972 research outputs found
Formation of Giant Planets by Concurrent Accretion of Solids and Gas inside an Anti-Cyclonic Vortex
We study the formation of a giant gas planet by the core--accretion
gas--capture process, with numerical simulations, under the assumption that the
planetary core forms in the center of an anti-cyclonic vortex. The presence of
the vortex concentrates particles of centimeter to meter size from the
surrounding disk, and speeds up the core formation process. Assuming that a
planet of Jupiter mass is forming at 5 AU from the star, the vortex enhancement
results in considerably shorter formation times than are found in standard
core--accretion gas--capture simulations. Also, formation of a gas giant is
possible in a disk with mass comparable to that of the minimum mass solar
nebula.Comment: 27 pages, 4 figures, ApJ in pres
Theory of planet formation and comparison with observation: Formation of the planetary mass-radius relationship
The planetary mass-radius diagram is an observational result of central
importance to understand planet formation. We present an updated version of our
planet formation model based on the core accretion paradigm which allows to
calculate planetary radii and luminosities during the entire formation and
evolution of the planets. We first study with it the formation of Jupiter, and
compare with previous works. Then we conduct planetary population synthesis
calculations to obtain a synthetic mass-radius diagram which we compare with
the observed one. Except for bloated Hot Jupiters which can be explained only
with additional mechanisms related to their proximity to the star, we find a
good agreement of the general shape of the observed and the synthetic
mass-radius diagram. This shape can be understood with basic concepts of the
core accretion model.Comment: Proceedings Haute Provence Observatory Colloquium: Detection and
Dynamics of Transiting Exoplanets (23-27 August 2010). Edited by F. Bouchy,
R. F. Diaz & C. Moutou. Extended version: 17 pages, 8 figure
Global magnetohydrodynamical models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and transport of solids
We present global 3D MHD simulations of disks of gas and solids, aiming at
developing models that can be used to study various scenarios of planet
formation and planet-disk interaction in turbulent accretion disks. A second
goal is to show that Cartesian codes are comparable to cylindrical and
spherical ones in handling the magnetohydrodynamics of the disk simulations, as
the disk-in-a-box models presented here develop and sustain MHD turbulence. We
investigate the dependence of the magnetorotational instability on disk scale
height, finding evidence that the turbulence generated by the magnetorotational
instability grows with thermal pressure. The turbulent stresses depend on the
thermal pressure obeying a power law of 0.24+/-0.03, compatible with the value
of 0.25 found in shearing box calculations. The ratio of stresses decreased
with increasing temperature. We also study the dynamics of boulders in the
hydromagnetic turbulence. The vertical turbulent diffusion of the embedded
boulders is comparable to the turbulent viscosity of the flow. Significant
overdensities arise in the solid component as boulders concentrate in high
pressure regions.Comment: Changes after peer review proces
Two-dimensional models of layered protoplanetary discs - II. The effect of a residual viscosity in the dead zone
We study axisymmetric models of layered protoplanetary discs taking radiative
transfer effects into account, and allowing for a residual viscosity in the
dead zone. We also explore the effect of different viscosity prescriptions. In
addition to the ring instability reported in the first paper of the series we
find an oscillatory instability of the dead zone, accompanied by variations of
the accretion rate onto the central star. We provide a simplified analytical
description explaining the mechanism of the oscillations. Finally, we find that
the residual viscosity enables stationary accretion in large regions of layered
discs. Based on results obtained with the help of a simple 1-D hydrocode we
identify these regions, and discuss conditions in which layered discs can give
rise to FU~Orionis phenomena.Comment: 9 pages, 5 figures, accepted for publication in MNRA
Tracing planet-induced structures in circumstellar disks using molecular lines
Circumstellar disks are considered to be the birthplace of planets. Specific
structures like spiral arms, gaps, and cavities are characteristic indicators
of planet-disk interaction. Investigating these structures can provide insights
into the growth of protoplanets and the physical properties of the disk. We
investigate the feasibility of using molecular lines to trace planet-induced
structures in circumstellar disks. Based on 3D hydrodynamic simulations of
planet-disk interactions, we perform self-consistent temperature calculations
and produce N-LTE molecular line velocity-channel maps and spectra of these
disks using our new N-LTE line radiative transfer code Mol3D. Subsequently, we
simulate ALMA observations using the CASA simulator. We consider two nearly
face-on inclinations, 5 disk masses, 7 disk radii, and 2 different typical
pre-main-sequence host stars (T Tauri, Herbig Ae). We calculate up to 141
individual velocity-channel maps for five molecules/isotopoloques in a total of
32 rotational transitions to investigate the frequency dependence of the
structures indicated above. We find that the majority of protoplanetary disks
in our parameter space could be detected in the molecular lines considered.
