794 research outputs found
Influence of viscosity and the adiabatic index on planetary migration
The strength and direction of migration of low mass embedded planets depends
on the disk's thermodynamic state, where the internal dissipation is balanced
by radiative transport, and the migration can be directed outwards, a process
which extends the lifetime of growing embryos. Very important parameters
determining the structure of disks, and hence the direction of migration, are
the viscosity and the adiabatic index. In this paper we investigate the
influence of different viscosity prescriptions (alpha-type and constant) and
adiabatic indices on disk structures and how this affects the migration rate of
planets embedded in such disks. We perform 3D numerical simulations of
accretion disks with embedded planets. We use the explicit/implicit
hydrodynamical code NIRVANA that includes full tensor viscosity and radiation
transport in the flux-limited diffusion approximation, as well as a proper
equation of state for molecular hydrogen. The migration of embedded 20Earthmass
planets is studied. Low-viscosity disks have cooler temperatures and the
migration rates of embedded planets tend toward the isothermal limit. In these
disks, planets migrate inwards even in the fully radiative case. The effect of
outward migration can only be sustained if the viscosity in the disk is large.
Overall, the differences between the treatments for the equation of state seem
to play a more important role in disks with higher viscosity. A change in the
adiabatic index and in the viscosity changes the zero-torque radius that
separates inward from outward migration. For larger viscosities, temperatures
in the disk become higher and the zero-torque radius moves to larger radii,
allowing outward migration of a 20 Earth-mass planet to persist over an
extended radial range. In combination with large disk masses, this may allow
for an extended period of the outward migration of growing protoplanetary
cores
Migration of Earth-size planets in 3D radiative discs
In this paper, we address the migration of small mass planets in 3D radiative
disks. Indeed, migration of small planets is known to be too fast inwards in
locally isothermal conditions. However, thermal effects could reverse its
direction, potentially saving planets in the inner, optically thick parts of
the protoplanetary disc. This effect has been seen for masses larger than 5
Earth masses, but the minimum mass for this to happen has never been probed
numerically, although it is of crucial importance for planet formation
scenarios. We have extended the hydro-dynamical code FARGO to 3D, with thermal
diffusion. With this code, we perform simulations of embedded planets down to 2
Earth masses. For a set of discs parameters for which outward migration has
been shown in the range of Earth masses, we find that the transition
to inward migration occurs for masses in the range Earth masses. The
transition appears to be due to an unexpected phenomenon: the formation of an
asymmetric cold and dense finger of gas driven by circulation and libration
streamlines. We recover this phenomenon in 2D simulations where we control the
cooling effects of the gas through a simple modeling of the energy equation.Comment: 17 pages, 20 figures, accepted. MNRAS, 201
The great dichotomy of the Solar System: small terrestrial embryos and massive giant planet cores
The basic structure of the solar system is set by the presence of low-mass
terrestrial planets in its inner part and giant planets in its outer part. This
is the result of the formation of a system of multiple embryos with
approximately the mass of Mars in the inner disk and of a few multi-Earth-mass
cores in the outer disk, within the lifetime of the gaseous component of the
protoplanetary disk. What was the origin of this dichotomy in the mass
distribution of embryos/cores? We show in this paper that the classic processes
of runaway and oligarchic growth from a disk of planetesimals cannot explain
this dichotomy, even if the original surface density of solids increased at the
snowline. Instead, the accretion of drifting pebbles by embryos and cores can
explain the dichotomy, provided that some assumptions hold true. We propose
that the mass-flow of pebbles is two-times lower and the characteristic size of
the pebbles is approximately ten times smaller within the snowline than beyond
the snowline (respectively at heliocentric distance and
, where is the snowline heliocentric distance), due to ice
sublimation and the splitting of icy pebbles into a collection of
chondrule-size silicate grains. In this case, objects of original sub-lunar
mass would grow at drastically different rates in the two regions of the disk.
