scale with stellar mass as M disk ≈ M * / 5 Q min . This interpretation of the gas and dust disk sizes simultaneously solves two long standing issues regarding the dust lifetime and exoplanet mass budget, and suggests that we consider millimeter wavelength observations as a window into an underlying population of particles with a wide size distribution in secular evolution with a massive planetesimal disk.
... There are no identifiable trends in the gas-to-dust size ratio with stellar mass, disk dust mass, or substructure (Long et al. 2022). Second, dust masses are low, typically much less than the minimum mass of solids required to form the Solar System, 30 M ⊕ (e.g., Ansdell et al. 2016;Pascucci et al. 2016) and the known exoplanet population (Manara et al. 2018). This implies either very rapid planet formation, late-stage infall onto disks around optically visible stars (Gupta et al. 2023), or a reservoir of material that is unseen in the observations possibly due to high optical depth (Zhu et al. 2019;Ribas et al. 2020). ...
Millimeter emitting dust grains have sizes that make them susceptible to drift in protoplanetary disks due to a difference between their orbital speed and that of the gas. The characteristic drift timescale depends on the surface density of the gas. By comparing disk radii measurements from ALMA CO and continuum observations at millimeter wavelengths, the gas surface density profile and dust drift time can be self-consistently determined. We find that profiles which match the measured dust mass have very short drift timescales, an order of magnitude or more shorter than the stellar age, whereas profiles for disks that are on the cusp of gravitational instability, defined via the minimum value of the Toomre parameter, Qmin ~ 1-2, have drift timescales comparable to the stellar lifetime. This holds for disks with masses of dust > 5 MEarth across a range of absolute ages from less than 1 Myr to over 10 Myr. The inferred disk masses scale with stellar mass as Mdisk ~ Mstar / 5Qmin. This interpretation of the gas and dust disk sizes simultaneously solves two long standing issues regarding the dust lifetime and exoplanet mass budget and suggests that we consider millimeter wavelength observations as a window into an underlying population of particles with a wide size distribution in secular evolution with a massive planetesimal disk.
... Protostellar disks are often observed around young protostars and are potential sites of planet formation (C. F. Manara et al. 2018;C.-H. Hsieh et al. 2024). ...
Protostellar disks around young protostars exhibit diverse properties, with their radii ranging from less than ten to several hundred astronomical units. To investigate the mechanisms shaping this disk radius distribution, we compiled a sample of 27 Class 0 and I single protostars with resolved disks and dynamically determined protostellar masses from the literature. Additionally, we derived the radial profile of the rotational-to-gravitational-energy ratio in dense cores from the observed specific angular momentum profiles in the literature. Using these observed protostellar masses and rotational energy profile, we computed theoretical disk radii from the hydrodynamic and nonideal magnetohydrodynamic (MHD) models in Y.-N. Lee et al. and generated synthetic samples to compare with the observations. In our theoretical model, the disk radii are determined by hydrodynamics when the central protostar+disk mass is low. After the protostars and disks grow and exceed certain masses, the disk radii become regulated by magnetic braking and nonideal MHD effects. The synthetic disk radius distribution from this model matches well with the observations. This result suggests that hydrodynamics and nonideal MHD can be dominant in different mass regimes (or evolutionary stages), depending on the rotational energy and protostar+disk mass. This model naturally explains the rarity of large (>100 au) disks and the presence of very small (<10 au) disks. It also predicts that the majority of protostellar disks have radii of a few tens of astronomical units, as observed.
... Indeed, substantial mass accumulation at least over the first ∼1 Myr of the disk lifetime is hinted at by the fact that disks younger than 1 Myr in ρ Ophiucus and Corona Australis are somewhat more compact and lower in mass than observed in ∼1−2 Myr old regions (Testi et al. 2016(Testi et al. , 2022Cazzoletti et al. 2019;Cieza et al. 2019;Williams et al. 2019). Infall may also help to resolve an apparent mass budget problem for protoplanetary disk populations compared with observed exoplanets (Manara et al. 2018). It is possible that disks contain a substantial fraction of recently accreted material for much of their lifetime, which would have significant consequences for disk evolution models. ...
Planet formation occurs over a few Myr within protoplanetary disks of dust and gas, which are often assumed to evolve in isolation. However, extended gaseous structures have been uncovered around many protoplanetary disks, suggestive of late-stage infall from the interstellar medium (ISM). To quantify the prevalence of late-stage infall, we apply an excursion set formalism to track the local density and relative velocity of the ISM over the disk lifetime. We then combine the theoretical Bondi–Hoyle–Lyttleton (BHL) accretion rate with a simple disk evolution model, anchoring stellar accretion timescales to observational constraints. Disk lifetimes, masses, stellar accretion rates, and gaseous outer radii as a function of stellar mass and age are remarkably well reproduced by our simple model that includes only ISM accretion. We estimate that 20%−70% of disks may be mostly composed of material accreted in the most recent half of their lifetime, suggesting that disk properties are not a direct test of isolated evolution models. Our calculations indicate that BHL accretion can also supply sufficient energy to drive turbulence in the outer regions of protoplanetary disks with viscous α SS ∼ 10 ⁻⁵ to 10 ⁻¹ , although we emphasize that angular momentum transport and particularly accretion onto the star may still be driven by internal processes. Our simple approach can be easily applied to semianalytic models. Our results represent a compelling case for regulation of planet formation by large-scale turbulence, with broad consequences for planet formation theory. This possibility urgently motivates deep observational surveys to confirm or refute our findings.
... Both gas and dust measurements show a "missing mass problem", in which protoplanetary disks appear not to be massive enough to generate the observed exoplanetary population (e.g. Ansdell et al., 2016, Manara et al., 2018, Parker et al., 2022. Mapping the chemical composition of protoplanetary discs is essential for understanding their masses, evolution and how planets form (e.g. ...
In this Roadmap, we present a vision for the future of submillimetre and millimetre astronomy in the United Kingdom over the next decade and beyond. This Roadmap has been developed in response to the recommendation of the Astronomy Advisory Panel (AAP) of the STFC in the AAP Astronomy Roadmap 2022. In order to develop our stragetic priorities and recommendations, we surveyed the UK submillimetre and millimetre community to determine their key priorities for both the near-term and long-term future of the field. We further performed detailed reviews of UK leadership in submillimetre/millimetre science and instrumentation. Our key strategic priorities are as follows: 1. The UK must be a key partner in the forthcoming AtLAST telescope, for which it is essential that the UK remains a key partner in the JCMT in the intermediate term. 2. The UK must maintain, and if possible enhance, access to ALMA and aim to lead parts of instrument development for ALMA2040. Our strategic priorities complement one another: AtLAST (a 50m single-dish telescope) and an upgraded ALMA (a large configurable interferometric array) would be in synergy, not competition, with one another. Both have identified and are working towards the same overarching science goals, and both are required in order to fully address these goals.
... Protostellar disks are often observed around young protostars and are potential sites of planet formation (Manara et al. 2018;Hsieh et al. 2024). Observations reveal that most protostellar disks have radii of a few tens of au, while large (>100 au) disks are rare (Tsukamoto et al. 2023). ...
Protostellar disks around young protostars exhibit diverse properties, with their radii ranging from less than ten to several hundred au. To investigate the mechanisms shaping this disk radius distribution, we compiled a sample of 27 Class 0 and I single protostars with resolved disks and dynamically determined protostellar masses from the literature. Additionally, we derived the radial profile of the rotational to gravitational energy ratio in dense cores from the observed specific angular momentum profiles in the literature. Using these observed protostellar masses and rotational energy profile, we computed theoretical disk radii from the hydrodynamic and non-ideal magnetohydrodynamic (MHD) models in Lee et al. (2021, 2024) and generated synthetic samples to compare with the observations. In our theoretical model, the disk radii are determined by hydrodynamics when the central protostar+disk mass is low. After the protostars and disks grow and exceed certain masses, the disk radii become regulated by magnetic braking and non-ideal MHD effects. The synthetic disk radius distribution from this model matches well with the observations. This result suggests that hydrodynamics and non-ideal MHD can be dominant in different mass regimes (or evolutionary stages) depending on the rotational energy and protostar+disk mass. This model naturally explains the rarity of large (>100 au) disks and the presence of very small (<10 au) disks. It also predicts that the majority of protostellar disks have radii of a few tens of au, as observed.
... Planet formation involves a variety of physical processes ranging from collisions of tiny dust grains to migration of massive planets within a circumstellar disk. It must be efficient and rapid; it requires that growth from micron-sized dust into fully fledged planets of up to 10 5 km in scale be completed within several Myr before gas dissipates, possibly even earlier as suggested by some observations (e.g., Manara et al. 2018;Segura-Cox et al. 2020). The intermediate stage of the process, where millimeter-to centimeter-sized pebbles coalesce into kilometer-scale planetesimals, entails one of the most challenging questions to answer: how do the planetesimals form out of their much smaller pebble constituents (see Simon et al. 2022 for a recent review)? ...
The streaming instability (SI) is a leading candidate for planetesimal formation, which can concentrate solids through two-way aerodynamic interactions with the gas. The resulting concentrations can become sufficiently dense to collapse under particle self-gravity, forming planetesimals. Previous studies have carried out large parameter surveys to establish the critical particle to gas surface density ratio ( Z ), above which SI-induced concentration triggers planetesimal formation. The threshold Z depends on the dimensionless stopping time ( τ s , a proxy for dust size). However, these studies neglected both particle self-gravity and external turbulence. Here, we perform 3D stratified shearing box simulations with both particle self-gravity and turbulent forcing, which we characterize via a turbulent diffusion parameter, α D . We find that forced turbulence, at amplitudes plausibly present in some protoplanetary disks, can increase the threshold Z by up to an order of magnitude. For example, for τ s = 0.01, planetesimal formation occurs when Z ≳ 0.06, ≳0.1, and ≳0.2 at α D = 10 ⁻⁴ , 10 −3.5 , and 10 ⁻³ , respectively. We provide a single fit to the critical Z required for the SI to work as a function of α D and τ s (although limited to the range τ s = 0.01–0.1). Our simulations also show that planetesimal formation requires a mid-plane particle-to-gas density ratio that exceeds unity, with the critical value being largely insensitive to α D . Finally, we provide an estimation of particle scale height that accounts for both particle feedback and external turbulence.
... Protostellar disks around Class 0 and I protostars can be more massive than protoplanetary disks around pre-mainsequence stars (Tychoniec et al. 2020;Sheehan et al. 2022), making them potential sites for planet formation (Manara et al. 2018). Based on hydrodynamics (HD), protostellar disks form and grow rapidly due to the conservation of angular momentum during the collapse of dense cores (Terebey et al. 1984). ...
The magnetic field of a molecular cloud core may play a role in the formation of circumstellar disks in the core. We present magnetic field morphologies in protostellar cores of 16 targets in the Atacama Large Millimeter/submillimeter Array large program “Early Planet Formation in Embedded Disks (eDisk),” which resolved their disks with 7 au resolutions. The 0.1 pc scale magnetic field morphologies were inferred from the James Clerk Maxwell Telescope POL-2 observations. The mean orientations and angular dispersions of the magnetic fields in the dense cores are measured and compared with the radii of the 1.3 mm continuum disks and the dynamically determined protostellar masses from the eDisk program. We observe a significant correlation between the disk radii and the stellar masses. We do not find any statistically significant dependence of the disk radii on the projected misalignment angles between the rotational axes of the disks and the magnetic fields in the dense cores, nor on the angular dispersions of the magnetic fields within these cores. However, when considering the projection effect, we cannot rule out a positive correlation between disk radii and misalignment angles in three-dimensional space. Our results suggest that the morphologies of magnetic fields in dense cores do not play a dominant role in the disk formation process. Instead, the sizes of protostellar disks may be more strongly affected by the amount of mass that has been accreted onto star+disk systems, and possibly other parameters, for example, magnetic field strength, core rotation, and magnetic diffusivity.
... Bate et al. 2010), and the disc may remain warped for much of its lifetime (e.g. Nixon & Pringle 2010; and much longer than the early timescales on which planets are expected to form; Nixon et al. 2018;Manara et al. 2018;Tychoniec et al. 2020). It is plausible that some, or all, of these mechanisms operate in different systems to produce the observed distribution of misaligned planets. ...
The Rossiter-McLaughlin effect measures the misalignment between a planet's orbital plane and its host star's rotation plane. Around 10 of planets exhibit misalignments in the approximate range , with their origin remaining a mystery. On the other hand, large misalignments may be common in eccentric circumbinary systems due to misaligned discs undergoing polar alignment. If the binary subsequently merges, a polar circumbinary disc -- along with any planets that form within it -- may remain inclined near 90 to the merged star's rotation. To test this hypothesis, we present N-body simulations of the evolution of a polar circumbinary debris disc comprised of test particles around an eccentric binary during a binary merger that is induced by tidal dissipation. After the merger, the disc particles remain on near-polar orbits. Interaction of the binary with the polar-aligned gas disc may be required to bring the binary to the small separations that trigger the merger by tides. Our findings imply that planets forming in discs that are polar-aligned to the orbit of a high-eccentricity binary may, following the merger of the binary, provide a possible origin for the population of near-polar planets around single stars.
... The lack of protoplanetary material in the inner region of an exosystem can/could be one of the crucial segments of rocky planet formation. As has been discussed in various studies, protoplanetary disks may function as "conveyor belts," transporting material from the outer regions toward the host star(s) [56]. Those inward-drifting pebbles (mm size, abundant dust aggregates) feed the growth of terrestrial and super-Earth-type inner planets [57], even from outside reservoirs from their protostellar cloud cores (e.g., tail-end accretion) [58]. ...
Considering the possibility of complex organic molecules and microbial life appearing under the ice shell of those satellites in the Solar system, this study investigates the possible analog sources (targeting the potential ice satellite hosting, Jupiter and Saturn-like planets in exoplanet databases) and the transport of such bioaerosols in an attempt to support or contradict Panspermia, a fringe theory about the fertilization of Earth. Along many general parameters of the candidate planets, the host star, and the star system, additional factors thought to be related to Panspermia were also considered (e.g., the evolution of icy satellites, the frequency of impact related ejection, the traveling time from a source, and so on), revealing the following results. Eleven exosystems, with candidate gas giants hosting icy satellites, were found in a database listing more than 5000 exoplanets. The exomoons of the oldest systems (c.a. > 8 Ga; HD 191939, HD 4203, and HD 34445) could have developed rapidly considering the short formation time (~100 Myrs), which may result in the overlap of the putative early biological evolution and asteroid bombardment phase, providing a higher chance to bioaerosol ejection to space due to frequent collisions. However, the direct transfer might have occurred too early, even before our Solar system was formed, which prevented the fertilization of the latter. A longer formation of the exomoons (~3 Gyrs) significantly reduces the chance of ejection into space by asteroid impacts, which become less frequent over time, but increased the chance of arrival in time to the Solar system. Younger systems, such as HD 217107, HD 219828, HD 140901, and HD 156279 (c.a. 4 to 6 Ga), are better candidates of putative microbe source if the short icy satellite formation, along the higher impact and ejection frequency are expected considering direct transport from those systems.
... arXiv:2405.09063v1 [astro-ph.SR] 15 May 2024 Protostellar disks around Class 0 and I protostars can be more massive than protoplanetary disks around pre-main sequence stars (Tychoniec et al. 2020;Sheehan et al. 2022), making them potential sites for planet formation (Manara et al. 2018). Based on hydrodynamics (HD), protostellar disks form and grow rapidly due to the conservation of angular momentum during the collapse of dense cores (Terebey et al. 1984). ...
