71 research outputs found
Gaps in Protoplanetary Disks as Signatures of Planets: I. Methodology and Validation
We examine the observational consequences of partial gaps being opened by
planets in protoplanetary disks. We model the disk using a static alpha-disk
model with detailed radiative transfer, parametrizing the shape and size of the
partially cleared gaps based on the results of hydrodynamic simulations.
Shadowing and illumination by stellar irradiation at the surface of the gap
leads to increased contrast as the gap trough is deepened by shadowing and
cooling and the far gap wall is puffed up by illumination and heating. In
calculating observables, we find that multiple scattering is important and
derive an approximation to include these effects. A gap produced by a 200
M_Earth (70 M_Earth) planet at 10 AU can lower/raise the midplane temperature
of the disk by up to ~-25/+29% (~-11/+19%) by shadowing in the gap trough and
illumination on the far shoulder of the gap. At the distance of Taurus, this
gap would be resolvable with ~0.01" angular resolution. The gap contrast is
most significant in scattered light and at thermal continuum wavelengths
characteristic of the surface temperature, reducing or raising the surface
brightness by up to order of magnitude. Since gaps sizes are correlated to
planet mass, this is a promising way of finding and determining the masses of
planets embedded in protoplanetary disks.Comment: 11 pages, 9 figures. Accepted to Ap
Planet Shadows in Protoplanetary Disks. I: Temperature Perturbations
Planets embedded in optically thick passive accretion disks are expected to
produce perturbations in the density and temperature structure of the disk. We
calculate the magnitudes of these perturbations for a range of planet masses
and distances. The model predicts the formation of a shadow at the position of
the planet paired with a brightening just beyond the shadow. We improve on
previous work on the subject by self-consistently calculating the temperature
and density structures under the assumption of hydrostatic equilibrium and
taking the full three-dimensional shape of the disk into account rather than
assuming a plane-parallel disk. While the excursion in temperatures is less
than in previous models, the spatial size of the perturbation is larger. We
demonstrate that a self-consistent calculation of the density and temperature
structure of the disk has a large effect on the disk model. In addition, the
temperature structure in the disk is highly sensitive to the angle of incidence
of stellar irradition at the surface, so accurately calculating the shape of
the disk surface is crucial for modeling the thermal structure of the disk.Comment: 14 pages, 14 figures. To appear in Ap
Radiative Transfer on Perturbations in Protoplanetary Disks
We present a method for calculating the radiative tranfer on a protoplanetary
disk perturbed by a protoplanet. We apply this method to determine the effect
on the temperature structure within the photosphere of a passive circumstellar
disk in the vicinity of a small protoplanet of up to 20 Earth masses. The
gravitational potential of a protoplanet induces a compression of the disk
material near it, resulting in a decrement in the density at the disk's
surface. Thus, an isodensity contour at the height of the photosphere takes on
the shape of a well. When such a well is illuminated by stellar irradiation at
grazing incidence, it results in cooling in a shadowed region and heating in an
exposed region. For typical stellar and disk parameters relevant to the epoch
of planet formation, we find that the temperature variation due to a
protoplanet at 1 AU separation from its parent star is about 4% (5 K) for a
planet of 1 Earth mass, about 14% (19 K) for planet of 10 Earth masses, and
about 18% (25 K) for planet of 20 Earth masses, We conclude that even such
relatively small protoplanets can induce temperature variations in a passive
disk. Therefore, many of the processes involved in planet formation should not
be modeled with a locally isothermal equation of state.Comment: 23 pages, 8 figures (including 3 color figs). Submitted to Ap
Constraints on the Formation of the Planet Around HD188753A
The claimed discovery of a Jupiter-mass planet in the close triple star
system HD 188753 poses a problem for planet formation theory. A circumstellar
disk around the planet's parent star would be truncated close to the star,
leaving little material available for planet formation. In this paper, we
attempt to model a protoplanetary disk around HD 188753A using a fairly simple
alpha-disk model, exploring a range of parameters constrained by observations
of T Tauri-type stars. The disk is truncated to within 1.5 to 2.7 AU, depending
on model parameters. We find that the in situ formation of the planet around HD
