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Abstract
Planets develop from the disk of dust and gas that surrounds a newly formed star. Observations of gaps in the disks of four such systems have allowed us to start unravelling the processes by which planets form.
... New ALMA observations focusing on CO-rotational lines have allowed to determine the gas content in the inner disk region in more detail. These results show that the inner disk gas depleted by factors of about 10 2 (van der Marel et al. 2015) or even to a factor of 10 4 with gas holes about a factor 2-3 smaller than the dust gaps (van der Marel et al. 2016) which is taken as another example of massive planets in disks (Ho 2016). The conclusion that all or the majority of Type II TDs are shaped by massive planets has been questioned by Dong & Dawson (2016) who argue that there may not be enough giant planets to explain all observed Type II TDs, see also Cumming et al. (2008) for the occurrence rate of massive planets at larger separations. ...
Transition disks form a special class of protoplanetary disks that are characterized by a deficiency of disk material close to the star. In a subgroup, inner holes in these disks can stretch out to a few tens of au while there is still mass accretion onto the central star observed. We analyse the proposition that this type of wide transition disks is generated by the interaction of the disk with a system of embedded planets. We performed 2D hydrodynamics simulations of a flat disk using either a locally isothermal equation of state or considering also radiative effects. Two 3 to 9 Jupiter mass planets were embedded in the disk and their dynamical evolution due to disk-planet interaction was followed for over 100 000 years. The simulations account for mass accretion onto the star and planets. We included models with parameters geared to the system PDS 70. To assess the observability of features in our models we performed synthetic ALMA observations. For systems with a more massive inner planet there are phases where both planets migrate outward engaged in a 2:1 mean motion resonance via the Masset-Snellgrove mechanism. In sufficiently massive disks the formation of a vortex in the outer disk can trigger rapid outward migration of the outer planet where its distance increases by tens of au within a few thousand years. Later, the outer planet migrates back inwards settling again into resonance with the inner planet. We call this emerging composite phenomenon a 'migration jump'. Outward migration and the migration jumps are accompanied by a high mass accretion rate onto the star. The synthetic images reveal numerous substructures depending on the type of dynamical behaviour. Our results suggest that the outward migration of two embedded planets is a prime candidate for the explanation of the observed high stellar mass accretion rate in wide transition disks.
... Planets are born from protoplanetary disks. Observations of protoplanetary disks provide us with clues concerning the formation and evolution of planets (e.g., Williams & Cieza 2011;Isella et al. 2013;Akiyama et al. 2015;Ho 2016;van der Marel et al. 2016). Recently, ALMA Partnership et al. (2015) provided the first images of the protoplanetary disk around the young star HL Tauri (hereafter HL Tau) by observing the millimeter and submillimeter continuum emission with the Atacama Large Millimeter/ submillimeter Array (ALMA). ...
Planet formation and photoevaporation have both been considered as gap opening mechanisms in protoplanetary disks. We have studied giant planet formation in a photoevaporating disk with long-term evolution. Our calculations suggest that the core accretion rate of a protoplanet declines and the trigger of the runaway gas accretion for a giant planet is delayed under the action of photoevaporation. We find that the final mass of a giant planet characterized by the "gap-limiting" case is not influenced by photoevaporation but the final mass of a giant planet characterized by the "diffusion-limiting" case is greatly influenced by photoevaporation. Considering the formation process of giant planets, we suggest that the locations of the gaps opened by giant planets are within 30–40 au and the gap width in the "gap-limiting" case is wider than that in the "diffusion-limiting" case. We also find that gaps in photoevaporating disks are wider than those in non-photoevaporating disks. Our calculations suggest that the origins of multiple gaps in a disk can be diverse depending on their formation locations. In the formation region of giant planets, gaps are opened by giant planets. The outer gap beyond the giant planet formation region may be opened under the action of photoevaporation. A gap may also be opened at 1–3 au under the actions of photoevaporating dissipation and gas accretion of the outer giant planets.
Context. Transition discs form a special class of protoplanetary discs that are characterised by a deficiency of disc material close to the star. In a subgroup, inner holes in these discs can stretch out to a few tens of au while there is still mass accretion onto the central star observed at the same time.
Aims. We analyse the proposition that this type of wide transition disc is generated by the interaction of the disc with a system of embedded planets.
Methods. We performed two-dimensional hydrodynamics simulations of a flat disc. Different equations of state were used including locally isothermal models and more realistic cases that consider viscous heating, radiative cooling, and stellar heating. Two massive planets (with masses of between three and nine Jupiter masses) were embedded in the disc and their dynamical evolution due to disc–planet interaction was followed for over 100 000 yr. The simulations account for mass accretion onto the star and planets. We included models with parameters reminiscent of the system PDS 70. To assess the observability of features in our models we performed synthetic ALMA observations.
