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Probing the Terrestrial Regions of Planetary Systems: Warm Debris Disks with Emission Features
Abstract
Observations of debris disks allow for the study of planetary systems, even
where planets have not been detected. However, debris disks are often only
characterized by unresolved infrared excesses that resemble featureless
blackbodies, and the location of the emitting dust is uncertain due to a
degeneracy with the dust grain properties. Here we characterize the Spitzer IRS
spectra of 22 debris disks exhibiting 10 micron silicate emission features.
Such features arise from small warm dust grains, and their presence can
significantly constrain the orbital location of the emitting debris. We find
that these features can be explained by the presence of an additional dust
component in the terrestrial zones of the planetary systems, i.e. an
exozodiacal belt. Aside from possessing exozodiacal dust, these debris disks
are not particularly unique; their minimum grain sizes are consistent with the
blowout sizes of their systems, and their brightnesses are comparable to those
of featureless warm debris disks. These disks are in systems with a range of
ages, although the older systems with features are found only around A-type
stars. The features in young systems may be signatures of terrestrial planet
formation. Analyzing the spectra of unresolved debris disks with emission
features may be one of the simplest and most accessible ways to study the
terrestrial regions of planetary systems.
... This suggests an enticing analogy to the structure of the Solar System, with a cold Kuiper belt and a warm asteroid belt, with the placement of the belts loosely associated with ice lines in the forming system 27 . Such structures remain by far the most popular explanation for the warmer component of debris disk SEDs in general [28][29][30][31] . However, inferences about debris disk structure from SEDs can be highly degenerate between the assumed dust grain absorption and emission properties, their distances from the star, scattering phase functions, porosity, sizes and size distributions. ...
Planetary debris disks around other stars are analogous to the asteroid and Kuiper belts in the Solar System. Their structure reveals the configuration of small bodies and provides hints for the presence of planets. The nearby star Fomalhaut hosts one of the most prominent debris disks, resolved by the Hubble Space Telescope, Spitzer, Herschel and the Atacama Large Millimeter Array. Images of this system at mid-infrared wavelengths using JWST/MIRI not only show the narrow Kuiper belt-analogue outer ring, but also that (1) what was thought from indirect evidence to be an asteroid-analogue structure is instead broad, extending outward into the outer system, and (2) there is an intermediate belt, probably shepherded by an unseen planet. The newly discovered belt is demarcated by an inner gap, located at ~78 au, and it is misaligned relative to the outer belt. The previously known collisionally generated dust cloud, Fomalhaut b, could have originated from this belt, suggesting increased dynamical stirring and collision rates there. We also discovered a large dust cloud within the outer ring, possible evidence of another dust-creating collision. Taken together with previous observations, Fomalhaut appears to be the site of a complex and possibly dynamically active planetary system.
... This limit is significantly higher than measured for all but the most extreme and rare excesses. Spectroscopic observations may slightly improve over this sensitivity if silicate emission features can be detected (Ballering et al. 2014). Detecting scattered light from dust very close to the star in visible-light aperture polarization measurements has been unsuccessful, which puts important constraints on the properties and origin of the hot, near-infrared-detected dust (Marshall et al. 2016). ...
The Large Binocular Telescope Interferometer (LBTI) enables nulling interferometric observations across the N band (8 to 13 μm) to suppress a star's bright light and probe for faint circumstellar emission. We present and statistically analyze the results from the LBTI/Hunt for Observable Signatures of Terrestrial Systems survey for exozodiacal dust. By comparing our measurements to model predictions based on the solar zodiacal dust in the N band, we estimate a 1σ median sensitivity of 23 zodis times the solar system dust surface density in its habitable zone (HZ; 23 zodis) for early-type stars and 48 zodis for Sun-like stars, where 1 zodi is the surface density of HZ dust in the solar system. Of the 38 stars observed, 10 show significant excess. A clear correlation of our detections with the presence of cold dust in the systems was found, but none with the stellar spectral type or age. The majority of Sun-like stars have relatively low HZ dust levels (best-fit median: 3 zodis, 1σ upper limit: 9 zodis, 95% confidence: 27 zodis based on our N band measurements), while ~20% are significantly more dusty. The solar system's HZ dust content is consistent with being typical. Our median HZ dust level would not be a major limitation to the direct imaging search for Earth-like exoplanets, but more precise constraints are still required, in particular to evaluate the impact of exozodiacal dust for the spectroscopic characterization of imaged exo-Earth candidates.
... This limit is significantly higher than measured for all but the most extreme and rare excesses. Spectroscopic observations may slightly improve over this sensitivity if silicate emission features can be detected (Ballering et al. 2014). Detecting scattered light from dust very close to the star in visible light aperture polarization measurements has been unsuccessful, which puts important constraints on the properties and origin of the hot, near-infrared detected dust (Marshall et al. 2016). ...
The Large Binocular Telescope Interferometer (LBTI) enables nulling interferometric observations across the N band (8 to 13 um) to suppress a star's bright light and probe for faint circumstellar emission. We present and statistically analyze the results from the LBTI/HOSTS (Hunt for Observable Signatures of Terrestrial Systems) survey for exozodiacal dust. By comparing our measurements to model predictions based on the Solar zodiacal dust in the N band, we estimate a 1 sigma median sensitivity of 23 zodis for early type stars and 48 zodis for Sun-like stars, where 1 zodi is the surface density of habitable zone (HZ) dust in the Solar system. Of the 38 stars observed, 10 show significant excess. A clear correlation of our detections with the presence of cold dust in the systems was found, but none with the stellar spectral type or age. The majority of Sun-like stars have relatively low HZ dust levels (best-fit median: 3 zodis, 1 sigma upper limit: 9 zodis, 95% confidence: 27 zodis based on our N band measurements), while ~20% are significantly more dusty. The Solar system's HZ dust content is consistent with being typical. Our median HZ dust level would not be a major limitation to the direct imaging search for Earth-like exoplanets, but more precise constraints are still required, in particular to evaluate the impact of exozodiacal dust for the spectroscopic characterization of imaged exo-Earth candidates.
... The final target, HD 98363, was observed in a follow-up program to the GPIES disk campaign (GS-2019A-Q-109) that Note. The radius of HD 143675 was found in Ballering et al. (2014); all other stellar radii were estimated with Siess et al. (2000) using measured photometry and distances. ...
We present the first spatially resolved scattered-light images of four debris disks around members of the Scorpius-Centaurus (Sco-Cen) OB association with high-contrast imaging and polarimetry using the Gemini Planet Imager (GPI). All four disks are resolved for the first time in polarized light, and one disk is also detected in total intensity. The three disks imaged around HD 111161, HD 143675, and HD 145560 are symmetric in both morphology and brightness distribution. The three systems span a range of inclinations and radial extents. The disk imaged around HD 98363 shows indications of asymmetries in morphology and brightness distribution, with some structural similarities to the HD 106906 planet–disk system. Uniquely, HD 98363 has a wide comoving stellar companion, Wray 15-788, with a recently resolved disk with very different morphological properties. HD 98363 A/B is the first binary debris disk system with two spatially resolved disks. All four targets have been observed with ALMA, and their continuum fluxes range from one nondetection to one of the brightest disks in the region. With the new results, a total of 15 A/F stars in Sco-Cen have resolved scattered-light debris disks, and approximately half of these systems exhibit some form of asymmetry. Combining the GPI disk structure results with information from the literature on millimeter fluxes and imaged planets reveals a diversity of disk properties in this young population. Overall, the four newly resolved disks contribute to the census of disk structures measured around A/F stars at this important stage in the development of planetary systems.
... Fomalhaut and HD 202628, Kalas et al. 2005;Krist et al. 2012;Schneider et al. 2016, Faramaz et al. in prep); or both inner and outer belts (e.g. Backman et al. 2009;Morales et al. 2009;Chen et al. 2009;Ballering et al. 2014;Kennedy & Wyatt 2014), which have been used as an argument for the presence of planets in between these two components (Shannon et al. 2016;Lazzoni et al. 2018;Matthews et al. 2018). Additionally, a few debris discs show evidence of single or multiple gaps in between broad outer belts which suggests the presence of planets formed within these icy planetesimals belts. ...
In the last few years, multiwavelength observations have revealed the ubiquity of gaps/rings in circumstellar discs. Here we report the first ALMA observations of HD 92945 at 0.86 mm, that reveal a gap at about 73$\pm$3 au within a broad disc of planetesimals that extends from 50 to 140 au. We find that the gap is $20^{+10}_{-8}$ au wide. If cleared by a planet in situ, this planet must be less massive than 0.6 $M_\mathrm{Jup}$, or even lower if the gap was cleared by a planet that formed early in the protoplanetary disc and prevented planetesimal formation at that radius. By comparing opposite sides of the disc we also find that the disc could be asymmetric. Motivated by the asymmetry and the fact that planets might be more frequent closer to the star in exoplanetary systems, we show that the gap and asymmetry could be produced by two planets interior to the disc through secular resonances. These planets excite the eccentricity of bodies at specific disc locations, opening radial gaps in the planetesimal distribution. New observations are necessary to confirm if the disc is truly asymmetric, thus favouring the secular resonance model, or if the apparent asymmetry is due to a background galaxy, favouring the in-situ planet scenario. Finally, we also report the non-detection of CO and HCN gas confirming that no primordial gas is present. The CO and HCN non-detections are consistent with the destruction of volatile-rich Solar System-like comets.
... While IR excess detections can be generally explained by a single temperature blackbody, there is a large number of systems that show evidence for a broad range of temperatures that are hard to explain simply as due to a single dusty narrow belt, even with temperatures varying as a function of grain size (e.g. Backman et al. 2009;Chen et al. 2009;Morales et al. 2009;Ballering, Rieke & Gáspár 2014;Kennedy & Wyatt 2014). Although this might be explained by material distributed over a broad range of radii (like in protoplanetary discs), an alternative and very attractive explanation for these systems is the presence of a two-temperature disc, with an inner asteroid belt and an outer exo-Kuiper belt, analogous to the Solar system (e.g. ...
While detecting low-mass exoplanets at tens of au is beyond current instrumentation, debris discs provide a unique opportunity to study the outer regions of planetary systems. Here, we report new ALMA observations of the 80-200 Myr old Solar analogue HD 107146 that reveal the radial structure of its exo-Kuiper belt at wavelengths of 1.1 and 0.86 mm. We find that the planetesimal disc is broad, extending from 40 to 140 au, and it is characterized by a circular gap extending from 60 to 100 au in which the continuum emission drops by about 50 per cent.We also report the non-detection of the CO J = 3-2 emission line, confirming that there is not enough gas to affect the dust distribution. To date, HD 107146 is the only gas-poor system showing multiple rings in the distribution of millimetre sized particles. These rings suggest a similar distribution of the planetesimals producing small dust grains that could be explained invoking the presence of one or more perturbing planets. Because the disc appears axisymmetric, such planets should be on circular orbits. By comparing N-body simulations with the observed visibilities we find that to explain the radial extent and depth of the gap, it would require the presence of multiple low-mass planets or a single planet that migrated through the disc. Interior to HD 107146's exo-Kuiper belt we find extended emission with a peak at ~20 au and consistent with the inner warm belt that was previously predicted based on 22 μm excess as in many other systems. This warm belt is the first to be imaged, although unexpectedly suggesting that it is asymmetric. This could be due to a large belt eccentricity or due to clumpy structure produced by resonant trapping with an additional inner planet. © 2018 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society.
... Strong constraints on the scattered light content of hot exozodi systems have been provided by parts per million accuracy polarimetric observations [29]. Additional constraints have been provided by ground and space based spectroscopic observations (e.g., [3,7,26]; Lisse et al. in prep.) and by studying WISE data of a large sample of stars to constrain the incidence rate of exozodis at the bright end of their luminosity function [20]. ...
