133 research outputs found
Diversity in the outcome of dust radial drift in protoplanetary discs
The growth of dust particles into planet embryos needs to circumvent the
radial-drift barrier, i.e. the accretion of dust particles onto the central
star by radial migration. The outcome of the dust radial migration is governed
by simple criteria between the dust-to-gas ratio and the exponents p and q of
the surface density and temperature power laws. The transfer of radiation
provides an additional constraint between these quantities because the disc
thermal structure is fixed by the dust spatial distribution. To assess which
discs are primarily affected by the radial-drift barrier, we used the radiative
transfer code MCFOST to compute the temperature structure of a wide range of
disc models, stressing the particular effects of grain size distributions and
vertical settling.
We find that the outcome of the dust migration process is very sensitive to
the physical conditions within the disc. For high dust-to-gas ratios (> 0.01)
or flattened disc structures (H/R < 0.05), growing dust grains can efficiently
decouple from the gas, leading to a high concentration of grains at a critical
radius of a few AU. Decoupling of grains can occur at a large fraction (> 0.1)
of the initial radius, for a dust-to-gas ratio greater than ~ 0.05. The exact
value of the required dust-to-gas ratio for dust to stop its migration is
strongly dependent on the disc temperature structure. Non growing dust grains
are accreted for discs with flat surface density profiles (p<0.7) while they
always remain in the disc if the surface density is steep enough (p>1.2). Both
the presence of large grains and vertical settling tend to favour the accretion
of non growing dust grains onto the central object, but it slows down the
migration of growing dust grains. All the disc configurations are found to have
favourable temperature profiles over most of the disc to retain their
planetesimals.Comment: 9 pages, 8 figures, accepted for publications in A&A, corrected typo
Super-Earths in the TW Hya disc
We test the hypothesis that the sub-millimetre thermal emission and scattered
light gaps seen in recent observations of TW Hya are caused by planet-disc
interactions. We perform global three-dimensional dusty smoothed particle
hydrodynamics simulations, comparing synthetic observations of our models with
dust thermal emission, CO emission and scattered light observations. We find
that the dust gaps observed at 24 au and 41 au can be explained by two
super-Earths (). A planet of approximately
Saturn-mass can explain the CO emission and the depth and width of the gap seen
in scattered light at 94 au. Our model produces a prominent spiral arm while
there are only hints of this in the data. To avoid runaway growth and migration
of the planets we require a disc mass of
in agreement with CO observations but 10100 times lower than the estimate
from HD line emission.Comment: 6 pages, 5 figures, accepted for publication in MNRA
Planet gaps in the dust layer of 3D proto-planetary disks: Observability with ALMA
Among the numerous known extrasolar planets, only a handful have been imaged
directly so far, at large orbital radii and in rather evolved systems. The
Atacama Large Millimeter/submillimeter Array (ALMA) will have the capacity to
observe these wide planetary systems at a younger age, thus bringing a better
understanding of the planet formation process. Here we explore the ability of
ALMA to detect the gaps carved by planets on wide orbits.Comment: 2 pages, 2 figures, to appear in the Proceedings of IAU Symp. 299:
Exploring the Formation and Evolution of Planetary Systems (Victoria, Canada
Radiative transfer in protoplanetary disks
We present a new 3D continuum radiative transfer code, MCFOST, based on a
Monte-Carlo method. The reliability and efficiency of the code is tested by
comparison with five different radiative transfer codes previously tested by
Pascucci et al., 2004, using a 2D disk configuration. When tested against the
same disk configuration, no significant difference is found between the
temperature and SED calculated with MCFOST and with the other codes. The
computed values are well within the range of values computed by the other
codes. The code-to-code differences are small, they rarely exceed 10% and are
usually much smaller.Comment: 20 pages, 11 figures, GRETA conference : "Radiative transfer and
Applications to Very Large Telescopes
A Herschel PACS survey of the dust and gas in Upper Scorpius disks
We present results of far-infrared photometric observations with Herschel
PACS of a sample of Upper Scorpius stars, with a detection rate of previously
known disk-bearing K and M stars at 70, 100, and 160 micron of 71%, 56%, and
50%, respectively. We fit power-law disk models to the spectral energy
distributions of K & M stars with infrared excesses, and have found that while
many disks extend in to the sublimation radius, the dust has settled to lower
scale heights than in disks of the less evolved Taurus-Auriga population, and
have much reduced dust masses. We also conducted Herschel PACS observations for