The magnetic field of a molecular cloud core may play a role in the formation of circumstellar disks in the core. We present magnetic field morphologies in protostellar cores of 16 targets in the Atacama Large Millimeter/submillimeter Array large program "Early Planet Formation in Embedded Disks (eDisk)", which resolved their disks with 7 au resolutions. The 0.1-pc scale magnetic field morphologies were inferred from the James Clerk Maxwell Telescope (JCMT) POL-2 observations. The mean orientations and angular dispersions of the magnetic fields in the dense cores are measured and compared with the radii of the 1.3 mm continuum disks and the dynamically determined protostellar masses from the eDisk program. We observe a significant correlation between the disk radii and the stellar masses. We do not find any statistically significant dependence of the disk radii on the projected misalignment angles between the rotational axes of the disks and the magnetic fields in the dense cores, nor on the angular dispersions of the magnetic fields within these cores. However, when considering the projection effect, we cannot rule out a positive correlation between disk radii and misalignment angles in three-dimensional space. Our results suggest that the morphologies of magnetic fields in dense cores do not play a dominant role in the disk formation process. Instead, the sizes of protostellar disks may be more strongly affected by the amount of mass that has been accreted onto star+disk systems, and possibly other parameters, for example, magnetic field strength, core rotation, and magnetic diffusivity.
... The vast majority have been discovered in surveys of relatively nearby starforming regions (see, e.g., the review of Manara et al. 2023) and specifically in star-forming regions such as Taurus (Andrews et al. 2013), Ophiuchus (Cieza et al. 2019), Lupus (Ansdell et al. 2016(Ansdell et al. , 2018Lovell et al. 2021), and Upper Scorpius (Carpenter et al. 2014; Barenfeld et al. 2016), enabling the detailed structural mapping of disk morphologies and substructures (e.g., Andrews et al. 2018;Long et al. 2018). Such detailed studies allow for connections to be made between these planetary birth sites and the observed population of planets that are now readily detected around mature stars (e.g., Manara et al. 2018;Mulders et al. 2021). ...
We present resolved images of IRAS 23077+6707 (“Dracula’s Chivito”) in 1.3 mm/225 GHz thermal dust and CO gas emission with the Submillimeter Array (SMA) and optical (0.5–0.8 μ m) scattered light with the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The Pan-STARRS data show a bipolar distribution of optically scattering dust that is characteristic for disks observed at high inclinations. Its scattered light emission spans ∼14″, with two highly asymmetric filaments extending along the upper bounds of each nebula by ∼9″. The SMA data measure 1.3 mm continuum dust as well as ¹² CO, ¹³ CO, and C ¹⁸ O J = 2 − 1 line emission over 12″–14″ extents, with the gas presenting the typical morphology of a disk in Keplerian rotation, in both position–velocity space and in each CO line spectrum. IRAS 23077+6707 has no reported distance estimate, but if it is located in the Cepheus star-forming region (180–800 pc), it would have a radius spanning thousands of astronomical units. Taken together, we infer IRAS 23077+6707 to be a giant and gas-rich edge-on protoplanetary disk, which to our knowledge is the largest in extent so far discovered.
... The required time delay, however, is unlikely because four million years is potentially longer than the disk lifetime (efolding time 2.5 Myr; e.g., Ribas et al. 2014), which would contradict evidence for KBO formation by gravitational collapse in a gas disk (Nesvorný et al. 2010(Nesvorný et al. , 2019Lisse et al. 2021), as well as Arrokoth needing nebular drag for the two lobes to come into contact (McKinnon et al. 2020;Lyra et al. 2021). The special timing also contradicts indication that planets might form early in protoplanetary disks (e.g., ALMA Partnership et al. 2015;Manara et al. 2018;Tobin et al. 2020;Sai et al. 2023;Yamato et al. 2023). Thus, we search for another explanation for the density trend, where the large change in density stems from compositional differences between large and small KBOs, with large KBOs containing a higher rock fraction than their smaller counterparts. ...
Kuiper Belt objects (KBOs) show an unexpected trend, whereby large bodies have increasingly higher densities, up to five times greater than their smaller counterparts. Current explanations for this trend assume formation at constant composition, with the increasing density resulting from gravitational compaction. However, this scenario poses a timing problem to avoid early melting by decay of ²⁶ Al. We aim to explain the density trend in the context of streaming instability and pebble accretion. Small pebbles experience lofting into the atmosphere of the disk, being exposed to UV and partially losing their ice via desorption. Conversely, larger pebbles are shielded and remain icier. We use a shearing box model including gas and solids, the latter split into ices and silicate pebbles. Self-gravity is included, allowing dense clumps to collapse into planetesimals. We find that the streaming instability leads to the formation of mostly icy planetesimals, albeit with an unexpected trend that the lighter ones are more silicate-rich than the heavier ones. We feed the resulting planetesimals into a pebble accretion integrator with a continuous size distribution, finding that they undergo drastic changes in composition as they preferentially accrete silicate pebbles. The density and masses of large KBOs are best reproduced if they form between 15 and 22 au. Our solution avoids the timing problem because the first planetesimals are primarily icy and ²⁶ Al is mostly incorporated in the slow phase of silicate pebble accretion. Our results lend further credibility to the streaming instability and pebble accretion as formation and growth mechanisms.
... The value ò = 0.2, with which the BDHI growth rate dominates, is mostly relevant for an early disk with moderate dust settling toward the midplane. As new evidence suggests that giant planets might have formed early and close-in (e.g., Manara et al. 2018;Morbidelli & Nesvorný 2020), the newly discovered BDHI could effectively be a plausible mechanism for early planetesimal formation during the class 0/I phases of PPD evolution. ...
Recent studies have shown that the large-scale gas dynamics of protoplanetary disks (PPDs) are controlled by nonideal magnetohydrodynamics (MHD), but how this influences dust dynamics is not fully understood. To this end, we investigate the stability of dusty, magnetized disks subject to the Hall effect, which applies to planet-forming regions of PPDs. We find a novel background drift Hall instability (BDHI) that may facilitate planetesimal formation in Hall-effected disk regions. Through a combination of linear analysis and nonlinear simulations, we demonstrate the viability and characteristics of BDHI. We find it can potentially dominate over the classical streaming instability (SI) and standard MHD instabilities at low dust-to-gas ratios and weak magnetic fields. We also identify magnetized versions of the classic SI, but these are usually subdominant. We highlight the complex interplay between magnetic fields and dust-gas dynamics in PPDs, underscoring the need to consider nonideal MHD like the Hall effect in the broader narrative of planet formation.
... But with the pressure bump suppressing the inward drift, pebbles can be converted to planets at order-unity efficiency once this chain formation process begins. Such high efficiency is favorable, given observational evidence that the dust mass in class II disks is comparable to the solid mass budget of planets (Najita & Kenyon 2014;Manara et al. 2018;Dai et al. 2020;Lu et al. 2020;Mulders et al. 2021). 5 ...
In protoplanetary disks, sufficiently massive planets excite pressure bumps, which can then be preferred locations for forming new planet cores. We discuss how this loop may affect the architecture of multiplanet systems and compare our predictions with observations. Our main prediction is that low-mass planets and giant planets can each be divided into two subpopulations with different levels of mass uniformity. Low-mass planets that can and cannot reach the pebble isolation mass (the minimum mass required to produce a pressure bump) develop into intra-system similarity “super-Earths” and more diverse “Earths,” respectively. Gas giants that do and do not accrete envelopes quickly develop into similar “Jupiters” and more diverse “Saturns,” respectively. Super-Earths prefer to form long chains via repeated pressure-bump planet formation, while Jupiter formation is usually terminated at pairs or triplets due to dynamical instability. These predictions are broadly consistent with observations. In particular, we discover a previously overlooked mass uniformity dichotomy among the observed populations of both low-mass planets (Earths versus super-Earths) and gas giants (Saturns versus Jupiters). For low-mass planets, planets well below the pebble isolation mass (≲3 M ⊕ or ≲1.5 R ⊕ for Sun-like stars) show significantly higher intra-system pairwise mass differences than planets around the pebble isolation mass. For gas giants, the period ratios of intra-system pairs show a bimodal distribution, which can be interpreted as two subpopulations with different levels of mass uniformity. These findings suggest that pressure-bump planet formation could be an important ingredient in shaping planetary architectures.
... Circumstellar disks around Class I objects are plausible sites for confirming the signs of planet formation because we can observe disks with less massive infalling envelopes (Manara et al. 2018;Williams et al. 2019;Mulders et al. 2020;Sanchis et al. 2020). In addition, a sufficient amount of dust remains in circumstellar disks around Class I objects (Tychoniec et al. 2020). ...
WL 17 is a Class I object and was considered to have a ring–hole structure. We analyzed the structure around WL 17 to investigate the detailed properties of this object. We used Atacama Large Millimeter/submillimeter Array archival data, which have a higher angular resolution than previous observations. We investigated the WL 17 system with the 1.3 mm dust continuum and ¹² CO and C ¹⁸ O ( J = 2–1) line emissions. The dust continuum emission showed a clear ring structure with inner and outer edges of ∼11 and ∼21 au, respectively. In addition, we detected an inner disk of <5 au radius enclosing the central star within the ring, the first observation of this structure. Thus, WL 17 has a ring–gap structure, not a ring–hole structure. We did not detect any marked emission in either the gap or inner disk, indicating that there is no sign of a planet, circumplanetary disk, or binary companion. We identified the source of both blueshifted and redshifted outflows based on the ¹² CO emission, which is clearly associated with the disk around WL 17. The outflow mass ejection rate is ∼3.6 × 10 ⁻⁷ M ⊙ yr ⁻¹ and the dynamical timescale is as short as ∼10 ⁴ yr. The C ¹⁸ O emission showed that an inhomogeneous infalling envelope, which can induce episodic mass accretion, is distributed in the region within ∼1000 au from the central protostar. With these new findings, we can constrain the scenarios of planet formation and dust growth in the accretion phase of star formation.
... To determine the fraction of FFPs produced by these different production mechanisms requires a population synthesis modeling that includes several parameters that are currently poorly constrained, including the efficiency of planet formation in circumbinary disks compared to circumsingle disks and the inclination distribution of circumbinary disks at the point at which planets decouple from their natal gas disks. However, given that (i) the broad range of inclinations observed for CBDs (Czekala et al. 2019); (ii) the realization that planet formation is a highly efficient process that begins early in the star-forming phase (Manara et al. 2018;Nixon et al. 2018;Tychoniec et al. 2020), implying that planet formation in CBDs is likely to be as efficient as planet formation around single stars; and (iii) binaries represent a significant fraction of stellar systems, we argue that it seems reasonable that instability in such systems may be the dominant mode of generating FFPs. As our understanding of the planet formation process improves we hope to address this question quantitatively in the future with detailed population synthesis models. ...
The dominant mechanism for generating free-floating planets has so far remained elusive. One suggested mechanism is that planets are ejected from planetary systems due to planet–planet interactions. Instability around a single star requires a very compactly spaced planetary system. We find that around binary star systems instability can occur even with widely separated planets that are on tilted orbits relative to the binary orbit due to combined effects of planet–binary and planet–planet interactions, especially if the binary is on an eccentric orbit. We investigate the orbital stability of planetary systems with various planet masses and architectures. We find that the stability of the system depends upon the mass of the highest-mass planet. The order of the planets in the system does not significantly affect stability, but, generally, the most massive planet remains stable and the lower-mass planets are ejected. The minimum planet mass required to trigger the instability is about that of Neptune for a circular orbit binary and a super-Earth of about 10 Earth masses for highly eccentric binaries. Hence, we suggest that planet formation around inclined binaries can be an efficient formation mechanism for free-floating planets. While most observed free-floating planets are giant planets, we predict that there should be more low-mass free-floating planets that are as of yet unobserved than higher-mass planets.
The composition of protoplanetary disks, and hence the initial conditions of planet formation, may be strongly influenced by the infall and thermal processing of material during the protostellar phase. The composition of dust and ice in protostellar envelopes, shaped by energetic processes driven by the protostar, serves as the fundamental building material for planets and complex organic molecules. As part of the JWST General Observers program, “Investigating Protostellar Accretion,” we observed an intermediate-mass protostar HOPS 370 (OMC2-FIR3) using NIRSpec integral field unit and Mid-Infrared Instrument medium-resolution spectroscopy. This study presents the gas and ice phase chemical inventory revealed with the JWST in the spectral range of ∼2.9–28 μ m and explores the spatial variation of volatile ice species in the protostellar envelope. We find evidence for the thermal processing of ice species throughout the inner envelope. We present the first high-spatial resolution (∼80 au) maps of key volatile ice species H 2 O, CO 2 , ¹³ CO 2 , CO, and OCN ⁻ , which reveal a highly structured and inhomogeneous density distribution of the protostellar envelope, with a deficiency of ice column density that coincides with the jet/outflow shocked knots. Further, we observe high relative crystallinity of H 2 O ice around the shocked knot seen in the H 2 and OH wind/outflow, which can be explained by a lack of outer colder material in the envelope along the line of sight due to the irregular structure of the envelope. These observations show clear evidence of thermal processing of the ices in the inner envelope, close to the outflow cavity walls, heated by the luminous protostar.
Protoplanetary disks naturally emerge during protostellar core collapse. In their early evolutionary stages, infalling material dominates their dynamical evolution. In the context of planet formation, this means that the conditions in young disks are different from the ones in the disks typically considered in which infall has subsided. High inward velocities are caused by the advection of accreted material that is deficient in angular momentum, rather than being set by viscous spreading, and accretion gives rise to strong velocity fluctuations. Therefore, we aim to investigate when it is possible for the first planetesimals to form and for subsequent planet formation to commence. We analyzed the disks obtained in numerical 3D nonideal magnetohydrodynamical simulations, which served as a basis for 1D models representing the conditions during the class 0/I evolutionary stages. We integrated the 1D models with an adapted version of the TwoPopPy code to investigate the formation of the first planetesimals via the streaming instability. In disks with temperatures such that the snow line is located at and in which it is assumed that velocity fluctuations felt by the dust are reduced by a factor of 10 compared to the gas, of planetesimals may be formed already during the first SI year after disk formation, implying the possible early formation of giant planet cores. The cold-finger effect at the snow line is the dominant driver of planetesimal formation, which occurs in episodes and utilizes solids supplied directly from the envelope, leaving the reservoir of disk solids intact. However, if the cold-finger effect is suppressed, early planetesimal formation is limited to cold disks with an efficient dust settling whose dust-to-gas ratio is initially enriched to ε_0≥ 0.03.
Dust growth is often indirectly inferred observationally in star-forming environments, is theoretically predicted to produce millimetre-sized particles in circumstellar discs, and has also acted on the predecessors of the terrestrial meteoritic record. For those reasons, it is believed that young gas giants under formation in protoplanetary discs that have putative circumplanetary discs (CPDs) surrounding them, such as PDS 70c, should contain millimetre-sized particles. We modelled the spectra of a set of CPDs, which we obtained from radiation hydrodynamic simulations at varying Rosseland opacities, κ_ R. The κ_ R from the hydrodynamic simulations are matched with consistent opacity sets of an interstellar-medium-like composition, but grown to larger sizes Our high κ_ R hydro data nominally corresponds to 10 μm-sized particles, and our low κ_ R cases correspond to millimetre-sized particles. We investigated the resulting broad spectral features at first, while keeping the overall optical depth in the planetary envelope constant. Dust growth to size distributions dominated by millimetre particles generally results in broad, featureless spectra with black-body like slopes in the far-infrared, while size distributions dominated by small dust develop steeper slopes in the far-infrared and maintain some features stemming from individual minerals. We find that significant dust growth from microns to millimetres can explain the broad features of the PDS 70c data, when upscaling the dust masses from our simulations by one hundred times. Furthermore, our results indicate that the spectral range of 30-500 μm is an ideal hunting ground for broadband features arising from the CPD, but that longer wavelengths observed with ALMA can also be used for massive CPDs.
Context. Young low-mass protostars often possess hot corinos, which are compact, hot, and dense regions that are bright in interstellar complex organic molecules (iCOMs). In addition to their prebiotic role, iCOMs can be used as a powerful tool to characterize the chemical and physical properties of hot corinos.
Aims. Using ALMA/FAUST data, our aim was to explore the iCOM emission at <50 au scale around the Class 0 prototypical hot corino IRAS 4A2.