Exo-zodiacal dust, exozodi for short, is warm (~300K) or hot (up to ~2000K) dust found in the inner regions of planetary systems around main sequence stars. In analogy to our own zodiacal dust, it may be located in or near the habitable zone or closer in, down to the dust sublimation distance. The study of the properties, distribution, and evolution of exozodis can inform about the architecture and dynamics of the innermost regions of planetary systems, close to their habitable zones. On the other hand, the presence of large amounts of exo-zodiacal dust may be an obstacle for future space missions aiming to image Earth-like exoplanets. The dust can be the most luminous component of extrasolar planetary systems, but predominantly emits in the near- to mid-infrared where it is outshone by the host star. Interferometry provides a unique method of separating the dusty from the stellar emission. We discuss the prospects of exozodi observations with the next generation VLTI instruments and summarize critical instrument specifications.
... While IR excess detections can be generally explained by a single temperature black body, there is a large number of systems that show evidence for a broad range of temperatures that are hard to explain simply as due to a single dusty narrow belt, even with temperatures varying as a function of grain size (e.g. Backman et al. 2009;Morales et al. 2009;Chen et al. 2009;Ballering et al. 2014;Kennedy & Wyatt 2014). Although this might be explained by material distributed over a broad range of radii (like in protoplanetary discs), an alternative and very attractive explanation for these systems is the presence of a two-temperature disc, with an inner asteroid belt and an outer exo-Kuiper belt, analogous to the Solar System (e.g. ...
While detecting low mass exoplanets at tens of au is beyond current instrumentation, debris discs provide a unique opportunity to study the outer regions of planetary systems. Here we report new ALMA observations of the 80-200 Myr old Solar analogue HD 107146 that reveal the radial structure of its exo-Kuiper belt at wavelengths of 1.1 and 0.86 mm. We find that the planetesimal disc is broad, extending from 40 to 140 au, and it is characterised by a circular gap extending from 60 to 100 au in which the continuum emission drops by about 50%. We also report the non-detection of the CO J=3-2 emission line, confirming that there is not enough gas to affect the dust distribution. To date, HD 107146 is the only gas-poor system showing multiple rings in the distribution of millimeter sized particles. These rings suggest a similar distribution of the planetesimals producing small dust grains that could be explained invoking the presence of one or more perturbing planets. Because the disk appears axisymmetric, such planets should be on circular orbits. By comparing N-body simulations with the observed visibilities we find that to explain the radial extent and depth of the gap, it would be required the presence of multiple low mass planets or a single planet that migrated through the disc. Interior to HD 107146's exo-Kuiper belt we find extended emission with a peak at ~20 au and consistent with the inner warm belt that was previously predicted based on 22$\mu$m excess as in many other systems. This warm belt is the first to be imaged, although unexpectedly suggesting that it is asymmetric. This could be due to a large belt eccentricity or due to clumpy structure produced by resonant trapping with an additional inner planet.
... While this section has focussed on just one system, there are many other systems for which compositional constraints exist, not all of which exhibit the same silica composition, even if they are interpreted within the framework of giant collisions (Lisse et al. 2008;Currie et al. 2011;Ballering et al. 2014). ...
2016, The Author(s).Giant impacts refer to collisions between two objects each of which is massive enough to be considered at least a planetary embryo. The putative collision suffered by the proto-Earth that created the Moon is a prime example, though most Solar System bodies bear signatures of such collisions. Current planet formation models predict that an epoch of giant impacts may be inevitable, and observations of debris around other stars are providing mounting evidence that giant impacts feature in the evolution of many planetary systems. This chapter reviews giant impacts, focussing on what we can learn about planet formation by studying debris around other stars. Giant impact debris evolves through mutual collisions and dynamical interactions with planets. General aspects of this evolution are outlined, noting the importance of the collision-point geometry. The detectability of the debris is discussed using the example of the Moon-forming impact. Such debris could be detectable around another star up to 10 Myr post-impact, but model uncertainties could reduce detectability to a few 100 yr window. Nevertheless the 3 % of young stars with debris at levels expected during terrestrial planet formation provide valuable constraints on formation models; implications for super-Earth formation are also discussed. Variability recently observed in some bright disks promises to illuminate the evolution during the earliest phases when vapour condensates may be optically thick and acutely affected by the collision-point geometry. The outer reaches of planetary systems may also exhibit signatures of giant impacts, such as the clumpy debris structures seen around some stars.
... In the three decades since, the detection of such infrared excesses has become routine; with many main sequence stars being found to host significant amounts of circumstellar debris at mid-and far-infrared wavelengths (e.g. Su et al. 2006;Fujiwara et al. 2013;Eiroa et al. 2013;Kennedy & Wyatt 2013;Thureau et al. 2014;Ballering et al. 2014; Montesinos et al. 2016). ...
Debris discs are the dusty aftermath of planet formation processes around main-sequence stars. Analysis of these discs is often hampered by the absence of any meaningful constraint on the location and spatial extent of the disc around its host star. Multi-wavelength, resolved imaging ameliorates the degeneracies inherent in the modelling process, making such data indispensable in the interpretation of these systems. The Herschel Space Observatory observed HD 105211 (Eta Cru, HIP 59072) with its PACS instrument in three far-infrared wavebands (70, 100 and 160 um). Here we combine these data with ancillary photometry spanning optical to far-infrared wavelengths in order to determine the extent of the circumstellar disc. The spectral energy distribution and multi-wavelength resolved emission of the disc are simultaneously modelled using a radiative transfer and imaging codes. Analysis of the Herschel/PACS images reveals the presence of extended structure in all three PACS images. From a radiative transfer model we derive a disc extent of 87.0 +/- 2.5 au, with an inclination of 70.7 +/- 2.2 degrees to the line of sight and a position angle of 30.1 +/- 0.5 degrees. Deconvolution of the Herschel images reveal a potential asymmetry but this remains uncertain as a combined radiative transfer and image analysis replicate both the structure and the emission of the disc using a single axisymmetric annulus.
... While this section has focussed on just one system, there are many other systems for which compositional constraints exist, not all of which exhibit the same silica composition, even if they are interpreted within the framework of giant collisions (Lisse et al. 2008;Currie et al. 2011;Ballering et al. 2014). ...
Giant impacts refer to collisions between two objects each of which is massive enough to be considered at least a planetary embryo. The putative collision suffered by the proto-Earth that created the Moon is a prime example, though most Solar System bodies bear signatures of such collisions. Current planet formation models predict that an epoch of giant impacts may be inevitable, and observations of debris around other stars are providing mounting evidence that giant impacts feature in the evolution of many planetary systems. This chapter reviews giant impacts, focussing on what we can learn about planet formation by studying debris around other stars. Giant impact debris evolves through mutual collisions and dynamical interactions with planets. General aspects of this evolution are outlined, noting the importance of the collision-point geometry. The detectability of the debris is discussed using the example of the Moon-forming impact. Such debris could be detectable around another star up to 10 Myr post-impact, but model uncertainties could reduce detectability to a few 100 yr window. Nevertheless the 3 % of young stars with debris at levels expected during terrestrial planet formation provide valuable constraints on formation models; implications for super-Earth formation are also discussed. Variability recently observed in some bright disks promises to illuminate the evolution during the earliest phases when vapour condensates may be optically thick and acutely affected by the collision-point geometry. The outer reaches of planetary systems may also exhibit signatures of giant impacts, such as the clumpy debris structures seen around some stars.
... The grain population is maintained by the disintegration of Jupiter Family Comets and the breakup of asteroids, with the inflow resulting mostly from PRD (Nesvorný et al. 2010). Hot dust is observed in many debris disks as subtle silicate emission features (Ballering et al. 2014;Mittal et al. 2015). In some cases, the hot dust component has been resolved (e.g., Mennesson et al. 2014); in the best-studied examples, the dust is relatively close to the star, inside the zone thermally equivalent to the orbit of the Earth (Stock et al. 2010;Defrère et al. 2015). ...
Planetesimals form in gas-rich protoplanetary disks around young stars.
However, protoplanetary disks fade in about 10 Myr. The planetesimals (and also
many of the planets) left behind are too dim to study directly. Fortunately,
collisions between planetesimals produce dusty debris disks. These debris disks
trace the processes of terrestrial planet formation for 100 Myr and of
exoplanetary system evolution out to 10 Gyr. This chapter begins with a summary
of planetesimal formation as a prelude to the epoch of planetesimal
destruction. Our review of debris disks covers the key issues, including dust
production and dynamics, needed to understand the observations. Our discussion
of extrasolar debris keeps an eye on similarities to and differences from Solar
System dust.
... Most spectral measurements of debris disks have been taken at infrared wavelengths where the flux ratio of the disk to star is most favorable. Minerals can be identified by emission features (e.g., Christensen et al., 2000) with silicates being the most common and easiest to identify (e.g., Beichman et al., 2005, Lisse et al., 2012, Ballering et al., 2014). Interpretation of these spectra, however, is challenging because many grain properties other than composition (size, shape) can affect the spectrum. ...
The past decade has brought major improvements in large-scale asteroid
discovery and characterization with over half a million known asteroids and
over 100,000 with some measurement of physical characterization. This explosion
of data has allowed us to create a new global picture of the Main Asteroid
Belt. Put in context with meteorite measurements and dynamical models, a new
and more complete picture of Solar System evolution has emerged. The question
has changed from "What was the original compositional gradient of the Asteroid
Belt?" to "What was the original compositional gradient of small bodies across
the entire Solar System?" No longer is the leading theory that two belts of
planetesimals are primordial, but instead those belts were formed and sculpted
through evolutionary processes after Solar System formation. This article
reviews the advancements on the fronts of asteroid compositional
characterization, meteorite measurements, and dynamical theories in the context
of the heliocentric distribution of asteroid compositions seen in the Main Belt
today. This chapter also reviews the major outstanding questions relating to
asteroid compositions and distributions and summarizes the progress and current
state of understanding of these questions to form the big picture of the
formation and evolution of asteroids in the Main Belt. Finally, we briefly
review the relevance of asteroids and their compositions in their greater
context within our Solar System and beyond.
... While we cannot formally rule out a substantial emission arising from within the 0.15 AU KIN IWA, the most likely explanation is that most of the IRS-N band excess flux actually comes from regions outside of the KIN 4 AU FOV. Such a location would also be in line with recent two-belt modeling of warm debris disks' IRS spectra by Ballering et al. (2014), which used emission features to derive additional information about the grain properties and found a best-fit inner belt location of 5-6 AU in the case of ζ Lep (significantly larger than the 3 AU previously suggested). Assuming a disk inclination of 30 • with a P.A. of 50 • , consistent with the Gemini South resolved observations of Moerchen et al. (2010), we find a zodi level of 243 ± 73 solar zodis. ...
Forty-seven nearby main-sequence stars were surveyed with the Keck Interferometer mid-infrared Nulling instrument (KIN) between 2008 and 2011, searching for faint resolved emission from exozodiacal dust. Observations of a subset of the sample have already been reported, focusing essentially on stars with no previously known dust. Here we extend this previous analysis to the whole KIN sample, including 22 more stars with known near-and/or far-infrared excesses. In addition to an analysis similar to that of the first paper of this series, which was restricted to the 8-9 mu m spectral region, we present measurements obtained in all 10 spectral channels covering the 8-13 mu m instrumental bandwidth. Based on the 8-9 mu m data alone, which provide the highest signal-to-noise measurements, only one star shows a large excess imputable to dust emission (eta Crv), while four more show a significant (> 3 sigma) excess: beta Leo, beta UMa, zeta Lep, and gamma Oph. Overall, excesses detected by KIN are more frequent around A-type stars than later spectral types. Astatistical analysis of the measurements further indicates that stars with known far-infrared (lambda >= 70 mu m) excesses have higher exozodiacal emission levels than stars with no previous indication of a cold outer disk. This statistical trend is observed regardless of spectral type and points to a dynamical connection between the inner (zodi-like) and outer (Kuiper-Belt-like) dust populations. The measured levels for such stars are clustering close to the KIN detection limit of a few hundred zodis and are indeed consistent with those expected from a population of dust that migrated in from the outer belt by Poynting-Robertson drag. Conversely, no significant mid-ilinfrared excess is found around sources with previously reported near-infrared resolved excesses, which typically have levels of the order of 1% over the photospheric flux. If dust emission is really at play in these near-infrared detections, the absence of a strong mid-infrared counterpart points to populations of very hot and small (submicron) grains piling up very close to the sublimation radius. For solar-type stars with no known infrared excess, likely to be the most relevant targets for a future exo-Earth direct imaging mission, we find that their median zodi level is 12 +/- 24 zodis and lower than 60 (90) zodis with 95% (99%) confidence, if a lognormal zodi luminosity distribution is assumed.