far-infrared line emission and JCMT observations for millimeter CO lines. Among
B and A stars, 0 of 5 debris disk hosts exhibit gas line emission, and among K
and M stars, only 2 of 14 dusty disk hosts are detected. The OI 63 micron and
CII 157 micron lines are detected toward [PZ99] J160421.7-213028 and [PBB2002]
J161420.3-190648, which were found in millimeter photometry to host two of the
most massive dust disks remaining in the region. Comparison of the OI line
emission and 63 micron continuum to that of Taurus sources suggests the
emission in the former source is dominated by the disk, while in the other
there is a significant contribution from a jet. The low dust masses found by
disk modeling and low number of gas line detections suggest that few stars in
Upper Scorpius retain sufficient quantities of material for giant planet
formation. By the age of Upper Scorpius, giant planet formation is essentially
complete.Comment: 48 pages, 14 figures, accepted A&
The theory of kinks -- I. A semi-analytic model of velocity perturbations due to planet-disc interaction
A new technique to detect protoplanets is by observing the kinematics of the
surrounding gas. Gravitational perturbations from a planet produce peculiar
`kinks' in channel maps of different gas species. In this paper, we show that
such kinks can be reproduced using semi-analytic models for the velocity
perturbation induced by a planet. In doing so we i) confirm that the observed
kinks are caused by the planet-induced wake; ii) show how to quantify the
planet mass from the kink amplitude; in particular, we show that the kink
amplitude scales with the square root of the planet mass for channels far from
the planet velocity, steepening to linear as the channels approach the planet;
iii) show how to extend the theory to include the effect of damping, which may
be needed in order to have localized kinks.Comment: Submitted to MNRAS, comments welcom
Characteristics of small protoplanetary disc warps in kinematic observations
Many circumstellar discs appear to have misaligned central regions that give
rise to shadows seen in scattered light observations. Small warps (
misalignment) are probably more common but are also more difficult to detect
than the large misalignments studied previously. We present the characteristics
of CO emission that may be used to identify a small disc warp, found from
synthetic CO maps of a model misaligned circumbinary disc. The spectra
are not symmetrical, so fitting a Keplerian model is not appropriate and can
hide a warp or lead to spurious features such as spirals appearing in the
residuals. We quantify the observed warp structure by fitting sinusoids to
concentric annuli of the disc. From this we can trace the radial variation of
the peak velocity and of the azimuth of the peak velocity, i.e., the twist. At
near face-on inclinations, these radial profiles reveal the warp structure. The
twist remains detectable at moderate inclinations () in the absence of radial flows but the measured inclination must
be accurate to to allow detection of the radial variation.
The observed twist does not provide a direct measure of the warp structure
because of its dependence on optical depth. The warp causes broad asymmetries
in the channel maps that span several channels and that are distinct from
localised features caused by embedded planets and gravitational instability. We
suspect that kinematic evidence of warps may have been missed and we suggest a
few examples where the data may be revisited.Comment: 16 pages. Accepted to MNRA
Continuum and line modelling of discs around young stars. I. 300000 disc models for Herschel/GASPS
We have combined the thermo-chemical disc code ProDiMo with the Monte Carlo
radiative transfer code MCFOST to calculate a grid of ~300000 circumstellar
disc models, systematically varying 11 stellar, disc and dust parameters
including the total disc mass, several disc shape parameters and the
dust-to-gas ratio. For each model, dust continuum and line radiative transfer
calculations are carried out for 29 far IR, sub-mm and mm lines of [OI], [CII],
12CO and o/p-H2O under 5 inclinations. The grid allows to study the influence
of the input parameters on the observables, to make statistical predictions for
different types of circumstellar discs, and to find systematic trends and
correlations between the parameters, the continuum fluxes, and the line fluxes.
The model grid, comprising the calculated disc temperatures and chemical
structures, the computed SEDs, line fluxes and profiles, will be used in
particular for the data interpretation of the Herschel open time key programme
GASPS. The calculated line fluxes show a strong dependence on the assumed UV
excess of the central star, and on the disc flaring. The fraction of models
predicting [OI] and [CII] fine-structure lines fluxes above Herschel/PACS and
Spica/SAFARI detection limits are calculated as function of disc mass. The
possibility of deriving the disc gas mass from line observations is discussed.Comment: accepted by MNRAS. 5 pages, 4 figures, 3 table
- …