Methods. We imaged IRAS 4A2 in six abundant common iCOMs (CH 3 OH, HCOOCH 3 , CH 3 CHO, CH 3 CH 2 OH, CH 2 OHCHO, and NH 2 CHO), and derived their emitting sizes. The column density and gas temperature for each species were derived at 1σ from a multiline analysis by applying a non-LTE approach for CH 3 OH, and LTE population or rotational diagram analysis for the other iCOMs. Thanks to the unique estimates of the absorption from foreground millimeter dust toward IRAS 4A2, we derived for the first time unbiased gas temperatures and column densities.
Results. We resolved the IRAS 4A2 hot corino, and found evidence for a chemical spatial distribution in the inner 50 au, with the outer emitting radius increasing from ∼22–23 au for NH 2 CHO and CH 2 OHCHO, followed by CH 3 CH 2 OH (∼27 au), CH 3 CHO (∼28 au), HCOOCH 3 (∼36 au), and out to ∼40 au for CH 3 OH. Combining our estimate of the gas temperature probed by each iCOM with their beam-deconvolved emission sizes, we inferred the gas temperature profile of the hot corino on scales of 20–50 au in radius, and found a power-law index q of approximately –1.
Conclusions. We observed, for the first time, a chemical segregation in iCOMs of the IRAS 4A2 hot corino, and derived the gas temperature profile of its inner envelope. The derived profile is steeper than when considering a simple spherical collapsing and optically thin envelope, hinting at a partially optically thick envelope or a gravitationally unstable disk-like structure.
While both observations and theories demonstrate that protoplanetary disks are not expected to live much longer than ∼10 Myr, several examples of prolonged disks have been observed in the past. In this work, we perform a systematic search for aged young stellar objects still surrounded by protoplanetary disks in the M-star catalog from the LAMOST archive. We identify 14 sources older than 10 Myr, still surrounded by protoplanetary disks and with ongoing accretion activities, significantly improving the census of the category known as the Peter Pan disks. The stellar parameters, variability, and accretion properties of these objects, as well as their spatial distribution, are investigated. Nearly all of these objects are distributed far away from nearby associations and star-forming regions but show evidence of being members of open clusters. Investigating the correlation between mass accretion rates and stellar masses, we find that these long-lived disks accrete at systematically lower levels, compared to their younger counterparts with similar stellar masses. Studying the evolution of mass accretion rates with stellar ages, we find that these aged disks follow a similar trend to young ones.
Context . While class II pre-main-sequence (PMS) stars have already accreted most of their mass, the continued inflow of fresh material via Bondi-Hoyle accretion acts as an additional mass reservoir for their circumstellar disks. This may explain the observed accretion rates of PMS stars, as well as observational inconsistencies in the mass and angular momentum balance of their disks.
Aims . Using a new simulation that reproduces the stellar initial mass function (IMF), we want to quantify the role of Bondi-Hoyle accretion in the formation of class II disks, as well as address the prospect of its observational detection with the James Webb Space Telescope (JWST).
Methods . We studied the mass and angular momentum of the accreting gas using passively advected tracer particles in the simulation, and we carried out radiative transfer calculations of near-infrared scattering to generate synthetic JWST observations of Bondi-Hoyle trails of PMS stars.
Results . Gas accreting on class II PMS stars approximately 1 Myr after their formation has enough mass and angular momentum to strongly affect the evolution of the preexisting disks. The accreted angular momentum is large enough to also explain the observed size of class II disks. The orientation of the angular momentum vector can differ significantly from that of the previously accreted gas, which may result in a significant disk warping or misalignment. We also predict that JWST observations of class II stars will be able to detect Bondi-Hoyle trails with a 80%-100% success rate with only a 2 min exposure time, depending on the filter, if stars with both an accretion rate Ṁ > 5 × 10 ⁻¹⁰ M ⊙ /yr and a luminosity of L > 0.5 L ⊙ are selected.
The last decade has witnessed remarkable advances in the characterization of the (sub-)millimeter emission from planet-forming disks. Conversely, the study of (sub-)centimeter emission has made more limited progress, to the point that only a few exceptional disk-bearing objects have been characterized in the centimeter regime. This work takes a broad view of the centimeter emission from a large sample with Karl G. Jansky Very Large Array (VLA) observations that is selected from previous Atacama Large (sub-)Millimeter Array (ALMA) surveys of more representative disks in brightness and extent . We report on the detection and characterization of flux at centimeter wavelengths from 21 sources in the Taurus star-forming region. Complemented by literature and archival data, the entire photometry from 0.85 mm to 6 cm is fit by a two-component model that determines the ubiquitous presence of free-free emission entangled with the dust emission. The flux density of the free-free emission is found to scale with the accretion rate but is independent of the outer-disk morphology depicted by ALMA. The dust emission at 2 cm is still appreciable and offers the possibility to extract an unprecedented large set of dust spectral indices in the centimeter regime. A pronounced change between the median millimeter indices (2.3) and centimeter indices (2.8) suggests that a large portion of the disk emission is optically thick up to 3 mm. The comparison of both indices and fluxes with the ALMA disk extent indicates that this portion can be as large as 40 au and suggests that the grain population within this disk region that emits the observed centimeter emission is similar in disks with different sizes and morphologies. All these results await confirmation and dedicated dust modeling once facilities such as next generation VLA (ngVLA) or Square Kilometre Array (SKA)-mid are able to resolve the centimeter emission from planet-forming disks and disentangle the various components.
Context. Multi-wavelength dust continuum observations of protoplanetary disks are essential for accurately measuring two key ingredients of planet formation theories: dust mass and grain size. Unfortunately, they are also extremely time-expensive.
Aims. Our aim is to investigate the most economic way of performing this analysis by identifying the optimal combination of multiband observations and angular resolution that provides accurate results.
Methods. We benchmarked the dust characterization analysis on multi-wavelength observations of a compact disk model with shallow rings, and an extended double-ringed disk model. We tested three different combinations of bands (in the 0.45 mm → 7.46 mm range) to see how optically thick and thin observations aid in the reconstruction of the dust properties for different morphologies and in three different dust mass regimes. We also tested different spatial resolutions (0.05″; 0.1″; 0.2″).
Results. Dust properties are robustly measured in a multi-band analysis if optically thin observations are included. For typical disks, this requires wavelengths longer than 3 mm. Instead, from fully optically thick observations alone the dust properties cannot be robustly constrained. A high resolution (<0.03″−0.05″) is fundamental in order to resolve the changes in dust content of substructures. However, lower-resolution results still provide an accurate measurement of the total dust mass and of the level of grain growth of rings. Additionally, we propose a new approach that successfully combines lower- and higher-resolution observations in the multi-wavelength analysis without losing spatial information. We also tested enhancing the resolution of each radial intensity profile individually with a flux reconstruction tool ( Frank ), but we note the presence of artifacts. Finally, we discuss the total dust mass that we derived from the SED analyses and compare it with the traditional method of deriving dust masses from millimeter fluxes. Accurate dust mass measurements from the SED analysis can be derived by including optically thin tracers. On the other hand, single-wavelength flux-based masses are always underestimated. For the 0.87 mm flux, the underestimation can be more than one order of magnitude.
Context . Mid-infrared spectra indicate considerable chemical diversity in the inner regions of protoplanetary discs, with some being H 2 O-dominated and others CO 2 -dominated. Sublimating ices from radially drifting dust grains are often invoked to explain some of this diversity, particularly with regards to H 2 O-rich discs.
Aims . We model the contribution made by radially drifting dust grains to the chemical diversity of the inner regions of protoplanetary discs. These grains transport ices – including those of H 2 O and CO 2 – inwards to snow lines, thus redistributing the molecular content of the disc. As radial drift can be impeded by dust trapping in pressure maxima, we also explore the difference between smooth discs and those with dust traps due to gas gaps, quantifying the effects of gap location and formation time.
Methods . We used a 1D protoplanetary disc evolution code to model the chemical evolution of the inner disc resulting from gas viscous evolution and dust radial drift. We post-processed these models to produce synthetic spectra, which we analyse with 0D LTE slab models to understand how this evolution may be expressed observationally.
Results . Discs evolve through an initial H 2 O-rich phase as a result of sublimating ices, followed by a CO 2 -rich phase as H 2 O vapour is advected onto the star and CO 2 is advected into the inner disc from its snow line. The introduction of traps hastens the transition between the phases, temporarily raising the CO 2 /H 2 O ratio. However, whether or not this evolution can be traced in observations depends on the contribution of dust grains to the optical depth. If the dust grains become coupled to the gas after crossing the H 2 O snow line – for example if bare grains fragment more easily than icy grains – then the dust that delivers the H 2 O adds to the continuum optical depth and obscures the H 2 O, preventing any evolution in its visible column density. However, the CO 2 /H 2 O visible column density ratio is only weakly sensitive to assumptions about the dust continuum obscuration, making it a more suitable tracer of the impact of transport on chemistry than either individual column density. This can be investigated with spectra that show weak features that probe deep enough into the disc. The least effective gaps are those that open close to the star on timescales competitive with dust growth and drift as they block too much CO 2 ; gaps opened later or further out lead to higher CO 2 /H 2 O. This leads to a potential correlation between CO 2 /H 2 O and gap location that occurs on million-year timescales for fiducial parameters.
Conclusions . Radial drift, especially when combined with dust trapping, produces CO 2 -rich discs on timescales longer than the viscous timescale at the H 2 O snow line (while creating H 2 O-rich discs at earlier times). Population analyses of the relationship between observed inner disc spectra and large-scale disc structure are needed to test the predicted role of traps.
The inferred dust masses from Class II protoplanetary disk observations are lower than or equal to the masses of the observed exoplanet systems. This poses the question of how planets form if their natal environments do not contain enough mass. This hypothesis has entered the literature as the ``mass budget problem'' of planet formation. We utilized numerical simulations of planet formation via pebble and gas accretion, including migration, in a viscously evolving protoplanetary disk, while tracing the time evolution of the dust mass. As expected, we found that the presence of a giant planet in the disk can influence the evolution of the disk itself and prevent rapid dust mass loss by trapping the dust outside its orbit. Early formation is crucial for giant planet formation, as we found in our previous work; therefore, our findings strengthen the hypothesis that planet formation has already occurred or is ongoing in Class II disks. More importantly, we find that the optically thin dust mass significantly underestimates the total dust mass in the presence of a dust-trapping deep gap. We also show that the beam convolution smears out the feature from a deep gap, especially if the planet forms in the inner disk. Such hidden dust mass, along with early planet formation, could be the answer to the hypothetical mass budget problem.
Context . Herbig disks are prime sites for the formation of massive exoplanets and looking into the precursors of these disks can offer clues for determining planet formation timescales. The precursors of Herbig stars, called intermediate-mass T Tauri (IMTT) stars, have spectral types later than F, but stellar masses between 1.5 and 5 M ⊙ . These stars will eventually become Herbig stars of spectral types A and B.
Aims . The aim of this work is to obtain the dust and gas masses and radii of all IMTT disks with ALMA archival data. The obtained disk masses are then compared to Herbig disks and T Tauri disks and the obtained disks sizes to those of Herbig disks.
Methods . ALMA Band 6 and 7 archival data were obtained for 34 IMTT disks with continuum observations, 32 of which have at least ¹² CO, ¹³ CO, or C ¹⁸ O observations, but with most of them at quite shallow integrations. The disk integrated flux together with a stellar luminosity-scaled disk temperature were used to obtain a total disk dust mass by assuming optically thin emission. Using thermochemical Dust And LInes (DALI) models drawn from previous works, we also obtained gas masses of 10 out of 35 of the IMTT disks based on the CO isotopologues. From the disk masses and sizes, we obtained the cumulative distributions.
Results . The IMTT disks in this study have the same dust mass and radius distributions as Herbig disks. The dust mass of the IMTT disks is higher compared to that of the T Tauri disks, as also found for the Herbig disks. No differences in dust mass were found for group I versus group II disks, in contrast to Herbig disks. The disks for which a gas mass could be determined display a similarly high-mass as to the Herbig disks. Comparing the disk dust and gas mass distributions to the mass distribution of exoplanets shows that there also is not enough dust mass in disks around intermediate-mass stars to form massive exoplanets. On the other hand, there is more than enough gas to form the atmospheres of exoplanets.
Conclusions . We conclude that the sampled IMTT disk population is almost indistinguishable compared to Herbig disks, as their disk masses are the same, even though the former objects are younger. Based on this study, we conclude that planet formation is already well underway in these objects and, thus, planet formation is expected to start early on in the lifetime of Herbig disks. Combined with our findings on group I and group II disks, we conclude that most disks around intermediate-mass pre-main sequence stars converge quickly to small disks, unless they are prevented from doing so by a nearby massive exoplanet.
Atacama Large Millimeter/submillimeter Array surveys have suggested that protoplanetary disks are not massive enough to form the known exoplanet population, based on the assumption that the millimeter continuum emission is optically thin. In this work, we investigate how the mass determination is influenced when the porosity of dust grains is considered in radiative transfer models. The results show that disks with porous dust opacities yield similar dust temperatures, but systematically lower millimeter fluxes, as compared to disks that incorporate compact dust grains. Moreover, we have recalibrated the relation between dust temperature and stellar luminosity for a wide range of stellar parameters. We also calculated the dust masses of a large sample of disks using the traditionally analytic approach. The median dust mass from our calculation is about six times higher than the literature result, and this is mostly driven by the different opacities of porous and compact grains. A comparison of the cumulative distribution function between disk dust masses and exoplanet masses shows that the median exoplanet mass is about two times lower than the median dust mass when grains are assumed to be porous and there are no exoplanetary systems with masses higher than the most massive disks. Our analysis suggests that adopting porous dust opacities may alleviate the mass budget problem for planet formation. As an example illustrating the combined effects of optical depth and porous dust opacities on the mass estimation, we conducted new IRAM/NIKA-2 observations toward the IRAS\,04370+2559 disk and performed a detailed radiative transfer modeling of the spectral energy distribution (SED). The best-fit dust mass is roughly 100 times higher than the value given by a traditionally analytic calculation. Future spatially resolved observations at various wavelengths are required to better constrain the dust mass.
Context . Growing evidence suggests that protoplanetary discs may be influenced by late stage infall from the interstellar medium (ISM). It remains unclear the degree to which infall shapes disc populations at ages ≳1 Myr.
Aims . We explored possible spatial correlations between stellar accretion rates in the Lupus star-forming region, which would support the hypothesis that infall can regulate stellar accretion.
Methods . We considered both the ‘clustered’ stars towards the centre of Lupus 3, and the ‘distributed’ stars that are more sparsely distributed across the Lupus complex. We took the observed accretion rates in the literature and explore spatial correlations. In particular, we tested whether the clustered stars exhibit a radial gradient in normalised accretion rates, and whether the distributed stars have spatially correlated accretion rates.
Results . We found statistically significant correlations for both the clustered and distributed samples. The clustered sample exhibits higher accretion rates in the central region, consistent with the expected Bondi-Hoyle-Lyttleton accretion rate. Stars that are spatially closer among the distributed population also exhibit more similar accretion rates. These results cannot be explained by the stellar mass distribution for either sample. Age gradients are disfavoured, though not discounted, because normalised disc dust masses are not spatially correlated across the region.
Conclusions . Spatially correlated stellar accretion rates within the Lupus star-forming region argue in favour of an environmental influence on stellar accretion, possibly combined with internal processes in the inner disc. Refined age measurements and searches for evidence of infalling material are potential ways to further test this finding.
Context . Stars and planets form in regions of enhanced stellar density, subjecting protoplanetary discs to gravitational perturbations from neighbouring stars. Observations in the Taurus star-forming region have uncovered evidence of at least three recent, star-disc encounters that have truncated discs (HV/DO Tau, RW Aurigae, and UX Tau), raising questions about the frequency of such events.
Aims . We aim to assess the probability of observing truncating star-disc encounters in Taurus.
Methods . We generated a physically motivated dynamical model including binaries and a spatial-kinematic substructure to follow the historical dynamical evolution of the Taurus star-forming region. We used this model to track star-disc encounters and the resulting outer disc truncation over the lifetime of Taurus.