Extreme debris discs (EDDs) are bright and warm circumstellar dusty structures around main sequence stars. They may represent the outcome of giant collisions occuring in the terrestrial region between large planetesimals or planetary bodies, and thus provide a rare opportunity to peer into the aftermaths of these events. Here, we report on results of a mini-survey we conducted with the aim to increase the number of known EDDs, investigate the presence of solid-state features around 10 μm in eight EDDs, and classify them into the silica or silicate dominated groups. We identify four new EDDs and derive their fundamental properties. For these, and for four other previously known discs, we study the spectral energy distribution around 10 μm by means of VLT/VISIR photometry in three narrow-band filters and conclude that all eight objects likely exhibit solid-state emission features from sub-micron grains. We find that four discs probably belong to the silicate dominated subgroup. Considering the age distribution of the entire EDD sample, we find that their incidence begins to decrease only after 300 Myr, suggesting that the earlier common picture that these objects are related to the formation of rocky planets may not be exclusive, and that other processes may be involved for older objects (≳100 Myr). Because most of the older EDD systems have wide, eccentric companions, we suggest that binarity may play a role in triggering late giant collisions.
We propose Sirius as an improved zero-point-defining star and calibrate its spectrum to an accuracy of ∼0.6% in both the visible and infrared. This result is based on a newly derived independent calibration in the visible of similar accuracy to the previous standard one, with which it is combined. We use a large variety of approaches in the infrared to reach about three times smaller error than for previous absolute calibrations. The results in the two wavelength regimes are in agreement, providing a consistent link from the visible throughout the near- and mid-infrared. The Sirius-based zero-point at 5557.5 Å (in vacuum) is 13.436 ± 0.081 × 10 ⁻¹² W cm ⁻² μ m ⁻¹ , based on the improved value for Vega of 3.473 ± 0.018 × 10 ⁻¹² W cm ⁻² μ m ⁻¹ and the measured magnitude difference between the two stars. At 2.1603 μ m, the zero-point is 4.225 ± 0.025 × 10 ⁻¹⁴ W cm ⁻² μ m ⁻¹ taking Sirius at a magnitude of −1.395. A jackknife analysis indicates that there are no serious systematic errors in these results. We consider selection of secondary standards that can extend the calibration over the sky. Despite more than a century in this role, normal A-stars are not suitable, although Am and Ap stars may be. G-stars older than ∼1 Gyr are good candidates if accurate temperatures can be measured. White dwarfs are suitable from the visible through the near-infrared, but their properties are unexplored at the necessary level at the longer infrared wavelengths, and for most facilities they are too faint there. Finally, as a further test of the calibration, we demonstrate an upgraded infrared flux method to determine accurate stellar diameters from K -band photometry.
We present characterization of the planetary system architecture for V488 Per, the dustiest main-sequence star known with a fractional infrared luminosity of ≈16%. Far-infrared imaging photometry confirms the existence of an outer planetary system dust population with a blackbody-fit temperature of ≈130 K. Mid-infrared spectroscopy probing the previously identified ≈800 K inner planetary system dust population does not detect any obvious solid-state emission features, suggesting either large grain sizes that mute such emission and/or grain compositions dominated by species like amorphous carbon and metallic iron, which do not produce such features. In the latter case, the presence of significant quantities of iron-rich material could be indicative of the active formation of a Mercury-like planet around V488 Per. In any event, the absence of solid-state emission features is very unusual among main-sequence stars with copious amounts of warm orbiting dust particles; we know of no other such star whose mid-infrared spectrum lacks such features. Combined radial velocity monitoring and adaptive optics imaging find no evidence for stellar/substellar companions within several hundred astronomical units of V488 Per.
We present the first spatially resolved scattered-light images of four debris disks around members of the Scorpius-Centaurus (Sco-Cen) OB Association with high-contrast imaging and polarimetry using the Gemini Planet Imager (GPI). All four disks are resolved for the first time in polarized light and one disk is also detected in total intensity. The three disks imaged around HD 111161, HD 143675, and HD 145560 are symmetric in both morphology and brightness distribution. The three systems span a range of inclinations and radial extents. The disk imaged around HD 98363 shows indications of asymmetries in morphology and brightness distribution, with some structural similarities to the HD 106906 planet-disk system. Uniquely, HD 98363 has a wide co-moving stellar companion Wray 15-788 with a recently resolved disk with very different morphological properties. HD 98363 A/B is the first binary debris disk system with two spatially resolved disks. All four targets have been observed with ALMA, and their continuum fluxes range from one non-detection to one of the brightest disks in the region. With the new results, a total of 15 A/F-stars in Sco-Cen have resolved scattered light debris disks, and approximately half of these systems exhibit some form of asymmetry. Combining the GPI disk structure results with information from the literature on millimeter fluxes and imaged planets reveals a diversity of disk properties in this young population. Overall, the four newly resolved disks contribute to the census of disk structures measured around A/F-stars at this important stage in the development of planetary systems.
Context. The presence of sub-micron grains has been inferred in several debris discs, usually because of a blue colour of the spectrum in scattered light or a pronounced silicate band around 10 μ m, even though these particles should be blown out by stellar radiation pressure on very short timescales. So far, no fully satisfying explanation has been found for this apparent paradox.
Aims. We investigate the possibility that the observed abundances of sub-micron grains could be naturally produced in bright debris discs, where the high collisional activity produces them at a rate high enough to partially compensate for their rapid removal. We also investigate to what extent this potential presence of small grains can affect our understanding of some debris disc characteristics.
Methods. We used a numerical collisional code to follow the collisional evolution of a debris disc down to sub-micron grains far below the limiting blow-out size sblow . We considered compact astrosilicates and explored different configurations: A and G stars, cold and warm discs, bright and very bright systems. We then produced synthetic spectra and spectral energy distributions, where we identified and quantified the signature of unbound sub-micron grains.
Results. We find that in bright discs (fractional luminosity ≳10 ⁻³ ) around A stars, the number of sub-micron grains is always high enough to leave detectable signatures in scattered light where the disc colour becomes blue, and also in the mid-IR (10 ≲ λ ≲ 20 μ m), where they boost the disc luminosity by at least a factor of 2 and induce a pronounced silicate solid-state band around 10 μ m. We also show that with this additional contribution of sub-micron grains, the spectral energy distribution can mimic that of two debris belts separated by a factor of ~2 in radial distance. For G stars, the effect of s ≤ sblow grains remains limited in the spectra although they dominate the geometrical cross section of the system. We also find that for all considered cases, the halo of small (bound and unbound) grains that extends far beyond the main disc contributes to ~50% of the flux up to λ ~ 50 μ m wavelengths.
Processes governing the evolution of planetesimals are critical to understanding how rocky planets are formed, how water is delivered to them, the origin of planetary atmospheres, how cores and magnetic dynamos develop, and ultimately, which planets have the potential to be habitable. Theoretical advances and new data from asteroid and meteorite observations, coupled with spacecraft missions such as Rosetta and Dawn, have led to major advances in this field over the last decade. This transdisciplinary volume presents an authoritative overview of the latest in our understanding of the processes of planet formation. Combining meteorite, asteroid and icy body observations with theory and modelling of accretion and orbital dynamics, this text also provides insights into the exoplanetary system and the search for habitable worlds. This is an essential reference for those interested in planetary formation, solar system dynamics, exoplanets and planetary habitability.
We describe a joint high contrast imaging survey for planets at Keck and VLT of the last large sample of debris disks identified by the Spitzer Space Telescope. No new substellar companions were discovered in our survey of 30 Spitzer-selected targets. We combine our observations with data from four published surveys to place constraints on the frequency of planets around 130 debris disk single stars, the largest sample to date. For a control sample, we assembled contrast curves from several published surveys targeting 277 stars which do not show infrared excesses. We assumed a double power law distribution in mass and semi-major axis of the form f(m,a) = $Cm^{\alpha}a^{\beta}$, where we adopted power law values and logarithmically flat values for the mass and semi-major axis of planets. We find that the frequency of giant planets with masses 5-20 $M_{\rm Jup}$ and separations 10-1000 AU around stars with debris disks is 6.27% (68% confidence interval 3.68 - 9.76%), compared to 0.73% (68% confidence interval 0.20 - 1.80%) for the control sample of stars without disks. These distributions differ at the 88% confidence level, tentatively suggesting distinctness of these samples.
The architectures of debris disks encode the history of planet formation in these systems. Studies of debris disks via their spectral energy distributions (SEDs) have found infrared excesses arising from cold dust, warm dust, or a combination of the two. The cold outer belts of many systems have been imaged, facilitating their study in great detail. Far less is known about the warm components, including the origin of the dust. The regularity of the disk temperatures indicates an underlying structure that may be linked to the water snow line. If the dust is generated from collisions in an exo-asteroid belt, the dust will likely trace the location of the water snow line in the primordial protoplanetary disk where planetesimal growth was enhanced. If instead the warm dust arises from the inward transport from a reservoir of icy material farther out in the system, the dust location is expected to be set by the current snow line. We analyze the SEDs of a large sample of debris disks with warm components. We find that warm components in single-component systems (those without detectable cold components) follow the primordial snow line rather than the current snow line, so they likely arise from exo-asteroid belts. While the locations of many warm components in two-component systems are also consistent with the primordial snow line, there is more diversity among these systems, suggesting additional effects play a role.
The conclusion of the WISE mission presents an opportune time to summarize the history of using excess emission in the infrared as a tracer of circumstellar material and exploit all available data for future missions such as JWST. We have compiled a catalog of infrared excess stars from peer-reviewed articles and perform an extensive search for new infrared excess stars by cross-correlating the Tycho-2 and AllWISE catalogs. We define a significance of excess in four spectral type divisions and select stars showing greater than either 3$\sigma$ or 5$\sigma$ significance of excess in the mid- and far-infrared. Through procedures including SED fitting and various image analyses, each potential excess source was rigorously vetted to eliminate false-positives. The infrared excess stars from the literature and the new stars found through the Tycho-2 and AllWISE cross-correlation produced nearly 500 `Prime' infrared excess stars and $\geq$1200 `Reserved' stars. The main catalog of infrared excess stars are nearby, bright, and either demonstrate excess in more than one passband or have infrared spectroscopy confirming the infrared excess. This study identifies stars that display a spectral energy distribution suggestive of a secondary or post-protoplanetary generation of dust and they are ideal targets for future optical and infrared imaging observations. The final catalogs of stars summarizes the past work using infrared excess to detect dust disks and with the most extensive compilation of infrared excess stars ($\sim$ 1750) to date, we investigate various relationships among stellar and disk parameters.
We investigate whether varying the dust composition (described by the optical constants) can solve a persistent problem in debris disk modeling--the inability to fit the thermal emission without over-predicting the scattered light. We model five images of the beta Pictoris disk: two in scattered light from HST/STIS at 0.58 microns and HST/WFC3 at 1.16 microns, and three in thermal emission from Spitzer/MIPS at 24 microns, Herschel/PACS at 70 microns, and ALMA at 870 microns. The WFC3 and MIPS data are published here for the first time. We focus our modeling on the outer part of this disk, consisting of a parent body ring and a halo of small grains. First, we confirm that a model using astronomical silicates cannot simultaneously fit the thermal and scattered light data. Next, we use a simple, generic function for the optical constants to show that varying the dust composition can improve the fit substantially. Finally, we model the dust as a mixture of the most plausible debris constituents: astronomical silicates, water ice, organic refractory material, and vacuum. We achieve a good fit to all datasets with grains composed predominantly of silicates and organics, while ice and vacuum are, at most, present in small amounts. This composition is similar to one derived from previous work on the HR 4796A disk. Our model also fits the thermal SED, scattered light colors, and high-resolution mid-IR data from T-ReCS for this disk. Additionally, we show that sub-blowout grains are a necessary component of the halo.