188753A is implausible.Comment: Accepted version, to appear in ApJ. 23 pages, 5 figures (3 in color
Disk Temperature Variations and Effects on the Snow Line in the Presence of Small Protoplanets
We revisit the computation of a "snow line" in a passive protoplanetary disk
during the stage of planetesimal formation. We examine how shadowing and
illumination in the vicinity of a planet affects where in the disk ice can
form, making use of our method for calculating radiative transfer on disk
perturbations with some improvements on the model. We adopt a model for the
unperturbed disk structure that is more consistent with observations and use
opacities for reprocessed dust instead of interstellar medium dust. We use the
improved disk model to calculate the temperature variation for a range of
planet masses and distances and find that planets at the gap-opening threshold
can induce temperature variations of up to +/-30%. Temperature variations this
significant may have ramifications for planetary accretion rates and migration
rates. We discuss in particular the effect of temperature variations near the
sublimation point of water, since the formation of ice can enhance the
accretion rate of disk material onto a planet. Shadowing effects can cool the
disk enough that ice will form closer to the star than previously expected,
effectively moving the snow line inward.Comment: 28 pages, 14 figures, to appear in the Astrophysical Journa
Type I Migration in a Non-Isothermal Protoplanetary Disk
We calculate rates of Type I migration of protoplanets in a non-isothermal
three-dimensional protoplanetary disk, building upon planet-disk models
developed in previous work. We find that including the vertical thickness of
the disk results in a decrease in the Type I migration rate by a factor of ~2
from a two-dimensional disk. The vertical temperature variation has only a
modest effect on migration rates since the torques at the midplane are weighted
heavily both because the density and the perturbing potential are maximized at
the midplane. However, temperature perturbations resulting from shadowing and
illumination at the disk's surface can decrease the migration rate by up to
another factor of 2 for planets at the gap-opening threshold at distances where
viscous heating is minimal. This would help to resolve the timescale mismatch
between the standard core-accretion scenario for planet formation and the
survival of planets, and could help explain some of the rich diversity of
planetary systems already observed.Comment: 9 pages, 8 figures. Accepted to the Astrophysical Journa
Dust Rings and Cavities in the Protoplanetary Disks around HD 163296 and DoAr 44
We model substructure in the protoplanetary disks around DoAr 44 and HD
163296 in order to better understand the conditions under which planets may
form. We match archival millimeter-wavelength thermal emission against models
of the disks' structure that are in radiation balance with the starlight
heating and in vertical hydrostatic equilibrium, and then compare to archival
polarized scattered near-infrared images of the disks. The millimeter emission
arises in the interior, while the scattered near-infrared radiation probes the
disks' outer layers. Our best model of the HD 163296 disk has dust masses
in the inner ring at 68 au and
in the outer ring at 102 au, both falling within the range of
estimates from previous studies. Our DoAr 44 model has total dust mass
. Unlike HD 163296, DoAr 44 as of yet has no
detected planets. If the central cavity in the DoAr 44 disk is caused by a
planet, the planet's mass must be at least 0.5 and is unlikely to be
greater than 1.6 . We demonstrate that the DoAr 44 disk's structure with a
bright ring offset within a fainter skirt can be formed by dust particles
drifting through a plausible distribution of gas.Comment: 18 pages, 19 figure
The 0.5 micrometer-2.2 micrometer Scattered Light Spectrum of the Disk Around TW Hya
We present a 0.5-2.2micron scattered light spectrum of the circumstellar disk around TW Hya from a combination of spatially resolved HST STIS spectroscopy and NICMOS coronagraphic images of the disk. \Ve investigate the morphology at the disk at distances> 40 AU over this wide range of wavelengths. We measure the surface brightness, azimuthal symmetry, and spectral character of the disk as a function of radius. We find that the scattering efficiency of the dust is largely neutral to blue over the observed wavelengths. We find a good fit to the data over a wide range of distances from the star if we use a model disk with a partial gap of 30% depth at 80 AU and with steep disk truncation exterior to 100 AU. If the gap is caused by a planetary companion in the process of accreting disk gas, it must be less than 20 Solar mass
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