Results . A quarter of discs are truncated below 30 au by dynamical encounters, but this truncation mostly occurs in binaries over the course of a few orbital periods, on a timescale ≲0.1 Myr. Nonetheless, some truncating encounters still occur up to the present age of Taurus. Strongly truncating encounters (ejecting ≳10 percent of the disc mass) occur at a rate ∼10 Myr ⁻¹ , sufficient to explain the encounter between HV and DO Tau ∼0.1 Myr ago. If encounters that eject only ∼1 percent of the disc mass are responsible for RW Aurigae and UX Tau, then they are also expected with encounter rate Γ enc ∼ 100–200 Myr ⁻¹ . However, the observed sample of recent encounters is probably incomplete, since these examples occurred in systems that are not consistent with a random drawing from the mass function. One more observed example would statistically imply additional physics, such as replenishment of the outer disc material.
Conclusions . The marginal consistency of the frequency of observed recent star-disc encounters with theoretical expectations underlines the value of future large surveys searching for external structures associated with recent encounters. The outcome of such a survey offers a highly constraining, novel probe of protoplanetary disc physics.
The mantra “know thy star, know thy planet” has proven to be very important for many aspects of exoplanet science. Here I review how stellar abundances inform our understanding of planet composition and, thus, formation and evolution. In particular, I discuss how: ▪ The strongest star–planet connection is still the giant planet–metallicity correlation, the strength of which may indicate a break point between the formation of planets versus brown dwarfs. ▪ We do not have very good constraints on the lower metallicity limit for planet formation, although new statistics from TESS are helping, and it appears that, at low [Fe/H], α elements can substitute for iron as seeds for planet formation. ▪ The depletion of refractory versus volatile elements in stellar photospheres (particularly the Sun) was initially suggested as a sign of small planet formation but is challenging to interpret, and small differences in binary star compositions can be attributed mostly to processes other than planet formation. ▪ We can and should go beyond comparisons of the carbon-to-oxygen ratio in giant planets and their host stars, incorporating other volatile and refractory species to better constrain planet formation pathways. ▪ There appears to be a positive correlation between small planet bulk density and host star metallicity, but exactly how closely small planet refractory compositions match those of their host stars—and their true diversity—is still uncertain.
We present the results of the observations made within the ALMA Large Program called Early Planet Formation in Embedded disks of the Class 0 protostar GSS30 IRS3. Our observations included the 1.3 mm continuum with a resolution of 0 (7.8 au) and several molecular species, including 12CO, 13CO, C18O _ H_ _ odot
Context. Planets form in the disks surrounding young stars. The time at which the planet formation process begins is still an open question. Annular substructures such as rings and gaps in disks are intertwined with planet formation, and thus their presence or absence is commonly used to investigate the onset of this process.
Aims. Current observations show that a limited number of disks surrounding protostars exhibit annular substructures, all of them in the Class I stage. The lack of observed features in most of these sources may indicate a late emergence of substructures, but it could also be an artifact of these disks being optically thick. To mitigate the problem of optical depth, we investigated substructures within a very young Class 0 disk characterized by low inclination using observations at longer wavelengths.
Methods. We used 3 mm ALMA observations tracing dust emission at a resolution of 7 au to search for evidence of annular substructures in the disk around the deeply embedded Class 0 protostar Oph A SM1.
Results. The observations reveal a nearly face-on disk (inclination ∼ 16°) extending up to 40 au. The radial intensity profile shows a clear deviation from a smooth profile near 30 au, which we interpret as the presence of either a gap at 28 au or a ring at 34 au with Gaussian widths of σ = 1.4 −1.2 +2.3 au and σ = 3.9 −1.9 +2.0 au, respectively. Crucially, the 3 mm emission at the location of the possible gap or ring is determined to be optically thin, precluding the possibility that this feature in the intensity profile is due to the emission being optically thick.
Conclusions. Annular substructures resembling those in the more evolved Class I and II disks could indeed be present in the Class 0 stage, which is earlier than suggested by previous observations. Similar observations of embedded disks in which the high-optical-depth problem can be mitigated are clearly needed to better constrain the onset of substructures in the embedded stages.
We present a new Submillimeter Array survey of 47 Class II sources in the Taurus–Auriga region. Our observations made 12 independent samples of flux densities over the 200–400 GHz frequency range. We tightly constrained the spectral indices of most sources to a narrow range of 2.0 ± 0.2; only a handful of spatially resolved (e.g., diameter >250 au) disks present larger spectral indices. The simplest interpretation for this result is that the (sub)millimeter luminosities of all of the observed target sources are dominated by very optically thick (e.g., τ ≳ 5) dust thermal emission. Some previous works that were based on the optically thin assumption thus might have underestimated optical depths by at least 1 order of magnitude. Assuming DSHARP dust opacities, this corresponds to underestimates of dust masses by a similar factor. For our specific selected sample, the lower limits of dust masses implied by the optically thick interpretation are 1–3 times higher than those previous estimates that were made based on the optically thin assumption. Moreover, some population synthesis models show that, to explain the observed, narrowly distributed spectral indices, the disks in our selected sample need to have very similar dust temperatures ( T dust ). Given a specific assumption of median T dust , the maximum grain sizes ( a max ) can also be constrained, which is a few times smaller than 0.1 mm for T dust ∼ 100 K and a few millimeters for T dust ∼ 24 K. The results may indicate that dust grain growth outside the water snow line is limited by the bouncing/fragmentation barriers. This is consistent with the recent laboratory experiments, which indicated that the coagulation of water-ice-coated dust is not efficient, and the water-ice-free dust is stickier and thus can coagulate more efficiently. In the Class II disks, the dust mass budget outside of the water snow line may be largely retained instead of being mostly consumed by planet formation. While Class II disks still possess sufficient dust masses to feed planet formation at a later time, it is unknown whether or not dust coagulation and planet formation can be efficient or natural outside of the water snow line.
The Rossiter–McLaughlin effect measures the misalignment between a planet’s orbital plane and its host star’s rotation plane. Around 10 per cent of planets exhibit misalignments in the approximate range 80°–125°, with their origin remaining a mystery. On the other hand, large misalignments may be common in eccentric circumbinary systems due to misaligned discs undergoing polar alignment. If the binary subsequently merges, a polar circumbinary disc – along with any planets that form within it – may remain inclined near 90 to the merged star’s rotation. To test this hypothesis, we present N-body simulations of the evolution of a polar circumbinary debris disc comprised of test particles around an eccentric binary during a binary merger that is induced by tidal dissipation. After the merger, the disc particles remain on near-polar orbits. Interaction of the binary with the polar-aligned gas disc may be required to bring the binary to the small separations that trigger the merger by tides. Our findings imply that planets forming in discs that are polar-aligned to the orbit of a high-eccentricity binary may, following the merger of the binary, provide a possible origin for the population of near-polar planets around single stars.
Context. The mass of protoplanetary discs determines the amount of material available for planet formation, the level of coupling between gas and dust, and possibly also sets gravitational instabilities. Measuring the mass of a disc is challenging, as it is not possible to directly detect H 2 , and CO-based estimates remain poorly constrained.
Aims. An alternative method has recently been proposed that does not rely on tracer-to-H 2 ratios. It allows dynamical measurement of the disc mass together with the star mass and the disc critical radius by looking at deviations from Keplerian rotation induced by the self-gravity of the disc. So far, this method has been used to weigh three protoplanetary discs: Elias 2-27, IM Lup, and GM Aurigae.
Methods. We provide a numerical benchmark of the above method. To this end, we simulated isothermal self-gravitating discs with a range of masses from 0.01 to 0.2 M ⊙ with the PHANTOM code and post-processed them with radiative transfer ( MCFOST ) to obtain synthetic observations.
Results. We find that dynamical weighing allows us to retrieve the expected disc masses as long as the disc-to-star mass ratio is larger than M d / M * = 0.05. We estimate an uncertainty for the disc mass measurement of ~25%.
Aims . We study a new mechanism of dust accumulation and planetesimal formation in a gravitationally unstable disk with suppressed magnetorotational instability and we compare it with the classical dead zone in a layered disk model.
Methods . We used numerical hydrodynamics simulations in the thin-disk limit ( FEOSAD code) to model the formation and long-term evolution of gravitationally unstable disks, including dust dynamics and growth.
Results . We found that in gravitationally unstable disks with a radially varying strength of gravitational instability (GI), an inner region (of several astronomical units) of low mass and angular momentum transport is formed. This region is characterized by a low effective value for the α GI parameter, often used to describe the efficiency of mass transport by GI in young protoplanetary disks. The inner region is also similar in terms of characteristics to the dead zone in the layered disk model. As the disk forms and evolves, the GI-induced dead zone accumulates a massive dust ring, which is susceptible to the development of the streaming instability. The model and observationally inferred dust masses and radii may differ significantly in gravitationally unstable disks with massive inner dust rings.
Conclusions . The early occurrence of the GI-induced dust ring, followed by the development of the streaming instability suggest that this mechanism may be behind the formation of the first generation of planetesimals in the inner terrestrial zone of the disk. The proposed mechanism, however, crucially depends on the susceptibility of the disk to gravitational instability and requires the magnetorotational instability to be suppressed.
Context . Recent observations of protostellar cores suggest that most of the material in the protostellar phase is accreted along streamers. Streamers in this context are defined as velocity coherent funnels of denser material potentially connecting the large-scale environment to the small scales of the forming accretion disk.
Aims . Using simulations that simultaneously resolve the driving of turbulence on the filament scale as well as the collapse of the core down to protostellar disk scales, we aim to understand the effect of the turbulent velocity field on the formation of overdensities in the accretion flow.
Methods . We performed a three-dimensional numerical study on a core collapse within a turbulent filament using the RAMSES code and analysed the properties of overdensities in the accretion flow.
Results . We find that overdensities are formed naturally by the initial turbulent velocity field inherited from the filament and subsequent gravitational collimation. This leads to streams that are not really filamentary but show a sheet-like morphology. Moreover, they have the same radial infall velocities as the low density material. As a main consequence of the turbulent initial condition, the mass accretion onto the disk does not follow the predictions for solid body rotation. Instead, most of the mass is funneled by the overdensities to intermediate disk radii.
Context . The observed diversity of exoplanets can possibly be traced back to the planet formation processes. Planet-disk interactions induce sub-structures in the circumstellar disk that can be revealed via scattered light observations. However, a high-contrast imaging technique such as polarimetric differential imaging (PDI) must first be applied to suppress the stellar diffraction halo.
Aims . In this work we present the PDI PiPelIne for NACO data (PIPPIN), which reduces the archival polarimetric observations made with the NACO instrument at the Very Large Telescope. Prior to this work, such a comprehensive pipeline to reduce polarimetric NACO data did not exist. We identify a total of 243 datasets of 57 potentially young stellar objects observed before NACO’s decommissioning.
Methods . The PIPPIN pipeline applies various levels of instrumental polarisation correction and is capable of reducing multiple observing setups, including half-wave plate or de-rotator usage and wire-grid observations. A novel template-matching method is applied to assess the detection significance of polarised signals in the reduced data.
Results . In 22 of the 57 observed targets, we detect polarised light resulting from a scattering of circumstellar dust. The detections exhibit a collection of known sub-structures, including rings, gaps, spirals, shadows, and in- or outflows of material. Since NACO was equipped with a near-infrared wavefront sensor, it made unique polarimetric observations of a number of embedded protostars. This is the first time detections of the Class I objects Elia 2-21 and YLW 16A have been published. Alongside the outlined PIPPIN pipeline, we publish an archive of the reduced data products, thereby improving the accessibility of these data for future studies.
The diversity of exoplanets has been linked to the disc environment in which they form, where the host star metallicity and the formation pathways play a crucial role. In the context of the core accretion paradigm, the initial stages of planet formation require the growth of dust material from micrometre-sized to planetesimal-sized bodies before core accretion can kick in. Although numerous studies have been conducted on planetesimal formation, it is still poorly understood how this process takes place in low-metallicity stellar environments. In this work, we explore how planetesimals are formed in stellar environments with primarily low metallicities. We performed global 1D viscous disc evolution simulations, including the growth of dust particles and the evaporation and condensation of chemical species at ice lines. We followed the formation of planetesimals during disc evolution and tested different metallicities, disc sizes, and turbulent viscosity strengths. We find that at solar and sub-solar metallicities, there is a significant increase in the midplane dust-to-gas mass ratios at the ice lines, but this leads to planetesimal formation only at the water--ice line. In our simulations Fe/H is the lower limit of metallicity for planetesimal formation where a few Earth masses of planetesimals can form. Our results further show that for such extreme disc environments, large discs are more conducive than small discs for forming large amounts of planetesimals at a fixed metallicity because the pebble flux can be maintained for a longer time, resulting in a longer and more efficient planetesimal formation phase. At lower metallicities, planetesimal formation is less supported in quiescent discs compared to turbulent discs, which produce larger amounts of planetesimals, because the pebble flux can be maintained for a longer time. The amount of planetesimals formed at sub-solar metallicities in our simulations places a limit on core sizes that could potentially result only in the formation of super-Earths.
Context . Elongated trails of infalling gas, often referred to as “streamers,” have recently been observed around young stellar objects (YSOs) at different evolutionary stages. This asymmetric infall of material can significantly alter star and planet formation processes, especially in the more evolved YSOs.
Aims . In order to ascertain the infalling nature of observed streamer-like structures and then systematically characterize their dynamics, we developed the code TIPSY (Trajectory of Infalling Particles in Streamers around Young stars).
Methods . Using TIPSY, the streamer molecular line emission is first isolated from the disk emission. Then the streamer emission, which is effectively a point cloud in three-dimensional (3D) position–position–velocity space, is simplified to a curve-like representation. The observed streamer curve is then compared to the theoretical trajectories of infalling material. The best-fit trajectories are used to constrain streamer features, such as the specific energy, the specific angular momenta, the infall timescale, and the 3D morphology.
Results . We used TIPSY to fit molecular-line ALMA observations of streamers around a Class II binary system, S CrA, and a Class I/II protostar, HL Tau. Our results indicate that both of the streamers are consistent with infalling motion. For the S CrA streamer, we could constrain the dynamical parameters well and find it to be on a bound elliptical trajectory. On the other hand, the fitting uncertainties are substantially higher for the HL Tau streamer, likely due to the smaller spatial scales of the observations. TIPSY results and mass estimates suggest that S CrA and HL Tau are accreting material at a rate of ≳27 M jupiter Myr –1 and ≳5 M jupiter Myr –1 , respectively, which can significantly increase the mass budget available to form planets.
Conclusions . TIPSY can be used to assess whether the morphology and kinematics of observed streamers are consistent with infalling motion and to characterize their dynamics, which is crucial for quantifying their impact on the protostellar systems.
Context . Due to the presence of magnetic fields, protostellar jets or outflows are a natural consequence of accretion onto protostars. They are expected to play an important role in star and protoplanetary disk formation.
Aims . We aim to determine the influence of outflows on star and protoplanetary disk formation in star-forming clumps.
Methods . Using RAMSES , we performed the first magnetohydrodynamics calculation of massive star-forming clumps with ambipolar diffusion; radiative transfer, including the radiative feedback of protostars; and protostellar outflows while systematically resolving the disk scales. We compared this simulation to a model without outflows.
Results . We found that protostellar outflows have a significant impact on both star and disk formation. They provide a significant amount of additional kinetic energy to the clump, with typical velocities of around a few 10 km s ⁻¹ ; impact the clump and disk temperatures; reduce the accretion rate onto the protostars; and enhance fragmentation in the filaments. We found that they promote a more numerous stellar population. They do not impact the low-mass end of the IMF much, which is probably controlled by the mass of the first Larson core; however, they have an influence on its peak and high-mass end.
Conclusions . Protostellar outflows appear to have a significant influence on both star and disk formation and should therefore be included in realistic simulations of star-forming environments.
Context. Hydrodynamical instabilities are likely the main source of turbulence in weakly ionized regions of protoplanetary disks. Among these, the vertical shear instability (VSI) stands out as a rather robust mechanism due to its few requirements to operate, namely a baroclinic stratification, which is enforced by the balance of stellar heating and radiative cooling, and short thermal relaxation timescales.