We analyze two new sets of coagulation calculations for solid particles
orbiting within the terrestrial zone of a solar-type star. In models of
collisional cascades, numerical simulations demonstrate that the total mass,
the mass in 1 mm and smaller particles, and the dust luminosity decline with
time more rapidly than predicted by analytic models, $\propto t^{-n}$ with $n
\approx$ 1.1-1.2 instead of 1. Size distributions derived from the numerical
calculations follow analytic predictions at radii less than 0.1 km but are
shallower than predicted at larger sizes. In simulations of planet formation,
the dust luminosity declines more slowly than in pure collisional cascades,
with $n \approx$ 0.5-0.8 instead of 1.1-1.2. Throughout this decline, giant
impacts produce large, observable spikes in dust luminosity which last roughly
0.01-0.1 Myr and recur every 1-10 Myr. If most solar-type stars have Earth mass
planets with $a \lesssim$ 1-2 AU, observations of debris around 1-100 Myr stars
allow interesting tests of theory. Current data preclude theories where
terrestrial planets form out of 1000 km or larger planetesimals. Although the
observed frequency of debris disks among $\gtrsim$ 30 Myr old stars agrees with
our calculations, the observed frequency of warm debris among 5-20 Myr old
stars is smaller than predicted.
HD 95086 is a young early-type star that hosts (1) a 5 MJ planet at the
projected distance of 56 AU revealed by direct imaging, and (2) a prominent
debris disk. Here we report the detection of 69 um crystalline olivine feature
from the disk using the Spitzer/MIPS-SED data covering 55-95 um. Due to the low
resolution of MIPS-SED mode, this feature is not spectrally resolved, but is
consistent with the emission from crystalline forsterite contributing 5% of the
total dust mass. We also present detailed analysis of the disk SED and
re-analysis of resolved images obtained by Herschel. Our results suggest that
the debris structure around HD 95086 consists of a warm (175 K) belt, a cold
(55 K) disk, and an extended disk halo (up to 800 AU), and is very similar to
that of HR 8799. We compare the properties of the three debris components, and
suggest that HD 95086 is a young analog of HR 8799. We further investigate and
constrain single-planet, two-planet, three-planet and four-planet architectures
that can account for the observed debris structure and are compatible with
dynamical stability constraints. We find that equal-mass four-planet
configurations of geometrically spaced orbits, with each planet of mass 5 MJ,
could maintain the gap between the warm and cold debris belts, and also be just
marginally stable for timescales comparable to the age of the system.
Context. Detecting and characterizing circumstellar dust is a way to study the architecture and evolution of planetary systems. Cold dust in debris disks only traces the outer regions. Warm and hot exozodiacal dust needs to be studied in order to trace regions close to the habitable zone.
Aims. We aim to determine the prevalence and to constrain the properties of hot exozodiacal dust around nearby main-sequence stars.
Methods. We searched a magnitude-limited (H ≤ 5) sample of 92 stars for bright exozodiacal dust using our VLTI visitor instrument PIONIER in the H band. We derived statistics of the detection rate with respect to parameters, such as the stellar spectral type and age or the presence of a debris disk in the outer regions of the systems. We derived more robust statistics by combining our sample with the results from our CHARA/FLUOR survey in the K band. In addition, our spectrally dispersed data allowed us to put constraints on the emission mechanism and the dust properties in the detected systems.
Results. We find an overall detection rate of bright exozodiacal dust in the H band of 11% (9 out of 85 targets) and three tentative detections. The detection rate decreases from early type to late type stars and increases with the age of the host star. We do not confirm the tentative correlation between the presence of cold and hot dust found in our earlier analysis of the FLUOR sample alone. Our spectrally dispersed data suggest that either the dust is extremely hot or the emission is dominated by the scattered light in most cases. The implications of our results for the target selection of future terrestrial planet-finding missions using direct imaging are discussed.
We have spatially resolved five debris disks (HD 30447, HD 35841, HD 141943,
HD 191089, and HD 202917) for the first time in near-infrared scattered light
by reanalyzing archival Hubble Space Telescope (HST)/NICMOS coronagraphic
images obtained between 1999 and 2006. One of these disks (HD 202917) was
previously resolved at visible wavelengths using HST/Advanced Camera for
Surveys. To obtain these new disk images, we performed advanced point-spread
function subtraction based on the Karhunen-Loeve Image Projection (KLIP)
algorithm on recently reprocessed NICMOS data with improved detector artifact
removal (Legacy Archive PSF Library And Circumstellar Environments Legacy
program). Three of the disks (HD 30447, HD 35841, and HD 141943) appear
edge-on, while the other two (HD 191089 and HD 202917) appear inclined. The
inclined disks have been sculpted into rings; in particular, the disk around HD
202917 exhibits strong asymmetries. All five host stars are young (8-40 Myr),
nearby (40-100 pc) F and G stars, and one (HD 141943) is a close analog to the
young sun during the epoch of terrestrial planet formation. Our discoveries
increase the number of debris disks resolved in scattered light from 19 to 23
(a 21% increase). Given their youth, proximity, and brightness (V = 7.2 to
8.5), these targets are excellent candidates for follow-up investigations of
planet formation at visible wavelengths using the HST/STIS coronagraph, at
near-infrared wavelengths with the Gemini Planet Imager (GPI) and Very Large
Telescope (VLT)/SPHERE, and at thermal infrared wavelengths with the James Webb
Space Telescope NIRCam and MIRI coronagraphs.
The quest for Earth-like planets is a major focus of current exoplanet research. Although planets that are Earth-sized and
smaller have been detected, these planets reside in orbits that are too close to their host star to allow liquid water on
their surfaces. We present the detection of Kepler-186f, a 1.11 ± 0.14 Earth-radius planet that is the outermost of five planets,
all roughly Earth-sized, that transit a 0.47 ± 0.05 solar-radius star. The intensity and spectrum of the star’s radiation
place Kepler-186f in the stellar habitable zone, implying that if Kepler-186f has an Earth-like atmosphere and water at its
surface, then some of this water is likely to be in liquid form.
Hot exozodiacal dust is thought to be responsible for excess near-infrared
(NIR) emission emanating from the innermost parts of some debris disks. The
origin of this dust, however, is still a matter of debate. We test whether hot
exozodiacal dust can be supplied from an exterior parent belt by
Poynting-Robertson (P-R) drag, paying special attention to the pile-up of dust
that occurs due to the interplay of P-R drag and dust sublimation.
Specifically, we investigate whether pile-ups still occur when collisions are
taken into account, and if they can explain the observed NIR excess. We compute
the steady-state distribution of dust in the inner disk by solving the
continuity equation. First, we derive an analytical solution under a number of
simplifying assumptions. Second, we develop a numerical debris disk model that
for the first time treats the complex interaction of collisions, P-R drag, and
sublimation in a self-consistent way. From the resulting dust distributions we
generate thermal emission spectra and compare these to observed excess NIR
fluxes. We confirm that P-R drag always supplies a small amount of dust to the
sublimation zone, but find that a fully consistent treatment yields a maximum
amount of dust that is about 7 times lower than that given by analytical
estimates. The NIR excess due this material is much smaller (<10^-3 for A-type
stars with parent belts at >1 AU) than the values derived from interferometric
observations (~10^-2). Pile-up of dust still occurs when collisions are
considered, but its effect on the NIR flux is insignificant. Finally, the
cross-section in the innermost regions is clearly dominated by barely bound
grains.
High levels of exozodiacal dust have been observed in the inner regions of a large fraction of main-sequence stars. Given
the short lifetime of the observed small dust grains, these ‘exozodis’ are difficult to explain, especially for old (>100 Myr)
stars. The exozodiacal dust may be observed as excess emission in the mid-infrared, or using interferometry. We hypothesize
that exozodi are sustained by planetesimals scattered by planets inwards from an outer planetesimal belt, where collision
time-scales are long. In this work, we use N-body simulations to show that the outward migration of a planet into a belt, driven by the scattering of planetesimals, can
increase, or sustain, the rate at which planetesimals are scattered from the outer belt to the exozodi region. We hypothesize
that this increase is sufficient to sustain the observed exozodi on Gyr time-scales. No correlation between observations of
an outer belt and an exozodi is required for this scenario to work, as the outer belt may be too faint to detect. If planetesimal-driven
migration does explain the observed exozodi, this work suggests that the presence of an exozodi indicates the presence of
outer planets and a planetesimal belt.
We present an adaptive optics imaging detection of the HD 32297 debris disk
at L' (3.8 \microns) obtained with the LBTI/LMIRcam infrared instrument at the
LBT. The disk is detected at signal-to-noise per resolution element ~ 3-7.5
from ~ 0.3-1.1" (30-120 AU). The disk at L' is bowed, as was seen at shorter
wavelengths. This likely indicates the disk is not perfectly edge-on and
contains highly forward scattering grains. Interior to ~ 50 AU, the surface
brightness at L' rises sharply on both sides of the disk, which was also
previously seen at Ks band. This evidence together points to the disk
containing a second inner component located at $\lesssim$ 50 AU. Comparing the
color of the outer (50 $< r$/AU $< 120$) portion of the disk at L' with
archival HST/NICMOS images of the disk at 1-2 \microns allows us to test the
recently proposed cometary grains model of Donaldson et al. 2013. We find that
the model fails to match the disk's surface brightness and spectrum
simultaneously (reduced chi-square = 17.9). When we modify the density
distribution of the model disk, we obtain a better overall fit (reduced
chi-square = 2.9). The best fit to all of the data is a pure water ice model
(reduced chi-square = 1.06), but additional resolved imaging at 3.1 \microns is
necessary to constrain how much (if any) water ice exists in the disk, which
can then help refine the originally proposed cometary grains model.
We have obtained intermediate-resolution (R ≍ 50) infrared
(2.6-13.5 μm) spectra of the particles in the circumstellar disk of
β Pic. The silicate dust feature near 10 microns is broader and
contains more structure than interstellar and most circumstellar
emission features. The silicate feature in β Pic is remarkably
similar to those in comets Halley, Bradfield 1987s, and Levy 1990 XX
which have emission features characteristic of crystalline silicates.
This result supports the inference based on IRAS results that cometary
bodies resupply the grains in the β Pic disk. Detailed models of
the dust disk and grains are used to derive plausible disk temperature
and density gradients. We discuss implications of these results on the
history and state of dust in disks and the solar system.
We present dual-band Herschel/Photodetector Array Camera and Spectrometer imaging for four stars whose spectral energy distributions (SEDs) suggest two-ring disk architectures that mirror that of the asteroid-Kuiper Belt geometry of our own solar system. The Herschel observations at 100 μm spatially resolve the cold/outer-dust component for each star-disk system for the first time, finding evidence of planetesimals at >100 AU, i.e., a larger size than assumed from a simple blackbody fit to the SED. By breaking the degeneracy between the grain properties and the dust's radial location, the resolved images help constrain the dust grain-size distribution for each system. Three of the observed stars are A-type and one solar-type. On the basis of the combined Spitzer/IRS+MIPS (5-70 μm), the Herschel/PACS (100 and 160 μm) dataset, and under the assumption of idealized spherical grains, we find that the cold/outer belts of the three A-type stars are well fit with a mixed ice/rock composition rather than pure rocky grains, while the debris around the solar-type star is consistent with either rock or ice/rock grains. For the solar-type star HD 104860, we find that the minimum grain size is larger than expected from the threshold set by radiative blowout. The A-type stars HD 71722 and HD 159492, on the other hand, require minimum grain sizes that are smaller than blowout for inner- and outer-ring populations. In the absence of spectral features for ice, we find that the behavior of the continuum can help constrain the composition of the grains (of icy nature and not pure rocky material) given the Herschel-resolved locations of the cold/outer-dust belts.