Results. For systems with a more massive inner planet, there are phases where both planets migrate outward engaged in a 2:1 mean motion resonance via the Masset-Snellgrove mechanism. In sufficiently massive discs, the resulting formation of a vortex and the interaction with it can trigger rapid outward migration of the outer planet where its distance can increase by tens of au in a few thousand years. After another few thousand years, the outer planet rapidly migrates back inwards into resonance with the inner planet. We call this emerging composite phenomenon a migration jump. Outward migration and the migration jumps are accompanied by a high mass accretion rate onto the star. The synthetic images reveal numerous substructures depending on the type of dynamical behaviour.
Conclusions. Our results suggest that the outward migration of two embedded planets is a prime candidate for the explanation of the observed high stellar mass accretion rate in wide transition discs. The models for PDS 70 indicate it is not currently undergoing a migration jump but might very well be in a phase of outward migration.
Transitional disks around young stars are promising candidates to look for
recently formed, embedded planets. Planet-disk interaction models predict that
planets clear a gap in the gas while trapping dust at larger radii. Other
physical mechanisms could be responsible for cavities as well. Previous
observations have revealed that gas is still present inside these cavities, but
the spatial distribution of this gas remains uncertain. We present high spatial
resolution observations with the Atacama Large Millimeter/submillimeter Array
(ALMA) of 13CO and C18O lines of four well-studied transitional disks. The
observations are used to set constraints on the gas surface density,
specifically cavity size and density drop inside the cavity. The
physical-chemical model DALI is used to analyze the gas images of SR21,
HD135344B, DoAr44 and IRS48. The main parameters of interest are the size,
depth and shape of the gas cavity. CO isotope-selective photodissociation is
included to properly constrain the surface density in the outer disk from C18O
emission. The gas cavities are up to 3 times smaller than those of the dust in
all four disks. Model fits indicate that the surface density inside the gas
cavities decreases by a factor of 100-10000 compared with the surface density
profile derived from the outer disk. A comparison with an analytical model of
gap depths by planet-disk interaction shows that the disk viscosities are
likely low, with a<1E-3 for planet masses <10 MJup. The resolved measurements
of the gas and dust in transition disk cavities support the predictions of
models that describe how planet-disk interactions sculpt gas disk structures
and influence the evolution of dust grains. These observed structures strongly
suggest the presence of giant planetary companions in transition disk cavities,
although at smaller orbital radii than is typically indicated from the dust
cavity radii alone.
We present Atacama Large Millimeter/submillimeter Array (ALMA) observations from the 2014 Long Baseline Campaign in dust continuum and spectral line emission from the HL Tau region. The continuum images at wavelengths of 2.9, 1.3, and 0.87 mm have unprecedented angular resolutions of 0 ''.075 (10 AU) to 0 ''.025 (3.5 AU), revealing an astonishing level of detail in the circumstellar disk surrounding the young solar analog HL Tau, with a pattern of bright and dark rings observed at all wavelengths. By fitting ellipses to the most distinct rings, we measure precise values for the disk inclination (46 degrees.72 +/- 0 degrees.05) and position angle (+138 degrees.02 +/- 0 degrees.07).We obtain a high-fidelity image of the 1.0 mm spectral index (alpha), which ranges from alpha similar to 2.0 in the optically thick central peak and two brightest rings, increasing to 2.3-3.0 in the dark rings. The dark rings are not devoid of emission, and we estimate a grain emissivity index of 0.8 for the innermost dark ring and lower for subsequent dark rings, consistent with some degree of grain growth and evolution. Additional clues that the rings arise from planet formation include an increase in their central offsets with radius and the presence of numerous orbital resonances. At a resolution of 35 AU, we resolve the molecular component of the disk in HCO+. (1-0) which exhibits a pattern over LSR velocities from 2-12 km s(-1) consistent with Keplerian motion around a similar to 1.3 M-circle dot star, although complicated by absorption at low blueshifted velocities. We also serendipitously detect and resolve the nearby protostars XZ Tau (A/B) and LkH alpha 358 at 2.9 mm.
The disks that surround young stars are mostly composed of molecular gas, which is harder to detect and interpret than the accompanying dust. Disk mass measurements have therefore relied on large and uncertain extrapolations from the dust to the gas. We have developed a grid of models to study the dependencies of isotopologue CO line strengths on disk structure and temperature parameters and find that a combination of 13CO and C18O observations provides a robust measure of the gas mass. We apply this technique to Submillimeter Array observations of nine circumstellar disks and published measurements of six well studied disks. We find evidence for selective photodissociation of C18O and determine masses to within a factor of about three. The inferred masses for the nine disks in our survey range from 0.7 to 6 M
Jup, and all are well below the extrapolation from the interstellar medium gas-to-dust ratio of 100. This is consistent with the low masses of planets found around such stars, and may be due to accretion or photoevaporation of a dust-poor upper atmosphere. However, the masses may be underestimated if there are more efficient CO depletion pathways than those known in molecular clouds and cold cores.