Aims. Our goal is to characterize the transport of angular momentum and the turbulent heating produced by the nonlinear evolution of the VSI in axisymmetric models of disks around T Tauri stars, considering varying degrees of depletion of small dust grains resulting from dust coagulation. We also explore the local applicability of both local and global VSI-stability criteria.
Methods. We modeled the gas-dust mixture in our disk models by means of high-resolution axisymmetric radiation-hydrodynamical simulations including stellar irradiation with frequency-dependent opacities. This is the first study of this instability to rely on two-moment radiative transfer methods. Not only are these able to handle transport in both the optically thin and thick limits, but also they can be integrated via implicit-explicit methods, thus avoiding the employment of expensive global matrix solvers. This is done at the cost of artificially reducing the speed of light, which, as we verified for this work, does not introduce unphysical phenomena.
Results. Given sufficient depletion of small grains (with a dust-to-gas mass ratio of 10% of our nominal value of 10 ⁻³ for < 0.25 μm grains), the VSI can operate in surface disk layers while being inactive close to the midplane, resulting in a suppression of the VSI body modes. The VSI reduces the initial vertical shear in bands of approximately uniform specific angular momentum, whose formation is likely favored by the enforced axisymmetry. Similarities with Reynolds stresses and angular momentum distributions in 3D simulations suggest that the VSI-induced angular momentum mixing in the radial direction may be predominantly axisymmetric. The stability regions in our models are well explained by local stability criteria, while the employment of global criteria is still justifiable up to a few scale heights above the midplane, at least as long as VSI modes are radially optically thin. Turbulent heating produces only marginal temperature increases of at most 0.1% and 0.01% in the nominal and dust-depleted models, respectively, peaking at a few (approximately three) scale heights above the midplane. We conclude that it is unlikely that the VSI can, in general, lead to any significant temperature increase since that would either require it to efficiently operate in largely optically thick disk regions or to produce larger levels of turbulence than predicted by models of passive irradiated disks.
A large planet orbiting a very low-mass star challenges theories of planet formation
Theories of planet formation predict that low-mass stars should rarely host exoplanets with masses exceeding that of Neptune. We used radial velocity observations to detect a Neptune-mass exoplanet orbiting LHS 3154, a star that is nine times less massive than the Sun. The exoplanet’s orbital period is 3.7 days, and its minimum mass is 13.2 Earth masses. We used simulations to show that the high planet-to-star mass ratio (>3.5 × 10 ⁻³ ) is not an expected outcome of either the core accretion or gravitational instability theories of planet formation. In the core-accretion simulations, we show that close-in Neptune-mass planets are only formed if the dust mass of the protoplanetary disk is an order of magnitude greater than typically observed around very low-mass stars.
Context. We present the second Gaia data release, Gaia DR2, consisting of astrometry, photometry, radial velocities, and information on astrophysical parameters and variability, for sources brighter than magnitude 21. In addition epoch astrometry and photometry are provided for a modest sample of minor planets in the solar system.
Aims. A summary of the contents of Gaia DR2 is presented, accompanied by a discussion on the differences with respect to Gaia DR1 and an overview of the main limitations which are still present in the survey. Recommendations are made on the responsible use of Gaia DR2 results.
Methods. The raw data collected with the Gaia instruments during the first 22 months of the mission have been processed by the Gaia Data Processing and Analysis Consortium (DPAC) and turned into this second data release, which represents a major advance with respect to Gaia DR1 in terms of completeness, performance, and richness of the data products. Results. Gaia DR2 contains celestial positions and the apparent brightness in G for approximately 1.7 billion sources. For 1.3 billion of those sources, parallaxes and proper motions are in addition available. The sample of sources for which variability information is provided is expanded to 0.5 million stars. This data release contains four new elements: broad-band colour information in the form of the apparent brightness in the GBP (330–680 nm) and GRP (630–1050 nm) bands is available for 1.4 billion sources; median radial velocities for some 7 million sources are presented; for between 77 and 161 million sources estimates are provided of the stellar effective temperature, extinction, reddening, and radius and luminosity; and for a pre-selected list of 14 000 minor planets in the solar system epoch astrometry and photometry are presented. Finally, Gaia DR2 also represents a new materialisation of the celestial reference frame in the optical, the Gaia -CRF2, which is the first optical reference frame based solely on extragalactic sources. There are notable changes in the photometric system and the catalogue source list with respect to Gaia DR1, and we stress the need to consider the two data releases as independent. Conclusions. Gaia DR2 represents a major achievement for the Gaia mission, delivering on the long standing promise to provide parallaxes and proper motions for over 1 billion stars, and representing a first step in the availability of complementary radial velocity and source astrophysical information for a sample of stars in the Gaia survey which covers a very substantial fraction of the volume of our galaxy.
We present ALMA Band 6 observations of a complete sample of protoplanetary disks in the young (1-3 Myr) Lupus star-forming region, covering the 1.33 mm continuum and the 12CO, 13CO, and C18O J=2-1 lines. The spatial resolution is 0.25 arcsec with a medium 3-sigma continuum sensitivity of 0.30 mJy, corresponding to M_dust ~ 0.2 M_earth. We apply "Keplerian masking" to enhance the signal-to-noise ratios of our 12CO zero-moment maps, enabling measurements of gas disk radii for 22 Lupus disks; we find that gas disks are universally larger than mm dust disks by a factor of two on average, likely due to a combination of the optically thick gas emission as well as the growth and inward drift of the dust. Using the gas disk radii, we calculate the dimensionless viscosity parameter, alpha_visc, finding a broad distribution and no correlations with other disk or stellar parameters, suggesting that viscous processes have not yet established quasi-steady states in Lupus disks. By combining our 1.33 mm continuum fluxes with our previous 890 micron continuum observations, we also calculate the mm spectral index, alpha_mm, for 70 Lupus disks; we find an anti-correlation between alpha_mm and mm flux for low-mass disks (M_dust < 5), followed by a flattening as disks approach alpha_mm = 2, which could indicate faster grain growth in higher-mass disks, but may also reflect their larger optically thick components. In sum, this work demonstrates the continuous stream of new insights into disk evolution and planet formation that can be gleaned from unbiased ALMA disk surveys.
Pebble accretion is the mechanism in which small particles (“pebbles”) accrete onto big bodies (planetesimals or planetary embryos) in gas-rich environments. In pebble accretion, accretion occurs by settling and depends only on the mass of the gravitating body, not its radius. I give the conditions under which pebble accretion operates and show that the collisional cross section can become much larger than in the gas-free, ballistic, limit. In particular, pebble accretion requires the pre-existence of a massive planetesimal seed. When pebbles experience strong orbital decay by drift motions or are stirred by turbulence, the accretion efficiency is low and a great number of pebbles are needed to form Earth-mass cores. Pebble accretion is in many ways a more natural and versatile process than the classical, planetesimal-driven paradigm, opening up avenues to understand planet formation in solar and exoplanetary systems.
The formation of planets strongly depends on the total amount as well as on the spatial distribution of solids in protoplanetary disks. Thanks to the improvements in resolution and sensitivity provided by ALMA, measurements of the surface density of mm-sized grains are now possible on large samples of disks. Such measurements provide statistical constraints that can be used to inform our understanding of the initial conditions of planet formation. We analyze spatially resolved observations of 36 protoplanetary disks in the Lupus star forming complex from our ALMA survey at 890 micron, aiming to determine physical properties such as the dust surface density, the disk mass and size and to provide a constraint on the temperature profile. We fit the observations directly in the uv-plane using a two-layer disk model that computes the 890 micron emission by solving the energy balance at each disk radius. For 22 out of 36 protoplanetary disks we derive robust estimates of their physical properties. The sample covers stellar masses between ~0.1 and ~2 Solar masses, and we find no trend between the average disk temperatures and the stellar parameters. We find, instead, a correlation between the integrated sub-mm flux (a proxy for the disk mass) and the exponential cut-off radii (a proxy of the disk size) of the Lupus disks. Comparing these results with observations at similar angular resolution of Taurus-Auriga/Ophiuchus disks found in literature and scaling them to the same distance, we observe that the Lupus disks are generally fainter and larger at a high level of statistical significance. Considering the 1-2 Myr age difference between these regions, it is possible to tentatively explain the offset in the disk mass/disk size relation with viscous spreading, however with the current measurements other mechanisms cannot be ruled out.
One focus of modern astronomy is to detect temperate terrestrial exoplanets well-suited for atmospheric characterisation. A milestone was recently achieved with the detection of three Earth-sized planets transiting (i.e. passing in front of) a star just 8% the mass of the Sun 12 parsecs away. Indeed, the transiting configuration of these planets with the Jupiter-like size of their host star - named TRAPPIST-1 - makes possible in-depth studies of their atmospheric properties with current and future astronomical facilities. Here we report the results of an intensive photometric monitoring campaign of that star from the ground and with the Spitzer Space Telescope. Our observations reveal that at least seven planets with sizes and masses similar to the Earth revolve around TRAPPIST-1. The six inner planets form a near-resonant chain such that their orbital periods (1.51, 2.42, 4.04, 6.06, 9.21, 12.35 days) are near ratios of small integers. This architecture suggests that the planets formed farther from the star and migrated inward. The seven planets have equilibrium temperatures low enough to make possible liquid water on their surfaces.
Gaia is a cornerstone mission in the science programme of the European Space Agency (ESA). The spacecraft construction was approved in 2006, following a study in which the original interferometric concept was changed to a direct-imaging approach. Both the spacecraft and the payload were built by European industry. The involvement of the scientific community focusses on data processing for which the international Gaia Data Processing and Analysis Consortium (DPAC) was selected in 2007. Gaia was launched on 19 December 2013 and arrived at its operating point, the second Lagrange point of the Sun-Earth-Moon system, a few weeks later. The commissioning of the spacecraft and payload was completed on 19 July 2014. The nominal five-year mission started with four weeks of special, ecliptic-pole scanning and subsequently transferred into full-sky scanning mode. We recall the scientific goals of Gaia and give a description of the as-built spacecraft that is currently (mid-2016) being operated to achieve these goals. We pay special attention to the payload module, the performance of which is closely related to the scientific performance of the mission. We provide a summary of the commissioning activities and findings, followed by a description of the routine operational mode. We summarise scientific performance estimates on the basis of in-orbit operations. Several intermediate Gaia data releases are planned and the data can be retrieved from the Gaia Archive, which is available through the Gaia home page at http://www.cosmos.esa.int/gaia.
The disk mass is among the most important input parameter for every planet formation model to determine the number and masses of the planets that can form. We present an ALMA 887micron survey of the disk population around objects from 2 to 0.03Msun in the nearby 2Myr-old Chamaeleon I star-forming region. We detect thermal dust emission from 66 out of 93 disks, spatially resolve 34 of them, and identify two disks with large dust cavities of about 45AU in radius. Assuming isothermal and optically thin emission, we convert the 887micron flux densities into dust disk masses, hereafter Mdust. We find that the Mdust-Mstar relation is steeper than linear with power law indices 1.3-1.9, where the range reflects two extremes of the possible relation between the average dust temperature and stellar luminosity. By re-analyzing all millimeter data available for nearby regions in a self-consistent way, we show that the 1-3 Myr-old regions of Taurus, Lupus, and Chamaeleon I share the same Mdust-Mstar relation, while the 10Myr-old Upper Sco association has a steeper relation. Theoretical models of grain growth, drift, and fragmentation reproduce this trend and suggest that disks are in the fragmentation-limited regime. In this regime millimeter grains will be located closer in around lower-mass stars, a prediction that can be tested with deeper and higher spatial resolution ALMA observations.
The terrestrial planets and the asteroids dominant in the inner asteroid belt
are water poor. However, in the protoplanetary disk the temperature should have
decreased below water condensation level well before the disk was
photoevaporated. Thus, the global water depletion of the inner Solar System is
puzling. We show that, even if the inner disk becomes cold, there cannot be
direct condensation of water. This is because the snowline moves towards the
Sun more slowly than the gas itself. The appearance of ice in a range of
heliocentric distances swept by the snowline can only be due to the radial
drift of icy particles from the outer disk. However, if a sufficiently massive
planet is present, the radial drift of particles is interrupted, because the
disk acquires a superKeplerian rotation just outside of the planetary orbit.
From this result, we propose that the precursor of Jupiter achieved about 20
Earth masses when the snowline was still around 3 AU. This effectively
fossilized the snowline at that location. Although cooling, the disk inside of
the Jovian orbit remained ice-depleted because the flow of icy particles from
the outer system was intercepted by the planet. This scenario predicts that
planetary systems without giant planets should be much more rich in water in
their inner regions than our system. We also show that the inner edge of the
planetesimal disk at 0.7AU, required in terrestrial planet formation models to
explain the small mass of Mercury and the absence of planets inside of its
orbit, could be due to the silicate condensation line, fossilized at the end of
the phase of streaming instability that generated the planetesimal seeds. Thus,
when the disk cooled, silicate particles started to drift inwards of 0.7AU
without being sublimated, but they could not be accreted by any pre-existing
planetesimals.
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0 ''.075 (10 AU) to 0 ''.025 (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analog HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46 degrees.72 +/- 0 degrees.05) and position angle (+138 degrees.02 +/- 0 degrees.07).We obtain a high-fidelity image of the 1.0 mm spectral index (alpha), which ranges from alpha similar to 2.0 in the optically thick central peak and two brightest rings, increasing to 2.3-3.0 in the dark rings. The dark rings are not devoid of emission, and we estimate a grain emissivity index of 0.8 for the innermost dark ring and lower for subsequent dark rings, consistent with some degree of grain growth and evolution. Additional clues that the rings arise from planet formation include an increase in their central offsets with radius and the presence of numerous orbital resonances. At a resolution of 35 AU, we resolve the molecular component of the disk in HCO+. (1-0) which exhibits a pattern over LSR velocities from 2-12 km s(-1) consistent with Keplerian motion around a similar to 1.3 M-circle dot star, although complicated by absorption at low blueshifted velocities. We also serendipitously detect and resolve the nearby protostars XZ Tau (A/B) and LkH alpha 358 at 2.9 mm.
We present new models for low-mass stars down to the hydrogen-burning limit
that consistently couple atmosphere and interior structures, thereby
superseding the widely used BCAH98 models. The new models include updated
molecular linelists and solar abundances, as well as atmospheric convection
parameters calibrated on 2D/3D radiative hydrodynamics simulations. Comparison
of these models with observations in various colour-magnitude diagrams for
various ages shows significant improvement over previous generations of models.
The new models can solve flaws that are present in the previous ones, such as
the prediction of optical colours that are too blue compared to M dwarf
observations. They can also reproduce the four components of the young
quadruple system LkCa 3 in a colour-magnitude diagram with one single
isochrone, in contrast to any presently existing model. In this paper we also
highlight the need for consistency when comparing models and observations, with
the necessity of using evolutionary models and colours based on the same
atmospheric structures.
The high rate of planet detection among solar-type stars argues that planet formation is common. It is also generally assumed
that planets form in protoplanetary discs like those observed in nearby star-forming regions. On what time-scale does the
transformation from discs to planets occur? Here, we show that current inventories of planets and protoplanetary discs are
sensitive enough to place basic constraints on the time-scale and efficiency of the planet formation process. A comparison
of planet detection statistics and the measured solid reservoirs in T Tauri discs suggests that planet formation is likely
already underway at the few Myr age of the discs in Taurus–Auriga, with a large fraction of solids having been converted into
large objects with low millimetre opacity and/or sequestered at small disc radii where they are difficult to detect at millimetre
wavelengths.
Young stars show evidence of accretion discs which evolve quickly and
disperse with an e-folding time of 3Myr. This is in striking contrast
with recent observations that suggest evidence for numerous Myr old stars
with an accretion disc in large star-forming complexes. We consider whether
these observations of apparently old accretors could be explained by invoking
Bondi-Hoyle accretion to rebuild a new disc around these stars during passage
through a clumpy molecular cloud. We combine a simple Monte Carlo model to
explore the capture of mass by such systems with a viscous evolution model to
infer the levels of accretion that would be observed. We find that a
significant fraction of stars may capture enough material via the Bondi-Hoyle
mechanism to rebuild a disc of mass 1 minimum-mass solar nebula, and
accrete at observable levels at any given time. A significant
fraction of the observed old accretors may be explained with our proposed
mechanism. Such accretion may provide a chance for a second epoch of planet
formation, and have unpredictable consequences for planetary evolution.