We describe Spitzer Infrared Spectrograph spectroscopic observations of the ~10 Myr old star, EF Cha. Compositional modeling of the spectra from 5 mum to 35 mum confirms that it is surrounded by a luminous debris disk with LD /L sstarf ~ 10-3, containing dust with temperatures between 225 K and 430 K, characteristic of the terrestrial zone. The EF Cha spectrum shows evidence for many solid-state features, unlike most cold, low-luminosity debris disks but like some other 10-20 Myr old luminous, warm debris disks (e.g., HD 113766A). The EF Cha debris disk is unusually rich in a species or combination of species whose emissivities resemble that of finely powdered, laboratory-measured phyllosilicate species (talc, saponite, and smectite), which are likely produced by aqueous alteration of primordial anhydrous rocky materials. The dust and, by inference, the parent bodies of the debris also contain abundant amorphous silicates and metal sulfides, and possibly water ice. The dust's total olivine to the pyroxene ratio of ~2 also provides evidence of aqueous alteration. The large mass volume of grains with sizes comparable to or below the radiation blow-out limit implies that planetesimals may be colliding at a rate high enough to yield the emitting dust but not so high as to devolatize the planetesimals via impact processing. Because phyllosilicates are produced by the interactions between anhydrous rock and warm, reactive water, EF Cha's disk is a likely signpost for water delivery to the terrestrial zone of a young planetary system.
Aims: Sixty percent of the A star members of the 12 Myr old beta Pictoris moving group (BPMG) show significant excess emission in the mid-infrared, several million years after the proto-planetary disc is thought to disperse. Theoretical models suggest this peak may coincide with the formation of Pluto-sized planetesimals in the disc, stirring smaller bodies into collisional destruction. Here we present resolved mid-infrared imaging of the disc of eta Tel (A0V in the BPMG) and consider its implications for the state of planet formation in this system. Methods: The source was observed at 11.7 and 18.3 mum using T-ReCS on Gemini South. The resulting images were compared to simple disc models to constrain the radial distribution of the emitting material. Results: The emission observed at 18.3 mum is shown to be significantly extended beyond the PSF along a position angle 8°. This is the first time dust emission has been resolved around eta Tel. Modelling indicates that the extension arises from an edge-on disc of radius 0.5 arcsec (~24 AU). Combining the spatial constraints from the imaging with those from the spectral energy distribution shows that >50% of the 18 mum emission comes from an unresolved dust component at ~4 AU. Conclusions: The radial structure of the eta Tel debris disc is reminiscent of the Solar System, suggesting that this is a young Solar System analogue. For an age of 12 Myr, both the radius and dust level of the extended cooler component are consistent with self-stirring models for a protoplanetary disc of 0.7 times minimum mass solar nebula. The origin of the hot dust component may arise in an asteroid belt undergoing collisional destruction or in massive collisions in ongoing terrestrial planet formation.
(Abridged) Dust is expected to be ubiquitous in extrasolar planetary systems
owing to the dynamical activity of minor bodies. Inner dust populations are,
however, still poorly known because of the high contrast and small angular
separation with respect to their host star. We aim to determine the level of
near-infrared exozodiacal dust emission around a sample of 42 nearby main
sequence stars with spectral types ranging from A to K and to investigate its
correlation with various stellar parameters and with the presence of cold dust
belts. We use high-precision K-band visibilities obtained with the FLUOR
interferometer on the shortest baseline of the CHARA array. The calibrated
visibilities are compared with the expected visibility of the stellar
photosphere to assess whether there is an additional, fully resolved
circumstellar emission. Near-infrared circumstellar emission amounting to about
1% of the stellar flux is detected around 13 of our 42 target stars. Follow-up
observations showed that one of them (eps Cep) is associated with a stellar
companion, while another one was detected around what turned out to be a giant
star (kap CrB). The remaining 11 excesses found around single main sequence
stars are most probably associated with hot circumstellar dust, yielding an
overall occurrence rate of 28+8-6% for our (biased) sample. We show that the
occurrence rate of bright exozodiacal discs correlates with spectral type,
K-band excesses being more frequent around A-type stars. It also correlates
with the presence of detectable far-infrared excess emission in the case of
solar-type stars. This study provides new insight into the phenomenon of bright
exozodiacal discs, showing that hot dust populations are probably linked to
outer dust reservoirs in the case of solar-type stars. For A-type stars, no
clear conclusion can be made regarding the origin of the detected near-infrared
excesses.
We have carried out high contrast imaging of 70 young, nearby B and A stars to search for brown dwarf and planetary companions as part of the Gemini NICI Planet-Finding Campaign. Our survey represents the largest, deepest survey for planets around high-mass stars (≈1.5-2.5 M
☉) conducted to date and includes the planet hosts β Pic and Fomalhaut. We obtained follow-up astrometry of all candidate companions within 400 AU projected separation for stars in uncrowded fields and identified new low-mass companions to HD 1160 and HIP 79797. We have found that the previously known young brown dwarf companion to HIP 79797 is itself a tight (3 AU) binary, composed of brown dwarfs with masses 58M
Jup and 55M
Jup, making this system one of the rare substellar binaries in orbit around a star. Considering the contrast limits of our NICI data and the fact that we did not detect any planets, we use high-fidelity Monte Carlo simulations to show that fewer than 20% of 2 M
☉ stars can have giant planets greater than 4 M
Jup between 59 and 460 AU at 95% confidence, and fewer than 10% of these stars can have a planet more massive than 10 M
Jup between 38 and 650 AU. Overall, we find that large-separation giant planets are not common around B and A stars: fewer than 10% of B and A stars can have an analog to the HR 8799 b (7 M
Jup, 68 AU) planet at 95% confidence. We also describe a new Bayesian technique for determining the ages of field B and A stars from photometry and theoretical isochrones. Our method produces more plausible ages for high-mass stars than previous age-dating techniques, which tend to underestimate stellar ages and their uncertainties.
Excess emission, associated with warm, dust belts, commonly known as exozodis, has been observed around a third of nearby
stars. The high levels of dust required to explain the observations are not generally consistent with steady-state evolution.
A common suggestion is that the dust results from the aftermath of a dynamical instability, an event akin to the Solar system's
Late Heavy Bombardment. In this work, we use a data base of N-body simulations to investigate the aftermath of dynamical instabilities between giant planets in systems with outer planetesimal
belts. We find that, whilst there is a significant increase in the mass of material scattered into the inner regions of the
planetary system following an instability, this is a short-lived effect. Using the maximum lifetime of this material, we determine
that even if every star has a planetary system that goes unstable, there is a very low probability that we observe more than
a maximum of 1 per cent of sun-like stars in the aftermath of an instability, and that the fraction of planetary systems currently
in the aftermath of an instability is more likely to be limited to ≤0.06 per cent. This probability increases marginally for
younger or higher mass stars. We conclude that the production of warm dust in the aftermath of dynamical instabilities is
too short-lived to be the dominant source of the abundantly observed exozodiacal dust.
We present the first characterisation of the 12um warm dust ("exo-Zodi")
luminosity function around Sun-like stars, focussing on the dustiest systems
that can be identified by WISE. We detect six new warm dust candidates, five of
which have unknown ages. We show that the dustiest old (>Gyr) systems like
BD+20 307 are 1 in 10,000 occurrences. Bright warm dust is more common around
young (<120Myr) systems, with a ~1% occurrence rate. We show that a two
component in situ model where all stars have initially massive warm disks and
in which warm debris is also generated at some random time along the stars'
main-sequence lifetime, perhaps due to a collision, can explain the
observations. However, if all stars only have initially massive warm disks
these would not be visible at Gyr ages, and random collisions on the
main-sequence are too infrequent to explain the high disk occurrence rate for
young stars. That is, neither component can explain the observations on their
own. Despite these conclusions, we cannot rule out an alternative model in
which comets are scattered from outer regions because the distribution of
systems with the appropriate dynamics is unknown. Our in situ model predicts
the fraction of stars with exo-Zodi bright enough to cause problems for future
exo-Earth imaging attempts is at least ~10%, and is higher for populations of
stars younger than a few Gyr. This prediction of ~10% applies to old stars
because bright systems like BD+20 307 imply a population of fainter systems
that were once bright but are now decaying through fainter levels. Our
prediction should be strongly tested by the Large Binocular Telescope
Interferometer, providing valuable input for more detailed evolution models. A
detection fraction lower than our prediction could indicate that the hot dust
in systems like BD+20 307 has a cometary origin due to the quirks of the
planetary dynamics.
Warm debris disks are a sub-sample of the large population of debris disks,
and display excess emission in the mid-IR. Around solar-type stars, very few
objects show emission features in mid-IR spectroscopic observations, that are
attributed to small, warm silicate dust grains. The origin of this warm dust
can possibly be explained either by a collision between several bodies or by
transport from an outer belt. We present and analyse new far-IR Herschel/Pacs
observations, supplemented by ground-based data in the mid-IR (VLTI/Midi and
VLT/Visir), for one of these rare systems: the 10-16 Myr old debris disk around
HD 113766 A. We improve an existing model to account for these new
observations, and better constrain the spatial distribution of the dust and its
composition. We underline the limitations of SED modelling and the need for
spatially resolved observations. We find that the system is best described by
an inner disk located within the first AU, well constrained by the Midi data,
and an outer disk located between 9-13 AU. In the inner dust belt, our previous
finding of Fe-rich crystalline olivine grains still holds. We do not observe
time variability of the emission features over at least a 8 years time span, in
a environment subjected to strong radiation pressure. The time stability of the
emission features indicates that {\mu}m-sized dust grains are constantly
replenished from the same reservoir, with a possible depletion of
sub-{\mu}m-sized grains. We suggest that the emission features may arise from
multi-composition aggregates. We discuss possible scenarios concerning the
origin of the warm dust. The compactness of the innermost regions as probed by
Midi, as well as the dust composition, suggest that we are witnessing the
outcomes of (at least) one collision between partially differentiated bodies,
in an environment possibly rendered unstable by terrestrial planetary
formation.
Context. Little is known about the properties of the warm (Tdust >~ 150 K)
debris disk material located close to the central star, which has a more direct
link to the formation of terrestrial planets than the low temperature debris
dust that has been detected to date. Aims. To discover new warm debris disk
candidates that show large 18 micron excess and estimate the fraction of stars
with excess based on the AKARI/IRC Mid-Infrared All-Sky Survey data. Methods.
We have searched for point sources detected in the AKARI/IRC All-Sky Survey,
which show a positional match with A-M dwarf stars in the Tycho-2 Spectral Type
Catalogue and exhibit excess emission at 18 micron compared to that expected
from the Ks magnitude in the 2MASS catalogue. Results. We find 24 warm debris
candidates including 8 new candidates among A-K stars. The apparent debris disk
frequency is estimated to be 2.8 +/- 0.6%. We also find that A stars and
solar-type FGK stars have different characteristics of the inner component of
the identified debris disk candidates --- while debris disks around A stars are
cooler and consistent with steady-state evolutionary model of debris disks,
those around FGK stars tend to be warmer and cannot be explained by the
steady-state model.
Spectral modeling of the large infrared excess in the Spitzer IRS spectra of
HD 172555 suggests that there is more than 10^19 kg of sub-micron dust in the
system. Using physical arguments and constraints from observations, we rule out
the possibility of the infrared excess being created by a magma ocean planet or
a circumplanetary disk or torus. We show that the infrared excess is consistent
with a circumstellar debris disk or torus, located at approximately 6 AU, that
was created by a planetary scale hypervelocity impact. We find that radiation
pressure should remove submicron dust from the debris disk in less than one
year. However, the system's mid-infrared photometric flux, dominated by
submicron grains, has been stable within 4 percent over the last 27 years, from
IRAS (1983) to WISE (2010). Our new spectral modeling work and calculations of
the radiation pressure on fine dust in HD 172555 provide a self-consistent
explanation for this apparent contradiction. We also explore the unconfirmed
claim that 10^47 molecules of SiO vapor are needed to explain an emission
feature at 8 um in the Spitzer IRS spectrum of HD 172555. We find that unless
there are 10^48 atoms or 0.05 Earth masses of atomic Si and O vapor in the
system, SiO vapor should be destroyed by photo-dissociation in less than 0.2
years. We argue that a second plausible explanation for the 8 um feature can be
emission from solid SiO, which naturally occurs in submicron silicate "smokes"
created by quickly condensing vaporized silicate.
