392 research outputs found
Can grain growth explain transition disks?
Aims: Grain growth has been suggested as one possible explanation for the
diminished dust optical depths in the inner regions of protoplanetary
"transition" disks. In this work, we directly test this hypothesis in the
context of current models of grain growth and transport.
Methods: A set of dust evolution models with different disk shapes, masses,
turbulence parameters, and drift efficiencies is combined with radiative
transfer calculations in order to derive theoretical spectral energy
distributions (SEDs) and images.
Results: We find that grain growth and transport effects can indeed produce
dips in the infrared SED, as typically found in observations of transition
disks. Our models achieve the necessary reduction of mass in small dust by
producing larger grains, yet not large enough to be fragmenting efficiently.
However, this population of large grains is still detectable at millimeter
wavelengths. Even if perfect sticking is assumed and radial drift is neglected,
a large population of dust grains is left behind because the time scales on
which they are swept up by the larger grains are too long. This mechanism thus
fails to reproduce the large emission cavities observed in recent
millimeter-wave interferometric images of accreting transition disks.Comment: 11 pages, 5 figures, accepted to A&
Testing particle trapping in transition disks with ALMA
We present new Atacama Large Millimeter/submillimeter Array (ALMA) continuum
observations at 336GHz of two transition disks, SR21 and HD135344B. In
combination with previous ALMA observations from Cycle 0 at 689GHz, we compare
the visibility profiles at the two frequencies and calculate the spectral index
(). The observations of SR21 show a clear shift in the
visibility nulls, indicating radial variations of the inner edge of the cavity
at the two wavelengths. Notable radial variations of the spectral index are
also detected for SR21 with values of in the
inner region ( AU) and outside. An
axisymmetric ring (which we call the ring model) or a ring with the addition of
an azimuthal Gaussian profile, for mimicking a vortex structure (which we call
the vortex model), is assumed for fitting the disk morphology. For SR21, the
ring model better fits the emission at 336GHz, conversely the vortex model
better fits the 689GHz emission. For HD135344B, neither a significant shift in
the null of the visibilities nor radial variations of are
detected. Furthermore, for HD135344B, the vortex model fits both frequencies
better than the ring model. However, the azimuthal extent of the vortex
increases with wavelength, contrary to model predictions for particle trapping
by anticyclonic vortices. For both disks, the azimuthal variations of
remain uncertain to confirm azimuthal trapping. The
comparison of the current data with a generic model of dust evolution that
includes planet-disk interaction suggests that particles in the outer disk of
SR21 have grown to millimetre sizes and have accumulated in a radial pressure
bump, whereas with the current resolution there is not clear evidence of radial
trapping in HD135344B, although it cannot be excluded either.Comment: Minor changes after language edition. Accepted for publication in A&A
(abstract slightly shortened for arXiv
A Multi-Wavelength Analysis of Dust and Gas in the SR 24S Transition Disk
We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm
continuum observations of the SR 24S transition disk with an angular resolution
(12 au radius). We perform a multi-wavelength investigation by
combining new data with previous ALMA data at 0.45 mm. The visibilities and
images of the continuum emission at the two wavelengths are well characterized
by a ring-like emission. Visibility modeling finds that the ring-like emission
is narrower at longer wavelengths, in good agreement with models of dust
trapping in pressure bumps, although there are complex residuals that suggest
potentially asymmetric structures. The 0.45 mm emission has a shallower profile
inside the central cavity than the 1.3 mm emission. In addition, we find that
the CO and CO (J=2-1) emission peaks at the center of the
continuum cavity. We do not detect either continuum or gas emission from the
northern companion to this system (SR 24N), which is itself a binary system.