We present X-Shooter/VLT observations of a sample of 36 accreting low-mass
stellar and sub-stellar objects (YSOs) in the Lupus star forming region,
spanning a range in mass from ~0.03 to ~1.2Msun, but mostly with 0.1Msun <
Mstar < 0.5Msun. Our aim is twofold: firstly, analyse the relationship between
excess-continuum and line emission accretion diagnostics, and, secondly, to
investigate the accretion properties in terms of the physical properties of the
central object. The accretion luminosity (Lacc), and from it the accretion rate
(Macc), is derived by modelling the excess emission, from the UV to the
near-IR, as the continuum emission of a slab of hydrogen. The flux and
luminosity (Ll) of a large number of emission lines of H, He, CaII, etc.,
observed simultaneously in the range from ~330nm to 2500nm, were computed. The
luminosity of all the lines is well correlated with Lacc. We provide empirical
relationships between Lacc and the luminosity of 39 emission lines, which have
a lower dispersion as compared to previous relationships in the literature. Our
measurements extend the Pab and Brg relationships to Lacc values about two
orders of magnitude lower than those reported in previous studies. We confirm
that different methodologies to measure Lacc and Macc yield significantly
different results: Ha line profile modelling may underestimate Macc by 0.6 to
0.8dex with respect to Macc derived from continuum-excess measures. Such
differences may explain the likely spurious bi-modal relationships between Macc
and other YSOs properties reported in the literature. We derive Macc in the
range 2e-12 -- 4e-8 Msun/yr and conclude that Macc is proportional to
Mstar^1.8(+/-0.2), with a dispersion lower by a factor of about 2 than in
previous studies. A number of properties indicate that the physical conditions
of the accreting gas are similar over more than 5 orders of magnitude in Macc.
The present observational understanding of the evolution of the mass accretion rates () in pre-main-sequence stars is limited by the lack of accurate measurements of over homogeneous and large statistical samples of young stars. Such observational effort is needed to properly constrain the theory of star formation and disk evolution. Based on Hubble Space Telescope/WFPC2 observations, we present a study of for a sample of ~700 sources in the Orion Nebula Cluster, ranging from the hydrogen-burning limit to M * ~ 2 M ☉. We derive from both the U-band excess and the Hα luminosity (L Hα), after determining empirically both the shape of the typical accretion spectrum across the Balmer jump and the relation between the accretion luminosity (L acc) and L Hα, which is L acc/L ☉ = (1.31 ± 0.03) L Hα/L ☉ + (2.63 ± 0.13). Given our large statistical sample, we are able to accurately investigate relations between and the parameters of the central star such as mass and age. We clearly find to increase with stellar mass and decrease over evolutionary time, but we also find strong evidence that the decay of with stellar age occurs over longer timescales for more massive PMS stars. Our best-fit relation between these parameters is given by log (/M ☉ yr) = (–5.12 ± 0.86) – (0.46 ± 0.13) log (t/yr) – (5.75 ± 1.47) log (M */M ☉) + (1.17 ± 0.23) log (t/yr) log (M */M ☉). These results also suggest that the similarity solution model could be revised for sources with M * 0.5 M ☉. Finally, we do not find a clear trend indicating environmental effects on the accretion properties of the sources.
We describe an online database for extra-solar planetary-mass candidates,
updated regularly as new data are available. We first discuss criteria for the
inclusion of objects in the catalog: "definition" of a planet and several
aspects of the confidence level of planet candidates. {\bf We are led to point
out the conflict between sharpness of belonging or not to a catalogue and
fuzziness of the confidence level.} We then describe the different tables of
extra-solar planetary systems, including unconfirmed candidates (which will
ultimately be confirmed, or not, by direct imaging). It also provides online
tools: histogrammes of planet and host star data, cross-correlations between
these parameters and some VO services. Future evolutions of the database are
presented.
We present initial result of a large spectroscopic survey aimed at measuring the timescale of mass accretion in young, pre-main-sequence stars in the spectral type range K0 - M5. Using multi-object spectroscopy with VIMOS at the VLT we identified the fraction of accreting stars in a number of young stellar clusters and associations of ages between 1 - 50 Myr. The fraction of accreting stars decreases from ~60% at 1.5 - 2 Myr to ~2% at 10 Myr. No accreting stars are found after 10 Myr at a sensitivity limit of Msun yr-1. We compared the fraction of stars showing ongoing accretion (f_acc) to the fraction of stars with near-to-mid infrared excess (f_IRAC). In most cases we find f_acc < f_IRAC, i.e., mass accretion appears to cease (or drop below detectable level) earlier than the dust is dissipated in the inner disk. At 5 Myr, 95% of the stellar population has stopped accreting material at a rate of > 10^{-11} Msun yr-1, while ~20% of the stars show near-infrared excess emission. Assuming an exponential decay, we measure a mass accretion timescale (t_acc) of 2.3 Myr, compared to a near-to-mid infrared excess timescale (t_IRAC) of 2.9 Myr. Planet formation, and/or migration, in the inner disk might be a viable mechanism to halt further accretion onto the central star on such a short timescale. Comment: Accepted for publication
We constrain the intrinsic architecture of Kepler planetary systems by modeling the observed multiplicities of the transiting planets (tranets) and their transit timing variations (TTVs). We robustly determine that the fraction of Sun-like stars with Kepler-like planets, , is . Here Kepler-like planets are planets that have radii and orbital periods ~days. Our result thus significantly revises previous claims that more than 50\% of Sun-like stars have such planets. Combining with the average number of Kepler planets per star (), we obtain that on average each planetary system has planets within 400 days. We also find that the dispersion in orbital inclinations of planets within a given planetary system, , is a steep function of its number of planets, k. This can be parameterized as and we find that at 2- level. Such a distribution well describes the observed multiplicities of both tranets and TTVs with no excess of single tranets. Therefore we do not find evidence supporting the so-called "Kepler dichotomy." Together with a previous study on orbital eccentricities, we now have a consistent picture: the fewer planets in a system, the hotter it is dynamically. We discuss briefly possible scenarios that lead to such a trend. Despite our Solar system not belonging to the Kepler club, it is interesting to notice that the Solar system also has three planets within 400 days and that the inclination dispersion is similar to Kepler systems of the same multiplicity.
In this article we introduce methods used to determine the gas and dust masses of protoplanetary disks, with an emphasis on the lesser characterized total gas mass. Our review encompasses all the indirect tracers and the methodology that is being used to isolate the hidden H2 via dust, CO, and HD emission. We discuss the overall calibration of gaseous tracers which is based on decades of study of the dense phases of the interstellar medium. At present, disk gas masses determined via CO and HD are (in a few instances) different by orders of magnitude, hinting at either significant evolution in total disk mass or in the CO abundance. Either of these would represent a fundamental physical or chemical process that appears to dominate the system on ∼ million year timescales. Efforts to reconcile these differences using existing and future facilities are discussed.
We present a high angular resolution (), high sensitivity ( mJy) survey of the 870 m continuum emission from the circumstellar material around 49 pre-main sequence stars in the Ophiuchus molecular cloud. Because most millimeter instruments have resided in the northern hemisphere, this represents the largest high-resolution, millimeter-wave survey of the circumstellar disk content of this cloud. Our survey of 49 systems comprises 63 stars; we detect disks associated with 29 single sources, 11 binaries, 3 triple systems and 4 transition disks. We present flux and radius distributions for these systems; in particular, this is the first presentation of a reasonably complete probability distribution of disk radii at millimeter-wavelengths. We also compare the flux distribution of these protoplanetary disks with that of the disk population of the Taurus-Auriga molecular cloud. We find that disks in binaries are both significantly smaller and have much less flux than their counterparts around isolated stars. We compute truncation calculations on our binary sources and find that these disks are too small to have been affected by tidal truncation and posit some explanations for this. Lastly, our survey found 3 candidate gapped disks, one of which is a newly identified transition disk with no signature of a dip in infrared excess in extant observations.
We have analysed the [OI]6300 A line in a sample of 131 young stars with discs in the Lupus, Chamaeleon and signa Orionis star forming regions, observed with the X-shooter spectrograph at VLT. The stars have mass accretion rates spanning from 10^{-12} to 10^{-7} Mo/yr. The line profile was deconvolved into a low velocity component (LVC, < 40 km/s) and a high velocity component (HVC, > 40 km/s ), originating from slow winds and high velocity jets, respectively. The LVC is by far the most frequent component, with a detection rate of 77%, while only 30% of sources have a HVC. The [OI]6300 luminosity of both the LVC and HVC, when detected, correlates with stellar and accretion parameters of the central sources (i.e. Lstar , Mstar , Lacc , Macc), with similar slopes for the two components. The line luminosity correlates better with the accretion luminosity than with the stellar luminosity or stellar mass. We suggest that accretion is the main drivers for the line excitation and that MHD disc-winds are at the origin of both components. In the sub-sample of Lupus sources observed with ALMA a relationship is found between the HVC peak velocity and the outer disc inclination angle, as expected if the HVC traces jets ejected perpendicularly to the disc plane. Mass loss rates measured from the HVC span from ~ 10^{-13} to ~10^{-7} Mo/yr. The corresponding Mloss/Macc ratio ranges from ~0.01 to ~0.5, with an average value of 0.07. However, considering the upper limits on the HVC, we infer a ratio < 0.03 in more than 40% of sources. We argue that most of these sources might lack the physical conditions needed for an efficient magneto-centrifugal acceleration in the star-disc interaction region. Systematic observations of populations of younger stars, that is, class 0/I, are needed to explore how the frequency and role of jets evolve during the pre-main sequence phase.
Context. The determination of the distance to dark star-forming clouds is a key parameter to derive the properties of the cloud itself and of its stellar content. This parameter is still loosely constrained even in nearby star-forming regions.
Aim. We want to determine the distances to the clouds in the Chamaeleon-Musca complex and explore the connection between these clouds and the large-scale cloud structures in the Galaxy.
Methods. We used the newly estimated distances obtained from the parallaxes measured by the Gaia satellite and included in the Tycho–Gaia Astrometric Solution catalog. When known members of a region are included in this catalog we used their distances to infer the distance to the cloud. Otherwise, we analyzed the dependence of the color excess on the distance of the stars and looked for a turn-on of this excess, which is a proxy of the position of the front-edge of the star-forming cloud.
Results. We are able to measure the distance to the three Chamaeleon clouds. The distance to Chamaeleon I is 179 -10-10 ⁺¹¹⁺¹¹ pc, where the quoted uncertainties are statistical and systematic uncertainties, respectively, ~20 pc further away than previously assumed. The Chamaeleon II cloud is located at the distance of 181 -5-10 ⁺⁶⁺¹¹ pc, which agrees with previous estimates. We are able to measure for the first time a distance to the Chamaeleon III cloud of 199 -7-11 ⁺⁸⁺¹² pc. Finally, the distance of the Musca cloud is smaller than 603603 -70-92 ⁺⁹¹⁺¹³³ pc. These estimates do not allow us to distinguish between the possibility that the Chamaeleon clouds are part of a sheet of clouds parallel to the Galactic plane, or perpendicular to it.
Conclusions. We measured a larger distance to the Chamaeleon I cloud than assumed in the past, confirmed the distance to the Chamaeleon II region, and measured for the first time the distance to the Chamaleon III cloud. These values are consistent with the scenario in which the three clouds are part of a single large-scale structure. Gaia Data Release 2 will allow us to put more stringent constraints on the distances to these clouds by giving us access to parallax measurements for a larger number of members of these regions.
We present a comprehensive evolutionary model of the Sun's protoplanetary disk. The model predicts from first principles the gas densities and temperatures, and abundances of calcium-rich, aluminum-rich inclusions (CAIs) and refractory lithophile elements (Ca, Al, Ti, etc.). A central assumption is that Jupiter's core formed early ( Myr) at 3 AU, opening a gap and creating a pressure maximum beyond it in which CAIs were trapped, thereby resolving the "CAI Storage Problem" of meteoritics. Carbonaceous chondrites formed in this pressure trap, while ordinary and enstatite chondrites formed from material inside Jupiter depleted in CAIs by aerodynamic drag. We match the model outputs at different times and locations to each of 11 chondrites, 5 achondrites, and the embryos of Earth and Mars, finding excellent agreement with known meteoritic constraints, and making specific, testable predictions where constraints are lacking. We predict the embryos of the terrestrial planets formed rapidly, in Myr, and that dynamical scattering of asteroids was limited. We predict CI chondrites are depleted in refractory elements relative to the Sun, by about 9\% (0.04 dex). The model demands low levels of turbulence () inside 1 AU, falling to lower levels () beyond 10 AU, suggesting angular momentum transport was dominated by hydrodynamical instabilities augmented by magnetic disk winds, and not by the magnetorotational instability. By 4 Myr, gas had vanished interior to Jupiter, but persisted beyond Jupiter, so that the solar nebula was a transition disk. The model demonstrates the power of meteoritic data to constrain astrophysical disk processes.
The detection and characterization of large populations of pebbles in protoplanetary disks have motivated the study of pebble accretion as a driver of planetary growth. This review covers all aspects of planet formation by pebble accretion, from dust growth over planetesimal formation to the accretion of protoplanets and fully grown planets with gaseous envelopes. Pebbles are accreted at a very high rate—orders of magnitude higher than planetesimal accretion—and the rate decreases only slowly with distance from the central star. This allows planetary cores to start their growth in much more distant positions than their final orbits. The giant planets orbiting our Sun and other stars, including systems of wide-orbit exoplanets, can therefore be formed in complete consistency with planetary migration. We demonstrate how growth tracks of planetary mass versus semimajor axis can be obtained for all the major classes of planets by integrating a relatively simple set of governing equations.
In this paper, we compare simple viscous diffusion models for the disc evolution with the results of recent surveys of the properties of young protoplanetary discs. We introduce the useful concept of `disc isochrones' in the accretion rate - disc mass plane and explore a set of Montecarlo realization of disc initial conditions. We find that such simple viscous models can provide a remarkable agreement with the available data in the Lupus star forming region, with the key requirement that the average viscous evolutionary timescale of the discs is comparable to the cluster age. Our models produce naturally a correlation between mass accretion rate and disc mass that is shallower than linear, contrary to previous results and in agreement with observations. We also predict that a linear correlation, with a tighter scatter, should be found for more evolved disc populations. Finally, we find that such viscous models can reproduce the observations in the Lupus region only in the assumption that the efficiency of angular momentum transport is a growing function of radius, thus putting interesting constraints on the nature of the microscopic processes that lead to disc accretion.
We present a sub-arcsecond resolution survey of the 340 GHz dust continuum emission from 50 nearby protoplanetary disks, based on new and archival observations with the Submillimeter Array. The observed visibility data were modeled with a simple prescription for the radial surface brightness profile. The results were used to extract intuitive, empirical estimates of the emission "size" for each disk, , defined as the radius that encircles a fixed fraction of the total continuum luminosity, . We find a significant correlation between the sizes and luminosities, such that , providing a confirmation and quantitative characterization of a putative trend that was noted previously. This correlation suggests that these disks have roughly the same average surface brightness interior to their given effective radius, ~0.2 Jy arcsec (or 8 K in brightness temperature). The same trend remains, but the 0.2dex of dispersion perpendicular to this relation essentially disappears, when we account for the irradiation environment of each disk with a crude approximation of the dust temperatures based on the stellar host luminosities. We consider two (not mutually exclusive) explanations for the origin of this size-luminosity relationship. Simple models of the growth and migration of disk solids can account for the observed trend for a reasonable range of initial conditions, but only on timescales that are much shorter than the nominal ages present in the sample. An alternative scenario invokes optically thick emission concentrated on unresolved scales, with filling factors of a few tens of percent, that are perhaps manifestations of localized particle traps.