The majority of debris discs discovered so far have only been detected
through infrared excess emission above stellar photospheres. While disc
properties can be inferred from unresolved photometry alone under various
assumptions for the physical properties of dust grains, there is a degeneracy
between disc radius and dust temperature that depends on the grain size
distribution and optical properties. By resolving the disc we can measure the
actual location of the dust. The launch of Herschel, with an angular resolution
superior to previous far-infrared telescopes, allows us to spatially resolve
more discs and locate the dust directly. Here we present the nine resolved
discs around A stars between 20 and 40 pc observed by the DEBRIS survey. We use
these data to investigate the disc radii by fitting narrow ring models to
images at 70, 100 and 160 {\mu}m and by fitting blackbodies to full spectral
energy distributions. We do this with the aim of finding an improved way of
estimating disc radii for unresolved systems. The ratio between the resolved
and blackbody radii varies between 1 and 2.5. This ratio is inversely
correlated with luminosity and any remaining discrepancies are most likely
explained by differences to the minimum size of grain in the size distribution
or differences in composition. We find that three of the systems are well fit
by a narrow ring, two systems are borderline cases and the other four likely
require wider or multiple rings to fully explain the observations, reflecting
the diversity of planetary systems.
High levels of exozodiacal dust are observed around a growing number of main
sequence stars. The origin of such dust is not clear, given that it has a short
lifetime against both collisions and radiative forces. Even a collisional
cascade with km-sized parent bodies, as suggested to explain outer debris
discs, cannot survive sufficiently long. In this work we investigate whether
the observed exozodiacal dust could originate from an outer planetesimal belt.
We investigate the scattering processes in stable planetary systems in order to
determine whether sufficient material could be scattered inwards in order to
retain the exozodiacal dust at its currently observed levels. We use N-body
simulations to investigate the efficiency of this scattering and its dependence
on the architecture of the planetary system. The results of these simulations
can be used to assess the ability of hypothetical chains of planets to produce
exozodi in observed systems. We find that for older (>100Myr) stars with
exozodiacal dust, a massive, large radii (>20AU) outer belt and a chain of
tightly packed, low-mass planets would be required in order to retain the dust
at its currently observed levels. This brings into question how many, if any,
real systems possess such a contrived architecture and are therefore capable of
scattering at sufficiently high rates to retain exozodi dust on long
timescales.
We present high spatial resolution mid- and far-infrared images of the Vega debris disk obtained with the Multiband Imaging Photometer for Spitzer (MIPS). The disk is well resolved and its angular size is much larger than found previously. The radius of the disk is at least 43" (330 AU), 70"(543 AU), and 105" (815 AU) in extent at 24, 70 and 160 um, respectively. The disk images are circular, smooth and without clumpiness at all three wavelengths. The radial surface brightness profiles imply an inner boundary at a radius of 11"+/-2" (86 AU). Assuming an amalgam of amorphous silicate and carbonaceous grains, the disk can be modeled as an axially symmetric and geometrically thin disk, viewed face-on, with the surface particle number density following an r^-1 power law. The disk radiometric properties are consistent with a range of models using grains of sizes ~1 to ~50 um. We find that a ring, containing grains larger than 180 um and at radii of 86-200 AU from the star, can reproduce the observed 850 um flux, while its emission does not violate the observed MIPS profiles. This ring could be associated with a population of larger asteroidal bodies analogous to our own Kuiper Belt. Cascades of collisions starting with encounters amongthese large bodies in the ring produce the small debris that is blown outward by radiation pressure to much larger distances where we detect its thermal emission. The dust production rate is >~10^15 g/s based on the MIPS results. This rate would require a very massive asteroidal reservoir for the dust to be produced in a steady state throughout Vega's life. Instead, we suggest that the disk we imaged is ephemeral and that we are witnessing the aftermath of a large and relatively recent collisional event, and subsequent collisional cascade.
We report new Spitzer 24 μm photometry of 76 main-sequence A-type stars. We combine these results with previously reported Spitzer 24 μm data and 24 and 25 μm photometry from the Infrared Space Observatory and the Infrared Astronomy Satellite. The result is a sample of 266 stars with mass close to 2.5 M☉, all detected to at least the ~7 σ level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K - 24 (or 25) μm photospheric color and then compared all stars in the sample to that color. Because we have excluded stars with strong emission lines or extended emission (associated with nearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emission more frequently and with higher fractional excess than do the older stars. However, as many as 50% of the younger stars do not show excess emission. The decline in the magnitude of excess emission, for those stars that show it, has a roughly t0/time dependence, with t0 ~ 150 Myr. If anything, stars in binary systems (including Algol-type stars) and λ Boo stars show less excess emission than the other members of the sample. Our results indicate that (1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10-60 AU region and (2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion end game, and that individual events often dominate the radiometric properties of a debris system. This latter behavior agrees generally with what we know about the evolution of the solar system, and also with theoretical models of planetary system formation.
The Infrared Spectrograph (IRS) is one of three science instruments on the Spitzer Space Telescope. The IRS comprises four separate spectrograph modules covering the wavelength range from 5.3 to 38 μm with spectral resolutions, R = λ/Δλ ≈ 90 and 600, and it was optimized to take full advantage of the very low background in the space environment. The IRS is performing at or better than the prelaunch predictions. An autonomous target acquisition capability enables the IRS to locate the mid-infrared centroid of a source, providing the information so that the spacecraft can accurately offset that centroid to a selected slit. This feature is particularly useful when taking spectra of sources with poorly known coordinates. An automated data-reduction pipeline has been developed at the Spitzer Science Center.
The Multiband Imaging Photometer for Spitzer (MIPS) provides long-wavelength capability for the mission in imaging bands at 24, 70, and 160 μm and measurements of spectral energy distributions between 52 and 100 μm at a spectral resolution of about 7%. By using true detector arrays in each band, it provides both critical sampling of the Spitzer point-spread function and relatively large imaging fields of view, allowing for substantial advances in sensitivity, angular resolution, and efficiency of areal coverage compared with previous space far-infrared capabilities. The 24 μm array has excellent photometric properties, and measurements with rms relative errors of about 1% can be obtained. The two longer-wavelength arrays use detectors with poor photometric stability, but a system of onboard stimulators used for relative calibration, combined with a unique data pipeline, produce good photometry with rms relative errors of less than 10%.
We have analyzed Spitzer and NASA/IRTF 2-35 μm spectra of the warm, ~350 K circumstellar dust around the nearby MS star η Corvi (F2V, 1.4 ± 0.3 Gyr). The spectra show clear evidence for warm, water- and carbon-rich dust at ~3 AU from the central star, in the system's terrestrial habitability zone. Spectral features due to ultra-primitive cometary material were found, in addition to features due to impact produced silica and high-temperature carbonaceous phases. At least 9 × 1018 kg of 0.1-100 μm warm dust is present in a collisional equilibrium distribution with dn/da ~ a –3.5, the equivalent of a 130 km radius Kuiper Belt object (KBO) of 1.0 g cm3 density and similar to recent estimates of the mass delivered to the Earth at 0.6-0.8 Gyr during the late-heavy bombardment. We conclude that the parent body was a Kuiper Belt body or bodies which captured a large amount of early primitive material in the first megayears of the system's lifetime and preserved it in deep freeze at ~150 AU. At ~1.4 Gyr they were prompted by dynamical stirring of their parent Kuiper Belt into spiraling into the inner system, eventually colliding at 5-10 km s–1 with a rocky planetary body of mass ≤M Earth at ~3 AU, delivering large amounts of water (>0.1% of M Earth 's Oceans) and carbon-rich material. The Spitzer spectrum also closely matches spectra reported for the Ureilite meteorites of the Sudan Almahata Sitta fall in 2008, suggesting that one of the Ureilite parent bodies was a KBO.
We report 24 and/or 70 μm measurements of ~160 A-type main-sequence stars using the Multiband Imaging Photometer for Spitzer (MIPS). Their ages range from 5 to 850 Myr, based on estimates from the literature (cluster or moving group associations) or from the H-R diagram and isochrones. The thermal infrared excess is identified by comparing the deviation (~3% and ~15% at the 1 σ level at 24 and 70 μm, respectively) between the measurements and the synthetic Kurucz photospheric predictions. Stars showing excess infrared emission due to strong emission lines or extended nebulosity seen at 24 μm are excluded from our sample; therefore, the remaining infrared excesses are likely to arise from circumstellar debris disks. At the 3 σ confidence level, the excess rate at 24 and 70 μm is 32% and ≥33% (with an uncertainty of 5%), considerably higher than what has been found for old solar analogs and M dwarfs. Our measurements place constraints on the fractional dust luminosities and temperatures in the disks. We find that older stars tend to have lower fractional dust luminosity than younger ones. While the fractional luminosity from the excess infrared emission follows a general 1/t relationship, the values at a given stellar age vary by at least 2 orders of magnitude. We also find that (1) older stars possess a narrow range of temperature distribution peaking at colder temperatures, and (2) the disk emission at 70 μm persists longer than that at 24 μm. Both results suggest that the debris disk clearing process is more effective in the inner regions.
The zodiacal cloud is a thick circumsolar disk of small debris particles produced by asteroid collisions and comets. Their relative contribution and how particles of different sizes dynamically evolve to produce the observed phenomena of light scattering, thermal emission, and meteoroid impacts are unknown. Until now, zodiacal cloud models have been phenomenological in nature, composed of ad hoc components with properties not understood from basic physical processes. Here, we present a zodiacal cloud model based on the orbital properties and lifetimes of comets and asteroids, and on the dynamical evolution of dust after ejection. The model is quantitatively constrained by Infrared Astronomical Satellite (IRAS) observations of thermal emission, but also qualitatively consistent with other zodiacal cloud observations, with meteor observations, with spacecraft impact experiments, and with properties of recovered micrometeorites (MMs). We find that particles produced by Jupiter-family comets (JFCs) are scattered by Jupiter before they are able to orbitally decouple from the planet and drift down to 1 AU. Therefore, the inclination distribution of JFC particles is broader than that of their source comets and leads to good fits to the broad latitudinal distribution of fluxes observed by IRAS. We find that 85%-95% of the observed mid-infrared emission is produced by particles from JFCs and <10% by dust from long-period comets. The JFC particles that contribute to the observed cross section area of the zodiacal cloud are typically D 100 μm in diameter. Asteroidal dust is found to be present at <10%. We suggest that spontaneous disruptions of JFCs, rather than the usual cometary activity driven by sublimating volatiles, is the main mechanism that liberates cometary particles into the zodiacal cloud. The ejected mm to cm-sized particles, which may constitute the basic grain size in comets, are disrupted on 10,000 yr to produce the 10-1000 μm grains that dominate the thermal emission and mass influx. Breakup products with D > 100 μm undergo a further collisional cascade with smaller fragments being progressively more affected by Poynting-Robertson (PR) drag. Upon reaching D < 100 μm, the particles typically drift down to <1 AU without suffering further disruptions. The resulting Earth-impact speed and direction of JFC particles is a strong function of particle size. While 300 μm to 1 mm sporadic meteoroids are still on eccentric JFC-like orbits and impact from antihelion/helion directions, which is consistent with the aperture radar observations, the 10-300 μm particles have their orbits circularized by PR drag, impact at low speeds, and are not detected by radar. Our results imply that JFC particles represent ~85% of the total mass influx at Earth. Since their atmospheric entry speeds are typically low (14.5 km s–1 mean for D = 100-200 μm with 12 km s–1 being the most common case), many JFC grains should survive frictional heating and land on Earth's surface. This explains why most MMs collected in antarctic ice have primitive carbonaceous composition. The present mass of the inner zodiacal cloud at <5 AU is estimated to be 1-2 × 1019 g, mainly in D = 100-200 μm particles. The inner zodiacal cloud should have been >104 times brighter during the Late Heavy Bombardment (LHB) epoch 3.8 Gyr ago, when the outer planets scattered numerous comets into the inner solar system. The bright debris disks with a large 24 μm excess observed around mature stars may be an indication of massive cometary populations existing in those systems. We estimate that at least ~1022, ~2 × 1021, and ~2 × 1020 g of primitive dark dust material could have been accreted during LHB by the Earth, Mars, and Moon, respectively.