However, unlike the continuum case, gap detection is not straightforward in
lines. For example, gaps are not seen in symmetric rings but are masked by the
pattern caused by the global (Keplerian) velocity field. We identify specific
regions in the velocity-channel maps that are characteristic of planet-induced
structures. Simulations of high angular resolution molecular line observations
demonstrate the potential of ALMA to provide complementary information about
the planet-disk interaction as compared to continuum observations. In
particular, the detection of planet-induced gaps is possible under certain
conditions.(abridged)Comment: 19 pages, 19 figures, accepted for publication in A&
Large-scale Vortices in Protoplanetary Disks: On the observability of possible early stages of planet formation
We investigate the possibility of mapping large-scale anti-cyclonic vortices,
resulting from a global baroclinic instability, as pre-cursors of planet
formation in proto-planetary disks with the planned Atacama Large Millimeter
Array (ALMA). On the basis of three-dimensional radiative transfer simulations,
images of a hydrodynamically calculated disk are derived which provide the
basis for the simulation of ALMA. We find that ALMA will be able to trace the
theoretically predicted large-scale anti-cyclonic vortex and will therefore
allow testing of existing models of this very early stage of planet formation
in circumstellar disks.Comment: Accepted by ApJ (Letters section). A preprint version with
high-quality figures can be downloaded from
http://spider.ipac.caltech.edu/staff/swolf/homepage/public/preprints/
vortex.ps.g
The baroclinic instability in the context of layered accretion. Self-sustained vortices and their magnetic stability in local compressible unstratified models of protoplanetary disks
Turbulence and angular momentum transport in accretion disks remains a topic
of debate. With the realization that dead zones are robust features of
protoplanetary disks, the search for hydrodynamical sources of turbulence
continues. A possible source is the baroclinic instability (BI), which has been
shown to exist in unmagnetized non-barotropic disks. We present shearing box
simulations of baroclinicly unstable, magnetized, 3D disks, in order to assess
the interplay between the BI and other instabilities, namely the
magneto-rotational instability (MRI) and the magneto-elliptical instability. We
find that the vortices generated and sustained by the baroclinic instability in
the purely hydrodynamical regime do not survive when magnetic fields are
included. The MRI by far supersedes the BI in growth rate and strength at
saturation. The resulting turbulence is virtually identical to an MRI-only
scenario. We measured the intrinsic vorticity profile of the vortex, finding
little radial variation in the vortex core. Nevertheless, the core is disrupted
by an MHD instability, which we identify with the magneto-elliptic instability.
This instability has nearly the same range of unstable wavelengths as the MRI,
but has higher growth rates. In fact, we identify the MRI as a limiting case of
the magneto-elliptic instability, when the vortex aspect ratio tends to
infinity (pure shear flow). We conclude that vortex excitation and
self-sustenance by the baroclinic instability in protoplanetary disks is viable
only in low ionization, i.e., the dead zone. Our results are thus in accordance
with the layered accretion paradigm. A baroclinicly unstable dead zone should
be characterized by the presence of large-scale vortices whose cores are
elliptically unstable, yet sustained by the baroclinic feedback. As magnetic
fields destroy the vortices and the MRI outweighs the BI, the active layers are
unmodified.Comment: 19+3 pages, 20+1 figures. Accepted by A&A, final versio
Grain opacity and the bulk composition of extrasolar planets. I. Results from scaling the ISM opacity
The opacity due to grains in the envelope of a protoplanet regulates the
accretion rate of gas during formation, thus the final bulk composition of
planets with primordial H/He is a function of it. Observationally, for
exoplanets with known mass and radius it is possible to estimate the bulk
composition via internal structure models. We first determine the reduction
factor of the ISM grain opacity f_opa that leads to gas accretion rates
consistent with grain evolution models. We then compare the bulk composition of
synthetic low-mass and giant planets at different f_opa with observations. For
f_opa=1 (full ISM opacity) the synthetic low-mass planets have too small radii,
i.e., too low envelope masses compared to observations. At f_opa=0.003, the
value calibrated with the grain evolution models, synthetic and actual planets
occupy similar mass-radius loci. The mean enrichment of giant planets relative
to the host star as a function of planet mass M can be approximated as
Z_p/Z_star = beta*(M/M_Jup)^alpha. We find alpha=-0.7 independent of f_opa in
synthetic populations in agreement with the observational result (-0.71+-0.10).
The absolute enrichment level decreases from beta=8.5 at f_opa=1 to 3.5 at
f_opa=0. At f_opa=0.003 one finds beta=7.2 which is similar to the
observational result (6.3+-1.0). We thus find observational hints that the
opacity in protoplanetary atmospheres is much smaller than in the ISM even if
the specific value of the grain opacity cannot be constrained here. The result
for the enrichment of giant planets helps to distinguish core accretion and
gravitational instability. In the simplest picture of core accretion where
first a critical core forms and afterwards only gas is added, alpha=-1. If a
core accretes all planetesimals inside the feeding zone, alpha=-2/3. The
observational result lies between these values, pointing to core accretion as
the formation mechanism.Comment: 21 pages, 15 figures. Accepted for A&
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