Significance
Jupiter is the most massive planet of the Solar System and its presence had an immense effect on the dynamics of the solar accretion disk. Knowing the age of Jupiter, therefore, is key for understanding how the Solar System evolved toward its present-day architecture. However, although models predict that Jupiter formed relatively early, until now, its formation has never been dated. Here we show through isotope analyses of meteorites that Jupiter’s solid core formed within only ∼1 My after the start of Solar System history, making it the oldest planet. Through its rapid formation, Jupiter acted as an effective barrier against inward transport of material across the disk, potentially explaining why our Solar System lacks any super-Earths.
The mass of a protoplanetary disk limits the formation and future growth of any planet. Masses of protoplanetary disks are usually calculated from measurements of the dust continuum emission by assuming an interstellar gas-to-dust ratio. To investigate the utility of CO as an alternate probe of disk mass, we use ALMA to survey CO and CO J = line emission from a sample of 93 protoplanetary disks around stars and brown dwarfs with masses from 0.03 -- 2 M in the nearby Chamaeleon I star-forming region. We detect CO emission from 17 sources and CO from only one source. Gas masses for disks are then estimated by comparing the CO line luminosities to results from published disk models that include CO freeze-out and isotope-selective photodissociation. Under the assumption of a typical ISM CO-to-H ratios of , the resulting gas masses are implausibly low, with an average gas mass of 0.05 M as inferred from the average flux of stacked CO lines. The low gas masses and gas-to-dust ratios for Cha I disks are both consistent with similar results from disks in the Lupus star-forming region. The faint CO line emission may instead be explained if disks have much higher gas masses, but freeze-out of CO or complex C-bearing molecules is underestimated in disk models. The conversion of CO flux to CO gas mass also suffers from uncertainties in disk structures, which could affect gas temperatures. CO emission lines will only be a good tracer of the disk mass when models for C and CO depletion are confirmed to be accurate.
The dependence of the mass accretion rate on the stellar properties is a key constraint for star formation and disk evolution studies. Here we present a study of a sample of stars in the Chamaeleon I star forming region carried out using the VLT/X-Shooter spectrograph. The sample is nearly complete down to M~0.1Msun for the young stars still harboring a disk in this region. We derive the stellar and accretion parameters using a self-consistent method to fit the broad-band flux-calibrated medium resolution spectrum. The correlation between the accretion luminosity to the stellar luminosity, and of the mass accretion rate to the stellar mass in the logarithmic plane yields slopes of 1.9 and 2.3, respectively. These slopes and the accretion rates are consistent with previous results in various star forming regions and with different theoretical frameworks. However, we find that a broken power-law fit, with a steeper slope for stellar luminosity smaller than ~0.45 Lsun and for stellar masses smaller than ~ 0.3 Msun, is slightly preferred according to different statistical tests, but the single power-law model is not excluded. The steeper relation for lower mass stars can be interpreted as a faster evolution in the past for accretion in disks around these objects, or as different accretion regimes in different stellar mass ranges. Finally, we find two regions on the mass accretion versus stellar mass plane empty of objects. One at high mass accretion rates and low stellar masses, which is related to the steeper dependence of the two parameters we derived. The second one is just above the observational limits imposed by chromospheric emission. This empty region is located at M~0.3-0.4Msun, typical masses where photoevaporation is known to be effective, and at mass accretion rates ~10^-10 Msun/yr, a value compatible with the one expected for photoevaporation to rapidly dissipate the inner disk.
The Orionis cluster is important for studying protoplanetary disk evolution, as its intermediate age (3-5 Myr) is comparable to the median disk lifetime. We use ALMA to conduct a high-sensitivity survey of dust and gas in 92 protoplanetary disks around Orionis members with . Our observations cover the 1.33 mm continuum and several CO lines: out of 92 sources, we detect 37 in the mm continuum and six in CO, three in CO, and none in CO. Using the continuum emission to estimate dust mass, we find only 11 disks with , indicating that after only a few Myr of evolution most disks lack sufficient dust to form giant planet cores. Stacking the individually undetected continuum sources limits their average dust mass to 5 lower than that of the faintest detected disk, supporting theoretical models that indicate rapid dissipation once disk clearing begins. Comparing the protoplanetary disk population in Orionis to those of other star-forming regions supports the steady decline in average dust mass and the steepening of the - relation with age; studying these evolutionary trends can inform the relative importance of different disk processes during key eras of planet formation. External photoevaporation from the central O9 star is influencing disk evolution throughout the region: dust masses clearly decline with decreasing separation from the photoionizing source, and the handful of CO detections exist at projected separations pc. Collectively, our findings indicate that giant planet formation is inherently rare and/or well underway by a few Myr of age.
Observations indicate that stars generally lose their protoplanetary discs on a timescale of about 5 Myr. Which mechanisms are responsible for the disc dissipation is still debated. Here we investigate the movement through an ambient medium as a possible cause of disc dispersal. The ram pressure exerted by the flow can truncate the disc and the accretion of material with no azimuthal angular momentum leads to further disc contraction. We derive a theoretical model from accretion disc theory that describes the evolution of the disc radius, mass, and surface density profile as a function of the density and velocity of the ambient medium. We test our model by performing hydrodynamical simulations of a protoplanetary disc embedded in a flow with different velocities and densities. We find that our model gives an adequate description of the evolution of the disc radius and accretion rate onto the disc. The total disc mass in the simulations follows the theoretically expected trend, except at the lowest density where our simulated discs lose mass owing to continuous stripping. This stripping may be a numerical rather than a physical effect. Some quantitative differences exist between the model predictions and the simulations. These are at least partly caused by numerical viscous effects in the disc and depend on the resolution of the simulation. Our model can be used as a conservative estimate for the process of face-on accretion onto protoplanetary discs, as long as viscous processes in the disc can be neglected. The model predicts that in dense gaseous environments, discs can shrink substantially in size and can, in theory, sweep up an amount of gas of the order of their initial mass. This process could be relevant for planet formation in dense environments.
We present the results of a study of the stellar and accretion properties of the (almost) complete sample of class II and transitional YSOs in the Lupus I, II, III and IV clouds, based on spectroscopic data acquired with the VLT/X-Shooter spectrograph. Our study combines the dataset from our previous work with new observations of 55 additional objects. We have investigated 92 YSO candidates in total, 11 of which have been definitely identified with giant stars unrelated to Lupus. The stellar and accretion properties of the 81 bona fide YSOs, which represent more than 90% of the whole class~II and transition disc YSO population in the aforementioned Lupus clouds, have been homogeneously and self-consistently derived, allowing for an unbiased study of accretion and its relationship with stellar parameters. The accretion luminosity, Lacc, increases with the stellar luminosity, Lstar, with an overall slope of ~1.6, similar but with a smaller scatter than in previous studies. There is a significant lack of strong accretors below Lstar~0.1Lsun, where Lacc is always lower than 0.01Lstar. We argue that the Lacc-Lstar slope is not due to observational biases, but is a true property of the Lupus YSOs. The logMacc-logMstar correlation shows a statistically significant evidence of a break, with a steeper relation for Mstar<0.2Msun and a flatter slope for higher masses. The bimodality of the Macc-Mstar relation is confirmed with four different evolutionary models used to derive the stellar mass. The bimodal behaviour of the observed relationship supports the importance of modelling self-gravity in the early evolution of the more massive discs, but other processes, such as photo evaporation and planet formation during the YSO's lifetime, may also lead to disc dispersal on different timescales depending on the stellar mass. We also refined the empirical Lacc vs. Lline relationships.
An era has started in which gas and dust can be observed independently in protoplanetary disks, thanks to the recent surveys with ALMA. The first near-complete high-resolution disk survey in both dust and gas in a single star-forming region has been carried out in Lupus, finding surprisingly low gas/dust ratios. The goal of this work is to fully exploit CO isotopologues observations in Lupus, comparing them with physical-chemical model results, in order to obtain gas masses for a large number of disks. We have employed physical-chemical models to analyze continuum and CO isotopologues observations of Lupus disks, including isotope-selective processes and freeze-out. Employing also the ALMA 13CO-only detections, disk gas masses have been calculated for a total of 34 sources, expanding the sample of 10 disks studied by Ansdell et al. (2016), where also C18O was detected. We confirm that overall gas-masses are very low, often smaller than 1 , if volatile carbon is not depleted. Accordingly, global gas/dust ratios predominantly between 1 and 10. Low CO-based gas masses and gas/dust ratios may indicate rapid loss of gas, or alternatively chemical evolution, e.g. via sequestering of carbon from CO to more complex molecules, or carbon locked up in larger bodies. Current ALMA observations cannot distinguish between these two hypotheses. We have simulated both scenarios, but chemical model results do not allow us to rule out one of the two. Assuming that all Lupus disks have evolved mainly due to viscous processes over the past few Myr, the observed correlation between the current mass accretion rate and dust mass found by Manara et al. (2016) implies a constant gas-to-dust ratio, which is close to 100 based on the observed ratio. This in turn points to a scenario in which carbon depletion is responsible for the low CO isotopologue line luminosities.
Protoplanetary disks form around stars as a consequence of pre-stellar cores collapsing in filaments of Giant Molecular Clouds, which makes them the smallest entity in a hierarchy of scales. Length scales range from tens of parsecs for Giant Molecular Clouds to protoplanetary disk sizes AU to AU. It is computationally very challenging to cover such a broad range of scales in a single simulation. Therefore, simulations of protostellar formation traditionally start from initial conditions representing a collapsing spherically symmetric cloud, as an approximation for the pre-stellar core. This approach allows detailed parameter studies, but neglecting the underlying turbulence in Giant Molecular Clouds and the potential interactions with the surroundings could potentially limit the applicability of such idealized initial conditions. Considering the Giant Molecular Cloud dynamics is important to investigate the protostellar formation process in a statistically unbiased sample of initial conditions. Fortunately, relevant spatial as well as temporal scales for large scale dynamics are larger than for smaller domains due to Larson's velocity law. Using extreme adaptive mesh refinement we carry out simulations with a maximal resolution of 2 AU around single protostars properly anchored in a 40 pc model of GMC using a modified version of the \ramses\ code to solve the equations of ideal magnetohydrodynamics including microphysics and a sub-grid sink particle representation of protostars. In this study, we present for the first time the formation process around nine different solar mass stars embedded in their large-scale environment determined by Giant Molecular Cloud dynamics. We find that protostellar accretion including the formation of protoplanetary disks is a heterogeneous process due to the different protostellar environments.
Globular clusters (GCs) are known to harbor multiple stellar populations. To explain these observations Bastian et al. suggested a scenario in which a second population is formed by the accretion of enriched material onto the low-mass stars in the initial GC population. The idea is that the low-mass, pre-main sequence stars sweep up gas expelled by the massive stars of the same generation into their protoplanetary disc as they move through the GC core. We perform simulations with 2 different smoothed particle hydrodynamics codes to investigate if a low-mass star surrounded by a protoplanetary disc can accrete the amount of enriched material required in this scenario. We focus on the gas loading rate onto the disc and star as well as on the lifetime of the disc. We find that the gas loading rate is a factor of 2 smaller than the geometric rate, because the effective cross section of the disc is smaller than its surface area. The loading rate is consistent for both codes, irrespective of resolution. The disc gains mass in the high resolution runs, but loses angular momentum on a time scale of 10^4 yrs. Two effects determine the loss of (specific) angular momentum in our simulations: 1) continuous ram pressure stripping and 2) accretion of material with no azimuthal angular momentum. Our study and previous work suggest that the former, dominant process is mainly caused by numerical rather than physical effects, while the latter is not. The latter process causes the disc to become more compact, increasing the surface density profile at smaller radii. The disc size is determined in the first place by the ram pressure when the flow first hits the disc. Further evolution is governed by the decrease in the specific angular momentum of the disc. We conclude that the size and lifetime of the disc are probably not sufficient to accrete the amount of mass required in Bastian et al.'s scenario.
The study of the properties of disks around young brown dwarfs can provide important clues on the formation of these very low mass objects and on the possibility of forming planetary systems around them. The presence of warm dusty disks around brown dwarfs is well known, based on near- and mid-infrared studies. High angular resolution observations of the cold outer disk are limited, we used ALMA to attempt a first survey of young brown dwarfs in the rho-Ophiuchi star forming region with ALMA. All 17 young brown dwarfs in our sample were observed at 890 um in the continuum at ~0.5" angular resolution. The sensitivity of our observations was chosen to detect ~0.5 MEarth of dust. We detect continuum emission in 11 disks (65% of the total), the estimated mass of dust in the detected disks ranges from ~0.5 to ~6 MEarth. These disk masses imply that planet formation around brown dwarfs may be relatively rare and that the supra-Jupiter mass companions found around some brown dwarfs are probably the result of a binary system formation. We find evidence that the two brightest disks in rho-Oph have sharp outer edges at R~25 AU, as opposite to disks around Taurus brown dwarfs. This difference may suggest that the different environment in rho-Oph may lead to significant differences in disk properties. A comparison of the Mdisk/Mstar ratio for brown dwarf and solar-mass systems also shows a possible deficit of mass in brown dwarfs, which could support the evidence for dynamical truncation of disks in the substellar regime. These findings are still tentative and need to be put on firmer grounds by studying the gaseous disks around brown dwarfs and by performing a more systematic and unbiased survey of the disk population around the more massive stars.
We present ALMA observations of 106 G-, K-, and M-type stars in the Upper Scorpius OB Association hosting circumstellar disks. With these data, we measure the 0.88 mm continuum and CO line fluxes of disks around low mass ( ) stars at an age of 5-11 Myr. Of the 75 primordial disks in the sample, 53 are detected in the dust continuum and 26 in CO. Of the 31 disks classified as debris/evolved transitional disks, 5 are detected in the continuum and none in CO. The lack of CO emission in approximately half of the disks with detected continuum emission can be explained if CO is optically thick but has a compact emitting area ( AU), or if the CO is heavily depleted by a factor of at least relative to interstellar medium abundances and is optically thin. The continuum measurements are used to estimate the dust mass of the disks. We find a correlation between disk dust mass and stellar host mass consistent with a power-law relation of . Disk dust masses in Upper Sco are compared to those measured in the younger Taurus star forming region to constrain the evolution of disk dust mass. We find that the difference in the mean of between Taurus and Upper Sco is , such that is lower in Upper Sco by a factor of .
A relation between the mass accretion rate onto the central young star and the mass of the surrounding protoplanetary disk has long been theoretically predicted and observationally sought. For the first time, we have accurately and homogeneously determined the photospheric parameters, the mass accretion rate, and the disk mass for an essentially complete sample of young stars with disks in the Lupus clouds. Our work combines the results of surveys conducted with VLT/X-Shooter and ALMA. With this dataset we are able to test a basic prediction of viscous accretion theory, the existence of a linear relation between the mass accretion rate onto the central star and the total disk mass. We find a correlation between the mass accretion rate and the disk dust mass, with a ratio that is roughly consistent with the expected viscous timescale when assuming an ISM gas-to-dust ratio. This confirms that mass accretion rates are related to the properties of the outer disk. We find no correlation between mass accretion rates and the disk mass measured by CO isotopologues emission lines, possibly due to the small number of measured disk gas masses. This suggests that the mm-sized dust mass better traces the total disk mass and that masses derived from CO may be underestimated, at least in some cases.
We present the first high-resolution sub-mm survey of both dust and gas for a large population of protoplanetary disks. Characterizing fundamental properties of protoplanetary disks on a statistical level is critical to understanding how disks evolve into the diverse exoplanet population. We use ALMA to survey 89 protoplanetary disks around stars with in the young (1-3 Myr), nearby (150-200 pc) Lupus complex. Our observations cover the 890 m continuum and the CO and CO 3-2 lines. We use the sub-mm continuum to constrain to a few Martian masses (0.2-0.4 ) and the CO isotopologue lines to constrain to roughly a Jupiter mass (assuming ISM-like abundance). Of 89 sources, we detect 62 in the continuum, 36 in CO, and 11 in CO at significance. Several new "transition disks" are found with relatively bright continuum and CO isotopologue emission. Stacking the individually undetected sources limits their average dust mass to Lunar masses (0.03 ), indicating rapid evolution once disk clearing begins. We find a positive but non-linear correlation between and , and also demonstrate for the first time a tentative positive correlation between and . The mean dust mass in Lupus is 3 higher than in Upper Sco, while the dust mass distributions in Lupus and Taurus are statistically indistinguishable from each other. Most detected disks have and gas-to-dust ratios when using our parameterized model framework; if confirmed, the inferred rapid gas depletion indicates that planet formation is largely complete by a few Myr, and may also explain the unexpected prevalence of super-Earths in the exoplanet population.