We compare the properties of warm dust emission from a sample of main-sequence A-type stars (B8-A7) to those of dust around solar-type stars (F5-K0) with similar Spitzer Space Telescope Infrared Spectrograph/MIPS data and similar ages. Both samples include stars with sources with infrared spectral energy distributions that show evidence of multiple components. Over the range of stellar types considered, we obtain nearly the same characteristic dust temperatures (~190 K and ~60 K for the inner and outer dust components, respectively)—slightly above the ice evaporation temperature for the inner belts. The warm inner dust temperature is readily explained if populations of small grains are being released by sublimation of ice from icy planetesimals. Evaporation of low-eccentricity icy bodies at ~150 K can deposit particles into an inner/warm belt, where the small grains are heated to T dust~ 190 K. Alternatively, enhanced collisional processing of an asteroid belt-like system of parent planetesimals just interior to the snow line may account for the observed uniformity in dust temperature. The similarity in temperature of the warmer dust across our B8-K0 stellar sample strongly suggests that dust-producing planetesimals are not found at similar radial locations around all stars, but that dust production is favored at a characteristic temperature horizon.
The Washington Double Star Catalog (WDS), maintained by the US Naval Observatory, is the world's principal database of astrometric double and multiple star information. The WDS contains positions (J2000), discoverer designations, epochs, position angles, separations, magnitudes, spectral types, proper motions, and, when available, Durchmusterung numbers and notes for the components of 84,486 systems based on 563,326 means. The current version, available on-line, is updated nightly. This catalog is one of four USNO double star catalogs to be included on a new CD-ROM. A brief summary and statistical analysis of the contents of the catalog are presented.
We present the Spitzer/Infrared Spectrograph spectrum of the main-sequence star HD165014, which is a warm (200 K) debris disk candidate discovered by the AKARI All-Sky Survey. The star possesses extremely large excess emission at wavelengths longer than 5 μm. The detected flux densities at 10 and 20 μm are ~10 and ~30 times larger than the predicted photospheric emission, respectively. The excess emission is attributable to the presence of circumstellar warm dust. The dust temperature is estimated as 300-750 K, corresponding to the distance of 0.7-4.4 AU from the central star. Significant fine-structured features are seen in the spectrum and the peak positions are in good agreement with those of crystalline enstatite. Features of crystalline forsterite are not significantly seen. HD165014 is the first debris disk sample that has enstatite as a dominant form of crystalline silicate rather than forsterite. Possible formation of enstatite dust from differentiated parent bodies is suggested according to the solar system analog. The detection of an enstatite-rich debris disk in the current study suggests the presence of large bodies and a variety of silicate dust processing in warm debris disks.
We have obtained Spitzer Space Telescope Infrared Spectrograph (IRS) 5.5-35 μm spectra of 59 main-sequence stars that possess IRAS 60 μm excess. The spectra of five objects possess spectral features that are well-modeled using micron-sized grains and silicates with crystalline mass fractions 0%-80%, consistent with T Tauri and Herbig AeBe stars. With the exception of η Crv, these objects are young with ages ≤50 Myr. Our fits require the presence of a cool blackbody continuum, Tgr = 80-200 K, in addition to hot, amorphous, and crystalline silicates, Tgr = 290-600 K, suggesting that multiple parent body belts are present in some debris disks, analogous to the asteroid and Kuiper belts in our solar system. The spectra for the majority of objects are featureless, suggesting that the emitting grains probably have radii a > 10 μm. We have modeled the excess continua using a continuous disk with a uniform surface density distribution, expected if Poynting-Robertson and stellar wind drag are the dominant grain removal processes, and using a single-temperature blackbody, expected if the dust is located in a narrow ring around the star. The IRS spectra of many objects are better modeled with a single-temperature blackbody, suggesting that the disks possess inner holes. The distribution of grain temperatures, based on our blackbody fits, peaks at Tgr = 110-120 K. Since the timescale for ice sublimation of micron-sized grains with Tgr > 110 K is a fraction of a Myr, the lack of warmer material may be explained if the grains are icy. If planets dynamically clear the central portions of debris disks, then the frequency of planets around other stars is probably high. We estimate that the majority of debris disk systems possess parent body masses, MPB < 1 M⊕. The low inferred parent body masses suggest that planet formation is an efficient process.
During the Spitzer Space Telescope cryogenic mission, Guaranteed Time Observers, Legacy Teams, and General Observers obtained Infrared Spectrograph (IRS) observations of hundreds of debris disk candidates. We calibrated the spectra of 571 candidates, including 64 new IRAS and Multiband Imaging Photometer for Spitzer (MIPS) debris disks candidates, modeled their stellar photospheres, and produced a catalog of excess spectra for unresolved debris disks. For 499 targets with IRS excess but without strong spectral features (and a subset of 420 targets with additional MIPS 70 μm observations), we modeled the IRS (and MIPS data) assuming that the dust thermal emission was well-described using either a one- or two-temperature blackbody model. We calculated the probability for each model and computed the average probability to select among models. We found that the spectral energy distributions for the majority of objects (~66%) were better described using a two-temperature model with warm (T
gr ~ 100-500 K) and cold (T
gr ~ 50-150 K) dust populations analogous to zodiacal and Kuiper Belt dust, suggesting that planetary systems are common in debris disks and zodiacal dust is common around host stars with ages up to ~1 Gyr. We found that younger stars generally have disks with larger fractional infrared luminosities and higher grain temperatures and that higher-mass stars have disks with higher grain temperatures. We show that the increasing distance of dust around debris disks is inconsistent with self-stirred disk models, expected if these systems possess planets at 30-150 AU. Finally, we illustrate how observations of debris disks may be used to constrain the radial dependence of material in the minimum mass solar nebula.
Main sequence stars, like the Sun, are often found to be orbited by
circumstellar material that can be categorized into two groups, planets and
debris. The latter is made up of asteroids and comets, as well as the dust and
gas derived from them, which makes debris disks observable in thermal emission
or scattered light. These disks may persist over Gyrs through steady-state
evolution and/or may also experience sporadic stirring and major collisional
breakups, rendering them atypically bright for brief periods of time. Most
interestingly, they provide direct evidence that the physical processes
(whatever they may be) that act to build large oligarchs from micron-sized dust
grains in protoplanetary disks have been successful in a given system, at least
to the extent of building up a significant planetesimal population comparable
to that seen in the Solar System's asteroid and Kuiper belts. Such systems are
prime candidates to host even larger planetary bodies as well. The recent
growth in interest in debris disks has been driven by observational work that
has provided statistics, resolved images, detection of gas in debris disks, and
discoveries of new classes of objects. The interpretation of this vast and
expanding dataset has necessitated significant advances in debris disk theory,
notably in the physics of dust produced in collisional cascades and in the
interaction of debris with planets. Application of this theory has led to the
realization that such observations provide a powerful diagnostic that can be
used not only to refine our understanding of debris disk physics, but also to
challenge our understanding of how planetary systems form and evolve.
We present new MIDI interferometric and VISIR spectroscopic observations
of HD 113766 and HD 172555. Additionally, we present VISIR 11-?m and
18-?m imaging observations of HD 113766. These sources represent the
youngest (16 and 12 Myr old, respectively) debris disc hosts with
emission on ≪10 au scales. We find that the disc of HD 113766 is
partially resolved on baselines of 42-102 m, with variations in
resolution with baseline length consistent with a Gaussian model for the
disc with a full width at half-maximum (FWHM) of 1.2-1.6 au (9-12 mas).
This is consistent with the VISIR observations which place an upper
limit of 0.14 arcsec (17 au) on the emission, with no evidence for
extended emission at larger distances. For HD 172555, the MIDI
observations are consistent with complete resolution of the disc
emission on all baselines of lengths 56-93 m, putting the dust at a
distance of >1 au (>35 mas). When combined with limits from TReCS
imaging, the dust at ˜10 ?m is constrained to lie somewhere in the
region of 1-8 au. Observations at ˜18 ?m reveal extended disc
emission which could originate from the outer edge of a broad disc, the
inner parts of which are also detected but not resolved at 10 ?m, or
from a spatially distinct component. These observations provide the most
accurate direct measurements of the location of the dust at 1-8 au that
might originate from the collisions expected during terrestrial planet
formation. These observations provide valuable constraints for models of
the composition of discs at this epoch and provide a foundation for
future studies to examine in more detail the morphology of debris discs.
Significance
A major question is whether planets suitable for biochemistry are common or rare in the universe. Small rocky planets with liquid water enjoy key ingredients for biology. We used the National Aeronautics and Space Administration Kepler telescope to survey 42,000 Sun-like stars for periodic dimmings that occur when a planet crosses in front of its host star. We found 603 planets, 10 of which are Earth size and orbit in the habitable zone, where conditions permit surface liquid water. We measured the detectability of these planets by injecting synthetic planet-caused dimmings into Kepler brightness measurements. We find that 22% of Sun-like stars harbor Earth-size planets orbiting in their habitable zones. The nearest such planet may be within 12 light-years.
Cold debris disks trace the limits of planet formation or migration in the
outer regions of planetary systems, and thus have the potential to answer many
of the outstanding questions in wide-orbit planet formation and evolution. We
characterized the infrared excess spectral energy distributions of 174 cold
debris disks around 546 main-sequence stars observed by both Spitzer IRS and
MIPS. We found a trend between the temperature of the inner edges of cold
debris disks and the stellar type of the stars they orbit. This argues against
the importance of strictly temperature-dependent processes (e.g. non-water ice
lines) in setting the dimensions of cold debris disks. Also, we found no
evidence that delayed stirring causes the trend. The trend may result from
outward planet migration that traces the extent of the primordial
protoplanetary disk, or it may result from planet formation that halts at an
orbital radius limited by the efficiency of core accretion.
We present a new high-mass membership of the nearby Sco OB2 association based
on HIPPARCOS positions, proper motions and parallaxes and radial velocities
taken from the Kharchenko et al. (2007) catalogue. The Bayesian membership
selection method developed makes no distinction between subgroups of Sco OB2
and utilises linear models in calculation of membership probabilities. We
select 436 members, 88 of which are new members not included in previous
membership selections. We include the classical non members Alpha-Cru and
Beta-Cru as new members as well as the pre-main-sequence stars HIP 79080 and
79081. We also show that the association is well mixed over distances of 8
degrees on the sky, and hence no determination can be made as to the formation
process of the entire association.
(Abridged) Studies of debris discs have shown that most systems are analogous
to the EKB. In this study we aim to determine how many IRAS 25um excesses
towards A stars are real, and investigate where the dust lies. We observe with
TIMMI2, VISIR, Michelle and TReCS a sample of A and B-type main sequence stars
reported as having mid-IR excess. We constrain the location of the debris
through combined modelling of the emission spectrum and a modelling technique
designed to constrain the radial extent of emission in mid-IR imaging. We
independently confirm the presence of warm dust around 3 of the candidates:
HD3003, HD80950 and eta Tel. For the binary HD3003 a stability analysis
indicates the dust is either circumstellar and lying at ~4 AU with the binary
orbiting at >14AU, or the dust lies in an unstable location; there is some
evidence for temporal evolution of its excess emission on a ~20 year timescale.
For 7 of the targets we present quantitative limits on the location of dust
around the star. We demonstrate that the disc around HD71155 must have
spatially distinct components at 2 and 60AU. We model the limits of current
instrumentation and show that most of the known A star debris discs which could
be readily resolved at 18um on 8m instruments have been resolved. Limits from
unresolved imaging can help distinguish between competing models of the disc
emission, but resolved imaging is key to the determination of the disc
location. Modelling of the detection limits for extended emission can be useful
for targeting future observational campaigns. MIRI on the JWST will be able to
resolve most of the known A star debris disc population. METIS on the E-ELT
will provide the opportunity to explore the hot disc population more thoroughly
by detecting extended emission where calibration accuracy limits disc detection
through photometry, reaching levels below 1 zodi for stars at <10pc.