Gap clearing by giant planets has been proposed to explain the optically thin cavities observed in many protoplanetary disks. How much material remains in the gap determines not only how detectable young planets are in their birth environments, but also how strong co-rotation torques are, which impacts how planets can survive fast orbital migration. We determine numerically how the average surface density inside the gap, Σgap, depends on planet-to-star mass ratio q, Shakura-Sunyaev viscosity parameter α, and disk height-to-radius aspect ratio h/r. Our results are derived from our new graphics processing unit accelerated Lagrangian hydrodynamical code PEnGUIn and are verified by independent simulations with ZEUS90. For Jupiter-like planets, we find Σgap∝q
–2.2α1.4(h/r)6.6, and for near brown dwarf masses, Σgap∝q
–1α1.3(h/r)6.1. Surface density contrasts inside and outside gaps can be as large as 104, even when the planet does not accrete. We derive a simple analytic scaling, Σgap∝q
–2α1(h/r)5, that compares reasonably well to empirical results, especially at low Neptune-like masses, and use discrepancies to highlight areas for progress.
Distinctive diffuse scattering in the form of diffuse rings around Bragg positions has been observed in the diffraction patterns of a crystal of the N-terminal fragment of the Gag protein from Feline Foamy Virus. It is shown that these are caused by geometric frustration as molecules try to pack on the triangular b–c mesh of the space group P6122. In order to explain the strong diffuse scattering it is necessary for the crystal to contain occupational disorder such that each unit cell contains one or other of two different molecular arrangements, A and B. The frustration arises because the nearest-neighbour packing prefers neighbouring cells to be AB or BA, which cannot be achieved on all three sides of a triangle simultaneously. To explain the observation that reciprocal sections hk5n, where n = integer, contain only Bragg peaks it is necessary that A and B are identical molecular arrangements differing only by a translation of 0.2c. The implications of the disorder for solving the structure of the protein by conventional techniques as well as the possibility of using the diffuse scattering for this purpose are discussed.
Several strategies in phase retrieval are unified by an iterative “difference map” constructed from a pair of elementary projections and three real parameters. For the standard application in optics, where the two projections implement Fourier modulus and object support constraints, respectively, the difference map reproduces the “hybrid” form of Fienup’s input–output map when a particular choice is made for two of the parameters. The geometric construction of the difference map illuminates the distinction between its fixed points and the recovered object, as well as the mechanism whereby the form of stagnation encountered by alternating projection schemes is avoided. When support constraints are replaced by object histogram or atomicity constraints, the difference map lends itself to crystallographic phase retrieval. Numerical experiments with synthetic data suggest that structures with hundreds of atoms can be solved.
The problem of reconstructing an object from diffraction data that has been incoherently averaged over a discrete group of symmetries is considered. A necessary condition for such data to uniquely specify the object is derived in terms of the object support and the symmetry group. An algorithm is introduced for reconstructing objects from symmetry-averaged data and its use with simulations is demonstrated. The results demonstrate the feasibility of structure determination using a recent proposal for aligning molecules by means of their anisotropic dielectric interaction with an intense light field [Spence et al. (2005). Acta Cryst. A61, 237–245].
We explain the axisymmetric gaps seen in recent long-baseline observations of
the HL Tau protoplanetary disc with the Atacama Large Millimetre/Submillimetre
Array (ALMA) as being due to the different response of gas and dust to embedded
planets in protoplanetary discs. We perform global, three dimensional dusty
smoothed particle hydrodynamics calculations of multiple planets embedded in
dust/gas discs which successfully reproduce most of the structures seen in the
ALMA image. We find a best match to the observations using three embedded
planets with masses of 0.2, 0.27 and 0.55 in the three main gaps
observed by ALMA, though there remain uncertainties in the exact planet masses
from the disc model.
X-ray diffuse scattering from protein crystals is, at the moment, the only available experimental process to be directly sensitive to long-range correlations between protein-atom displacements. It is shown here that calculations based on independent rigid-body displacements of individual molecules yield a description in good agreement with the experimental diffuse-scattering pattern displayed by tetragonal crystals of hen egg-white lysozyme (HEWL) In particular, it appears that molecular rigid-body translations and rigid- body rotations appear roughly in the same proportion as the average atomic mean-square positional fluctuations. The crystallographic temperature-factor analysis by TLS (translation/libration/screw) refinement, performed by Sternberg, Grace & Phillips [Sternberg, Grace, & Phillips (1979). J. Mol. Biol. 130, 231-253], is then confirmed and completed by a quantitative estimation of the molecular rigid-body translation contributions. The major contribution of molecular rigid- body displacements to the average atomic mean-square positional fluctuations, contradicts a previous analysis of the tetragonal HEWL diffuse-scattering data by Clarage, Clarage, Phillips, Sweet & Caspar [Clarage, Clarage, Phillips, Sweet & Caspar (1992). Proteins Struct. Funct. Genet. 12, 145-157] which concluded that short-range correlations dominate. The origin of these opposite conclusions mostly lies in the different hypotheses made to model diffuse scattering, underlying the limits of the 'homogeneous disorder' model.
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