We have conducted the first comprehensive numerical investigation of the relative velocity distribution of dust particles
in self-gravitating protoplanetary discs with a view to assessing the viability of planetesimal formation via direct collapse in such environments.
The viability depends crucially on the large sizes that are preferentially collected in pressure maxima produced by transient
spiral features (Stokes numbers, ); growth to these size scales requires that collision velocities remain low enough that grain growth is not reversed by fragmentation.
We show that, for a single sized dust population, velocity driving by the disc's gravitational perturbations is only effective
for St > 3, while coupling to the gas velocity dominates otherwise. We develop a criterion for understanding this result in terms
of the stopping distance being of order the disc scale height. Nevertheless, the relative velocities induced by differential
radial drift in multi-sized dust populations are too high to allow the growth of silicate dust particles beyond or 10−1 (10 cm to m sizes at 30 au), such Stokes numbers being insufficient to allow concentration of solids in spiral features.
However, for icy solids (which may survive collisions up to several 10 m s−1), growth to (10 m size) may be possible beyond 30 au from the star. Such objects would be concentrated in spiral features and could potentially
produce larger icy planetesimals/comets by gravitational collapse. These planetesimals would acquire moderate eccentricities
and remain unmodified over the remaining lifetime of the disc.
Exoplanet discoveries of recent years have provided a great deal of new data
for studying the bulk compositions of giant planets. Here we identify 38
transiting giant planets () whose stellar
insolation is low enough (, or roughly ) that they are not affected
by the hot Jupiter radius inflation mechanism(s). We compute a set of new
thermal and structural evolution models and use these models in comparison with
properties of the 38 transiting planets (mass, radius, age) to determine their
heavy element masses. A clear correlation emerges between the planetary heavy
element mass and the total planet mass, approximately of the form . This finding is consistent with the core accretion model of
planet formation. We also study how stellar metallicity [Fe/H] affects
planetary metal-enrichment and find a weaker correlation than has been
previously reported from studies with smaller sample sizes. Our results suggest
that planets with large heavy element masses are more common around stars with
a high iron abundance, but are not found there exclusively. We confirm a strong
relationship between the planetary metal-enrichment relative to the parent star
and the planetary mass, but see no relation in
with planet orbital properties or stellar mass.
Suggestively, circumbinary planets are more enriched in heavy elements than
similar mass single-star planets, but with only four such planets the effect is
not yet significant. The large heavy element masses of many planets () suggest significant amounts of heavy elements in H/He envelopes,
rather than cores, such that metal-enriched giant planet atmospheres should be
the rule.
We present the analysis of 34 new VLT/X-Shooter spectra of young stellar
objects in the Chamaeleon I star forming region, together with four more
spectra of stars in Taurus and two in Chamaeleon II. The broad wavelength
coverage and accurate flux calibration of our spectra allow us to estimate
stellar and accretion parameters for our targets by fitting the photospheric
and accretion continuum emission from the Balmer continuum down to 700 nm. The
dependence of accretion with stellar properties for this sample is consistent
with previous results from the literature. The accretion rates for transitional
disks are consistent with those of full disks in the same region. The spread of
mass accretion rates at any given stellar mass is found to be smaller than in
many studies, but is larger than that derived in the Lupus clouds using similar
data and techniques. Differences in the stellar mass range and in the
environmental conditions between our sample and that of Lupus may account for
the discrepancy in scatter between Chamaeleon I and Lupus. Complete samples in
Chamaeleon I and Lupus are needed to determine whether the difference in
scatter of accretion rates and the lack of evolutionary trends are robust to
sample selection.
Trends in the planet population with host star mass provide an avenue to
constrain planet formation theories. We derive the planet radius distribution
function for Kepler stars of different spectral types, sampling a range in host
star masses. We find that M dwarf stars have 3.5 times more small planets
(1.0-2.8 R_Earth) than main-sequence FGK stars, but two times fewer
Neptune-sized and larger planets (>2.8 R_Earth). We find no systematic trend in
the planet size distribution between spectral types F, G, and K to explain the
increasing occurrence rates. Taking into account the mass-radius relationship
and heavy-element mass of observed exoplanets, and assuming those are
independent of spectral type, we derive the inventory of the heavy-element mass
locked up in exoplanets at short orbits. The overall higher planet occurrence
rates around M stars are not consistent with the redistribution of the same
mass into more, smaller planets. At the orbital periods and planet radii where
Kepler observations are complete for all spectral types, the average
heavy-element mass locked up in exoplanets increases roughly inversely with
stellar mass from 4 M_Earth in F stars to 5 M_Earth in G and K stars to 7
M_Earth in M stars. This trend stands in stark contrast with observed
protoplanetary disk masses that decrease towards lower mass stars, and provides
a challenge for current planet formation models. Neither models of in situ
formation nor migration of fully-formed planets are consistent with these
results. Instead, these results are indicative of large-scale inward migration
of planetary building blocks --- either through type-I migration or radial
drift of dust grains --- that is more efficient for lower mass stars, but does
not result in significantly larger or smaller planets.
We explain the axisymmetric gaps seen in recent long-baseline observations of
the HL Tau protoplanetary disc with the Atacama Large Millimetre/Submillimetre
Array (ALMA) as being due to the different response of gas and dust to embedded
planets in protoplanetary discs. We perform global, three dimensional dusty
smoothed particle hydrodynamics calculations of multiple planets embedded in
dust/gas discs which successfully reproduce most of the structures seen in the
ALMA image. We find a best match to the observations using three embedded
planets with masses of 0.2, 0.27 and 0.55 in the three main gaps
observed by ALMA, though there remain uncertainties in the exact planet masses
from the disc model.
We identify a new hydrodynamical instability in protoplanetary discs that may arise due to variations in the dust-to-gas ratio
and may lead to concentration of dust grains within a disc. The instability can arise due to dust settling, which produces
a vertical compositional entropy gradient. The entropy gradient drives a baroclinic instability that is capable of creating
toroidal gas vortices that gather dust into rings. Such dust rings are potentially observable via continuum emission of the
dust or scattered light. Indeed, this instability may offer an explanation for the rings recently observed in the discs around
the young stars HL Tau and TW Hya that does not rely on clearing by protoplanets. The instability may also have wider ramifications,
potentially aiding dust agglomeration, altering the radial migration of larger planetesimals, and modifying angular momentum
transport within a disc.
Context. Circumstellar disks are known to contain a significant mass in dust
ranging from micron to centimeter size. Meteorites are evidence that individual
grains of those sizes were collected and assembled into planetesimals in the
young solar system. Aims. We assess the efficiency of dust collection of a
swarm of non-drifting planetesimals {\rev with radii ranging from 1 to
\,km and beyond. Methods. We calculate the collision probability of dust
drifting in the disk due to gas drag by planetesimal accounting for several
regimes depending on the size of the planetesimal, dust, and orbital distance:
the geometric, Safronov, settling, and three-body regimes. We also include a
hydrodynamical regime to account for the fact that small grains tend to be
carried by the gas flow around planetesimals. Results. We provide expressions
for the collision probability of dust by planetesimals and for the filtering
efficiency by a swarm of planetesimals. For standard turbulence conditions
(i.e., a turbulence parameter ), filtering is found to be
inefficient, meaning that when crossing a minimum-mass solar nebula (MMSN) belt
of planetesimals extending between 0.1 AU and 35 AU most dust particles are
eventually accreted by the central star rather than colliding with
planetesimals. However, if the disk is weakly turbulent ()
filtering becomes efficient in two regimes: (i) when planetesimals are all
smaller than about 10 km in size, in which case collisions mostly take place in
the geometric regime; and (ii) when planetary embryos larger than about 1000 km
in size dominate the distribution, have a scale height smaller than one tenth
of the gas scale height, and dust is of millimeter size or larger in which case
most collisions take place in the settling regime. These two regimes have very
different properties: we find that the local filtering efficiency
scales with (where r is the orbital distance) in
the geometric regime, but with to in the settling regime.
This implies that the filtering of dust by small planetesimals should occur
close to the central star and with a short spread in orbital distances. On the
other hand, the filtering by embryos in the settling regime is expected to be
more gradual and determined by the extent of the disk of embryos. Dust
particles much smaller than millimeter size tend only to be captured by the
smallest planetesimals because they otherwise move on gas streamlines and their
collisions take place in the hydrodynamical regime. Conclusions. Our results
hint at an inside-out formation of planetesimals in the infant solar system
because small planetesimals in the geometrical limit can filter dust much more
efficiently close to the central star. However, even a fully-formed belt of
planetesimals such as the MMSN only marginally captures inward-drifting dust
and this seems to imply that dust in the protosolar disk has been filtered by
planetesimals even smaller than 1 km (not included in this study) or that it
has been assembled into planetesimals by other mechanisms (e.g., orderly
growth, capture into vortexes). Further refinement of our work concerns, among
other things: a quantitative description of the transition region between the
hydro and settling regimes; an assessment of the role of disk turbulence for
collisions, in particular in the hydro regime; and the coupling of our model to
a planetesimal formation model.
We investigate the role of mass infall in the formation and evolution of
protostars. To avoid ad hoc initial and boundary conditions, we consider the
infall resulting self-consistently from modeling the formation of stellar
clusters in turbulent molecular clouds. We show that protostellar infall rates
in turbulent clouds are always comparable to or larger than observed accretion
rates, and thus cannot be neglected in modeling the luminosity of protostars
and the evolution of disks, even after the embedded phase of protostars. We
find large variations of infall rates from protostar to protostar, and large
fluctuations during the evolution of individuals protostars. In most cases, the
accretion rate is initially of order 10 M yr, and may
either decay rapidly in the formation of low-mass stars, or remain relatively
large when more massive stars are formed. The simulation reproduces well the
observed characteristic values and scatter of protostellar luminosities and
matches the observed protostellar luminosity function. The luminosity problem
is therefore solved once realistic protostellar infall histories are accounted
for, with no need for extreme accretion episodes. These results are based on a
simulation of randomly-driven magneto-hydrodynamic turbulence on a scale of 4
pc, including self-gravity, adaptive-mesh refinement to a resolution of 50 AU,
and accreting sink particles. The simulation yields a low star formation rate,
consistent with the observations, and a mass distribution of sink particles
consistent with the observed stellar initial mass function during the whole
duration of the simulation, forming nearly 1,300 sink particles over 3.2 Myr.
It has recently been noted that many discs around T Tauri stars appear
to comprise only a few Jupiter masses of gas and dust. Using millimetre
surveys of discs within six local star formation regions, we confirm
this result, and find that only a few per cent of young stars have
enough circumstellar material to build gas giant planets, in standard
core accretion models. Since the frequency of observed exoplanets is
greater than this, there is a `missing-mass' problem. As alternatives to
simply adjusting the conversion of dust flux to disc mass, we
investigate three other classes of solution. Migration of planets could
hypothetically sweep up the disc mass reservoir more efficiently, but
trends in multiplanet systems do not support such a model, and
theoretical models suggest that the gas accretion time-scale is too
short for migration to sweep the disc. Enhanced inner-disc mass
reservoirs are possible, agreeing with predictions of disc evolution
through self-gravity, but not adding to millimetre dust flux as the
inner disc is optically thick. Finally, the incidence of massive discs
is shown to be higher at the protostellar stages, Classes 0 and I, where
discs substantial enough to form planets via core accretion are abundant
enough to match the frequency of exoplanets. Gravitational instability
may also operate in the Class 0 epoch, where half the objects have
potentially unstable discs of >~30 per cent of the stellar mass.
However, recent calculations indicate that forming gas giants inside 50
au by instability is unlikely, even in such massive discs. Overall, the
results presented suggest that the canonically `protoplanetary' discs of
Class II T Tauri stars have globally low masses in dust observable at
millimetre wavelengths, and conversion to larger bodies (anywhere from
small rocks up to planetary cores) must already have occurred.
Refractory objects such as Calcium, Aluminum-rich Inclusions, Amoeboid Olivine Aggregates, and crystalline silicates, are found in primitive bodies throughout our Solar System. It is believed that these objects formed in the hot, inner solar nebula and were redistributed during the mass and angular momentum transport that took place during its early evolution. The ages of these objects thus offer possible clues about the timing and duration of this transport. Here we study how the dynamics of these refractory objects in the evolving solar nebula affected the age distribution of the grains that were available to be incorporated into planetesimals throughout the Solar System. It is found that while the high temperatures and conditions needed to form these refractory objects may have persisted for millions of years, it is those objects that formed in the first 105 years that dominate (make up over 90%) those that survive throughout most of the nebula. This is due to two effects: (1) the largest numbers of refractory grains are formed at this time period, as the disk is rapidly drained of mass during subsequent evolution and (2) the initially rapid spreading of the disk due to angular momentum transport helps preserve this early generation of grains as opposed to later generations. This implies that most refractory objects found in meteorites and comets formed in the first 105 years after the nebula formed. As these objects contained live 26Al, this constrains the time when short-lived radionuclides were introduced to the Solar System to no later than 105 years after the nebula formed. Further, this implies that the t=0 as defined by meteoritic materials represents at most, the instant when the solar nebula finished accreting significant amounts of materials from its parent molecular cloud.
We present a substantial extension of the millimeter (mm) wave continuum photometry catalog for circumstellar dust disks in the Taurus star-forming region, based on a new "snapshot" λ = 1.3 mm survey with the Submillimeter Array. Combining these new data with measurements in the literature, we construct a mm-wave luminosity distribution, f(L
mm), for Class II disks that is statistically complete for stellar hosts with spectral types earlier than M8.5 and has a 3σ depth of roughly 3 mJy. The resulting census eliminates a longstanding selection bias against disks with late-type hosts, and thereby demonstrates that there is a strong correlation between L
mm and the host spectral type. By translating the locations of individual stars in the Hertzsprung-Russell diagram into masses and ages, and adopting a simple conversion between L
mm and the disk mass, Md, we confirm that this correlation corresponds to a statistically robust relationship between the masses of dust disks and the stars that host them. A Bayesian regression technique is used to characterize these relationships in the presence of measurement errors, data censoring, and significant intrinsic scatter: the best-fit results indicate a typical 1.3 mm flux density of ~25 mJy for 1 M
☉ hosts and a power-law scaling . We suggest that a reasonable treatment of dust temperature in the conversion from L
mm to Md favors an inherently linear Md∝M
* scaling, with a typical disk-to-star mass ratio of ~0.2%-0.6%. The measured rms dispersion around this regression curve is ±0.7 dex, suggesting that the combined effects of diverse evolutionary states, dust opacities, and temperatures in these disks imprint a full width at half-maximum range of a factor of ~40 on the inferred Md (or L
mm) at any given host mass. We argue that this relationship between Md and M
* likely represents the origin of the inferred correlation between giant planet frequency and host star mass in the exoplanet population, and provides some basic support for the core accretion model for planet formation. Moreover, we caution that the effects of incompleteness and selection bias must be considered in comparative studies of disk evolution, and illustrate that fact with statistical comparisons of f(L
mm) between the Taurus catalog presented here and incomplete subsamples in the Ophiuchus, IC 348, and Upper Sco young clusters.
I summarize recent surveys of protoplanetary disks at millimeter wavelengths
and show that the distribution of luminosity, equivalent to the mass in small
dust grains, declines rapidly. This contrasts with statistics on the lifetime
of disks from infrared observations and the high occurrence of planets from
radial velocity and transit surveys. I suggest that these disparate results can
be reconciled if most of the dust in a disk is locked up in millimeter and
larger sized particles within about 2 Myr. This statistical result on disk
evolution agrees with detailed modeling of a small number of individual disks
and with cosmochemical measurements of rapid planetesimal formation.