1) Introduction. Cox 2)General Constants and Units. Cox 3) Atoms and Molecules. Dappen 4) Spectra. Cowley, et al 5) Radiation. Keady & Kilcrease 6) Radio and Microwave Astronomy. Hjellming 7) Infrared Astronomy. Tokunaga 8) Ultraviolet Astronomy. Teays 9) X-Ray Astronomy. Seward 10) Gamma-Ray and Neutrino Astronomy. Lingenfelter & Rothschild 11) Earth. Schubert & Walterscheid 12) Planets and Satellites. Tholen 13) Solar System Small Bodies. Binzel, et al 14) Sun. Livingston 15) Normal Stars. Drilling & Landolt 16) Stars with Special Characteristics. Fernie 17) Cataclysmic and Symbiotic Variables. Sparks, et al. 18) Supernovae. Wheeler & Bennetti 19) Star Populations and the Solar Neighborhood. Gilmore & Zeilik 20) Theoretical Stellar Evolution. Becker/Pensell/Cox 21) Circumstellar and Interstellar Material. Mathis 22) Star Clusters. Harris & Harris 23) Milky Way Galaxies. Trimble 24) Quasars and Active Galactic Nuclei. Wilkes 25) Clusters and Groups of Galaxies. Bahcall 26) Cosmology. Scott, et al 27) Incidental Tables. Fiala, et al.
We have used the Keck I telescope to image at 11.7 and 17.9 mm the dust emission around z Leporis, a main- sequence A-type star at 21.5 pc from the Sun with an infrared excess. The excess is at most marginally resolved at 17.9 mm. The dust distance from the star is probably ≤6 AU, although some dust may extend to 9 AU. The mass of observed dust is ∼1022 g. Since the lifetime of dust particles is about 10 4 yr because of the Poynting- Robertson effect, we robustly estimate at least Vega-like systems, first discovered by IRAS, are main- sequence stars surrounded by dust. The operation of the Poynt- ing-Robertson effect on micron-sized grains, responsible for much of the observed infrared excess, requires that the lifetime of these particles be significantly shorter than the age of the stars (Backman & Paresce 1993). Thus, the dust grains must be re- plenished from a reservoir such as collisions between larger bodies or the sublimation of comets. Studies of Vega-like systems thus afford the opportunity to study the evolution of large solids such as planets, comets, and asteroids. The identification of dust around main-sequence A-type stars is usually made from the IRAS colors (e.g., Mannings & Barlow 1998; Silverstone 2000). Identifying objects with 12 mm excess is difficult because the photosphere usually dominates the total flux at this wavelength. The bulk of the dust associated with main-sequence A-type stars typically has temperatures T ∼ gr K and semimajor axes greater than 50 AU. For example,
The Large Binocular Telescope
Interferometer (LBTI) is designed to probe nearby stars for dust disks, indicative of planetesimal belts, as well as giant planets in wide orbits (>3 AU). The instrument provides a unique combination of resolution and sensitivity in the thermal infrared to achieve these objectives. Both of these programs will provide important insights into the architecture for other planetary systems. The discovery space of the LBTI is presented, as well as recent observational results from prototype instruments now being carried out at the MMT Observatory.
"An introduction to modern astrophysics, 2nd Edition" has been
thoroughly revised to reflect the dramatic changes and advancements in
astrophysics that have occurred over the past decade. This book has been
updated to include the latest results from relevant fields of
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astrophysical phenomena.
We have obtained Spitzer IRS 5.5-35 μm spectroscopy of the debris disk around β Pictoris. In addition to the 10 μm silicate emission feature originally observed from the ground, we also detect the crystalline silicate emission bands at 28 and 33.5 μm. This is the first time that the silicate bands at wavelengths longer than 10 μm have ever been seen in the β Pictoris disk. The observed dust emission is well reproduced by a dust model consisting of fluffy cometary and crystalline olivine aggregates. We searched for line emission from molecular hydrogen and atomic [S I], Fe II, and Si II gas but detected none. We place a 3 σ upper limit of <17 M⊕ on the H2 S(1) gas mass, assuming an excitation temperature of Tex = 100 K. This suggests that there is less gas in this system than is required to form the envelope of Jupiter. We hypothesize that some of the atomic Na I gas observed in Keplerian rotation around β Pictoris may be produced by photon-stimulated desorption from circumstellar dust grains.
We present the spectral atlas of sources observed in low resolution with the Infrared Spectrograph on board the Spitzer
Space Telescope. More than 11,000 distinct sources were extracted using a dedicated algorithm based on the SMART software with an optimal extraction (AdOpt package). These correspond to all 13,000 low-resolution observations of fixed objects (both single source and cluster observations). The pipeline includes image cleaning, individual exposure combination, and background subtraction. Particular attention is given to bad pixel and outlier rejection at the image and spectra levels. Most sources are spatially unresolved so that optimal extraction reaches the highest possible signal-to-noise ratio. For all sources, an alternative extraction is also provided that accounts for all of the source flux within the aperture. CASSIS provides publishable quality spectra through an online database together with several important diagnostics, such as the source spatial extent and a quantitative measure of detection level. Ancillary data such as available spectroscopic redshifts are also provided. The database interface will eventually provide various ways to interact with the spectra, such as on-the-fly measurements of spectral features or comparisons among spectra.
We have used the infrared mineralogical model derived from the Spitzer IRS observations of the Deep Impact experiment to study the nature of the dust in the debris found around the K0 V star HD 69830. Using a robust approach to determine the bulk average mineralogical composition of the dust, we show it to be substantially different from that found for comets 9P/Tempel 1 and C/Hale-Bopp 1995 O1 or for the comet-dominated YSO HD 100546. Lacking in carbonaceous and ferrous materials but including small icy grains, the composition of the HD 69830 dust most closely resembles that of a disrupted P- or D-type asteroid. The amount of mass responsible for the observed emission is the equivalent of a 30 km radius, 2500 kg m-3 sphere, while the radiative temperature of the dust implies that the bulk of the observed material is at ~1.0 AU from the central source, coincident with the 2 : 1 and 5 : 2 mean motion resonances of the outermost of three Neptune-sized planets detected by Lovis and coworkers. In our solar system, P- and D-type asteroids are both large and numerous in the outer main belt and near Jupiter (e.g., the Hildas and Trojans) and have undergone major disruptive events to produce debris disk-like structures (cf. the Karin and Veritas families 5-8 Myr ago). The short-lived nature of the small and icy dust implies that the disruption occurred within the last year, or that replenishment due to ongoing collisional fragmentation is occurring.
We have observed the 8-13 μm spectrum (R ~ 250) of the Vega-like star candidate HD 145263 using Subaru/COMICS. The spectrum of HD 145263 shows the broad trapezoidal silicate feature with the shoulders at 9.3 and 11.44 μm, indicating the presence of crystalline silicate grains. This detection implies that crystalline silicate may also be commonly present around Vega-like stars. The 11.44 μm feature is slightly shifted to a longer wavelength compared to the usual 11.2-3 μm crystalline forsterite feature detected toward Herbig Ae/Be stars and T Tauri stars. Although the peak shift due to the effects of the grain size cannot be ruled out, we suggest that Fe-bearing crystalline olivine explains the observed peak wavelength fairly well. Fe-bearing silicates are commonly found in meteorites and most interplanetary dust particles, which originate from planetesimal-like asteroids. According to studies of meteorites, Fe-bearing silicate must have been formed in asteroidal planetesimals, supporting the scenario that dust grains around Vega-like stars are of planetesimal origin, if the observed 11.44 μm peak is due to Fe-bearing silicates.
We obtained small-aperture (4''–5'' diameter) infrared (2–20 μm) photometry of 10 early-type main-sequence stars with infrared excesses from circumstellar dust. These systems possibly exemplify the β Pictoris phenomenon. We observed them with either the NASA Marshall Space Flight Center bolometer array camera ("Big Mac") or the Infrared Telescope Facility 2–30 μm single-channel bolometer system. Measurements were obtained in the KLMNQ filters and the narrowband (Δλ ≈ 1 μm) 10 μm "silicate" filters. We fitted Kurucz photospheric models to the photometric data to determine excess-emission spectra. We report the nondetection of small-aperture circumstellar dust emission from HR 10 and 21 LMi. We confirmed previous nondetections of near-infrared or 10 μm excess emission from 68 Oph, α PsA, and HR 4796A. We did not detect prominent silicate emission from any of the sources. The spectra of γ Oph, σ Her, HR 2174A, β UMa, and ζ Lep show weak 10 μm excesses. We fitted simple models to these data, together with IRAS excess fluxes, to determine plausible distributions of temperature and density of circumstellar dust grains. Significant quantities of these grains around HR 2174A, ζ Lep, and β UMa are at temperatures similar to terrestrial material in the solar system.
Observations at 70 μm with the Spitzer Space Telescope have detected several stellar systems within 65 pc of the Sun. Of 18 presumably young systems detected in this study, as many as 15 have 70 μm emission in excess of that expected from their stellar photospheres. Five of the systems with excesses are members of the Tucanae association. The 70 μm excesses range from a factor of ~2 to nearly 30 times the expected photospheric emission from these stars. In contrast to the 70 μm properties of these systems, there is evidence for an emission excess at 24 μm for only HD 3003, confirming previous results for this star. The lack of a strong 24 μm excess in most of these systems suggests that the circumstellar dust producing the IR excesses is relatively cool (Tdust 150 K) and that there is little IR-emitting material within the inner few AU of the primary stars. Many of these systems lie close enough to Earth that the distribution of the dust producing the IR excesses might be imaged in scattered light at optical and near-IR wavelengths.
Vega-like sources are main-sequence stars that exhibit IR fluxes in excess of expectations for stellar photospheres, most likely due to reradiation of stellar emission intercepted by orbiting dust grains. We have identified a large sample of main-sequence stars with possible excess IR radiation by cross-correlating the Michigan Catalog of Two-dimensional Spectral Types for the HD Stars with the IRAS Faint Source Survey Catalog. Some 60 of these Vega-like sources were not found during previous surveys of the IRAS database, the majority of which employed the lower sensitivity Point Source Catalog. Here, we provide details of our search strategy, together with a preliminary examination of the full sample of Vega-like sources.
Spitzer photometry and spectroscopy of the star HD 69830 reveal an excess of emission relative to the stellar photosphere between 8 and 35 μm dominated by strong features attributable to crystalline silicates with an emitting surface area more than 1000 times that of our zodiacal cloud. The spectrum closely resembles that of the comet C/1995 O1 (Hale-Bopp). Since no excess is detected at 70 μm, the emitting material must be quite warm, be confined within a few AU of the star, and originate in grains with low, long-wavelength emissivity, i.e., grains much smaller than 70 μm/2π ~ 10 μm. The strong mineralogical features are evidence for even smaller, possibly submicron-sized grains. This small grain size is in direct contrast to the 10-100 μm grains that dominate the relatively featureless spectra of our zodiacal dust cloud and most other main-sequence stars with excesses. The upper limit at 70 μm also implies that any Kuiper Belt analog must be either very cold or less massive than ~5 times our own Kuiper Belt. With collisional and Poynting-Robertson drag times of less than 1000 yr for small grains, the emitting material must either (1) be created through continual grinding down of material in a dense asteroid belt, or (2) originate in cometary debris arising from either a single "supercomet" or a very large number of individual comets arriving from a distant reservoir. In the case of a cometary origin for the emission, the mass requirements for continuous generation by many individual comets are unreasonable, and we favor the capture of a single super comet into a 0.5-1 AU orbit, where it can evolve a large number of small grains over a 2 Myr period.
We have conducted deep mid-infrared imaging of a relatively nearby sample of candidate Vega-like stars using the OSCIR instrument on the Cerro Tololo Inter-American Observatory 4 m and Keck II 10 m telescopes. Our discovery of a spatially resolved disk around HR 4796A has already been reported in 1998 by Jayawardhana et al. Here we present imaging observations of the other members of the sample, including the discovery that only the primary in the HD 35187 binary system appears to harbor a substantial circumstellar disk and the possible detection of extended disk emission around 49 Ceti. We derive global properties of the dust disks, place constraints on their sizes, and discuss several interesting cases in detail. Although our targets are believed to be main-sequence stars, we note that several have large infrared excesses compared with prototype Vega-like systems and may therefore be somewhat younger. The disk size constraints we derive, in many cases, imply emission from relatively large (10 μm) particles at mid-infrared wavelengths.