The upper limit for the dust disk mass of SR 24N is , which gives a disk mass ratio in dust between the two
components of . The current ALMA observations may imply that either
planets have already formed in the SR 24N disk or that dust growth to mm-sizes
is inhibited there and that only warm gas, as seen by ro-vibrational CO
emission inside the truncation radii of the binary, is present.Comment: Accepted for publication in Ap
A Major Asymmetric Dust Trap in a Transition Disk
The statistics of discovered exoplanets suggest that planets form
efficiently. However, there are fundamental unsolved problems, such as
excessive inward drift of particles in protoplanetary disks during planet
formation. Recent theories invoke dust traps to overcome this problem. We
report the detection of a dust trap in the disk around the star Oph IRS 48
using observations from the Atacama Large Millimeter/submillimeter Array
(ALMA). The 0.44-millimeter-wavelength continuum map shows high-contrast
crescent-shaped emission on one side of the star originating from
millimeter-sized grains, whereas both the mid-infrared image (micrometer-sized
dust) and the gas traced by the carbon monoxide 6-5 rotational line suggest
rings centered on the star. The difference in distribution of big grains versus
small grains/gas can be modeled with a vortex-shaped dust trap triggered by a
companion.Comment: 25 pages, 7 figures (accepted version prior to language editing
Type I Migration in Radiatively Efficient Discs
We study Type I migration of a planet in a radiatively efficient disk using
global two dimensional hydrodynamic simulations. The large positive corotation
torque is exerted on a planet by an adiabatic disk at early times when the disk
has the steep negative entropy gradient. The gas on the horseshoe orbit of the
planet is compressed adiabatically during the change of the orbit from the slow
orbit to the fast orbit, increasing its density and exerting the positive
torque on the planet. The planet would migrate outward in the adiabatic disk
before saturation sets in. We further study the effect of energy dissipation by
radiation on Type I migration of the planet. The corotation torque decreases
when the energy dissipates effectively because the density of the gas on the
horseshoe orbit does not increase by the compression compared with the gas of
the adiabatic disk. The total torque is mainly determined by the negative
Lindblad torque and becomes negative. The planet migrates inward toward the
central star in the radiatively efficient disk. The migration velocity is
dependent on the radiative efficiency and greatly reduced if the radiative
cooling works inefficiently.Comment: 12 pages, 10 figures, 1 table, Accepted for publication in MNRA
Type I migration in optically thick accretion discs
We study the torque acting on a planet embedded in an optically thick
accretion disc, using global two-dimensional hydrodynamic simulations. The
temperature of an optically thick accretion disc is determined by the energy
balance between the viscous heating and the radiative cooling. The radiative
cooling rate depends on the opacity of the disc. The opacity is expressed as a
function of the temperature. We find the disc is divided into three regions
that have different temperature distributions. The slope of the entropy
distribution becomes steep in the inner region of the disc with the high
temperature and the outer region of the disc with the low temperature, while it
becomes shallow in the middle region with the intermediate temperature. Planets
in the inner and outer regions move outward owing to the large positive
corotation torque exerted on the planet by an adiabatic disc, on the other
hand, a planet in the middle region moves inward toward the central star.
Planets are expected to accumulate at the boundary between the inner and middle
regions of the adiabatic disc. The positive corotation torque decreases with an
increase in the viscosity of the disc. We find that the positive corotation
torque acting on the planet in the inner region becomes too small to cancel the
negative Lindblad torque when we include the large viscosity, which destroys
the enhancement of the density in the horseshoe orbit of the planet. This leads
to the inward migration of the planet in the inner region of the disc. A planet
with 5 Earth masses in the inner region can move outward in a disc with the
surface density of 100 g/cm^2 at 1 AU when the accretion rate of a disc is
smaller than 2x10^{-8} solar mass/yr.Comment: 17 pages, 15 figure
Impact of grain evolution on the chemical structure of protoplanetary disks
We study the impact of dust evolution in a protoplanetary disk around a T
Tauri star on the disk chemical composition. For the first time we utilize a
comprehensive model of dust evolution which includes growth, fragmentation and
sedimentation. Specific attention is paid to the influence of grain evolution
on the penetration of the UV field in the disk. A chemical model that includes
a comprehensive set of gas phase and grain surface chemical reactions is used
to simulate the chemical structure of the disk. The main effect of the grain
evolution on the disk chemical composition comes from sedimentation, and, to a
lesser degree, from the reduction of the total grain surface area. The net
effect of grain growth is suppressed by the fragmentation process which
maintains a population of small grains, dominating the total grain surface
area. We consider three models of dust properties. In model GS both growth and
sedimentation are taken into account. In models A5 and A4 all grains are
assumed to have the same size (10(-5) cm and 10(-4) cm, respectively) with
constant gas-to-dust mass ratio of 100. Like in previous studies, the
"three-layer" pattern (midplane, molecular layer, hot atmosphere) in the disk
chemical structure is preserved in all models, but shifted closer to the
midplane in models with increased grain size (GS and A4). Unlike other similar
studies, we find that in models GS and A4 column densities of most gas-phase
species are enhanced by 1-3 orders of magnitude relative to those in a model
with pristine dust (A5), while column densities of their surface counterparts
are decreased. We show that column densities of certain species, like C2H,
HC(2n+1)N (n=0-3), H2O and some other molecules, as well as the C2H2/HCN
abundance ratio which are accessible with Herschel and ALMA can be used as
observational tracers of early stages of the grain evolution process in
protoplanetary disks.Comment: 50 pages, 4 tables, 11 figures, accepted to the Ap
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