Within the snowline these bodies would reach approximately the mass of Mars
while beyond the snowline they would grow to Earth masses. The
results may change quantitatively with changes to the assumed parameters, but
the establishment of a clear dichotomy in the mass distribution of protoplanets
appears robust, provided that there is enough turbulence in the disk to prevent
the sedimentation of the silicate grains into a very thin layer.Comment: In press in Icaru
Surface waves in protoplanetary disks induced by outbursts: Concentric rings in scattered light
Context: Vertically hydrostatic protoplanetary disk models are based on the
assumption that the main heating source, stellar irradiation, does not vary
much with time. However, it is known that accreting young stars are variable
sources of radiation. This is particularly evident for outbursting sources such
as EX Lupi and FU Orionis stars. Aim: We investigate how such outbursts affect
the vertical structure of the outer regions of the protoplanetary disk, in
particular their appearance in scattered light at optical and near-infrared
wavelengths. Methods: We employ the 3D FARGOCA radiation-hydrodynamics code, in
polar coordinates, to compute the time-dependent behavior of the axisymmetric
disk structure. The outbursting inner disk region is not included explicitly.
Instead, its luminosity is added to the stellar luminosity and is thus included
in the irradiation of the outer disk regions. For time snapshots of interest we
insert the density structure into the RADMC-3D radiative transfer code and
compute the appearance of the disk at optical/near-infrared wavelengths.
Results: We find that, depending on the amplitude of the outbursts, the
vertical structure of the disk can become highly dynamic, featuring circular
surface waves of considerable amplitude. These "hills" and "valleys" on the
disk's surface show up in the scattered light images as bright and dark
concentric rings. Initially these rings are small and act as standing waves,
but they subsequently lead to outward propagating waves, like the waves
produced by a stone thrown into a pond. These waves continue long after the
actual outburst has died out. Conclusions: We propose that some of the
multi-ringed structures seen in optical/infrared images of several
protoplanetary disks may have their origin in outbursts that occurred decades
or centuries ago.Comment: Accepted for publication in A&A Letter
Universal and non-universal behavior in Dirac spectra
We have computed ensembles of complete spectra of the staggered Dirac
operator using four-dimensional SU(2) gauge fields, both in the quenched
approximation and with dynamical fermions. To identify universal features in
the Dirac spectrum, we compare the lattice data with predictions from chiral
random matrix theory for the distribution of the low-lying eigenvalues. Good
agreement is found up to some limiting energy, the so-called Thouless energy,
above which random matrix theory no longer applies. We determine the dependence
of the Thouless energy on the simulation parameters using the scalar
susceptibility and the number variance.Comment: LATTICE98(confine), 9 pages, 11 figure
Meridional circulation of gas into gaps opened by giant planets in three-dimensional low-viscosity disks
We examine the gas circulation near a gap opened by a giant planet in a
protoplanetary disk. We show with high resolution 3D simulations that the gas
flows into the gap at high altitude over the mid-plane, at a rate dependent on
viscosity. We explain this observation with a simple conceptual model. From
this model we derive an estimate of the amount of gas flowing into a gap opened
by a planet with Hill radius comparable to the scale-height of a layered disk
(i. e. a disk with viscous upper layer and inviscid midplane). Our estimate
agrees with modern MRI simulations(Gressel et al., 2013). We conclude that gap
opening in a layered disk can not slow down significantly the runaway gas
accretion of Saturn to Jupiter-mass planets.Comment: in press as a Note in Icaru
Spectrum of the SU(3) Dirac operator on the lattice: Transition from random matrix theory to chiral perturbation theory
We calculate complete spectra of the Kogut-Susskind Dirac operator on the
lattice in quenched SU(3) gauge theory for various values of coupling constant
and lattice size. From these spectra we compute the connected and disconnected
scalar susceptibilities and find agreement with chiral random matrix theory up
to a certain energy scale, the Thouless energy. The dependence of this scale on
the lattice volume is analyzed. In the case of the connected susceptibility
this dependence is anomalous, and we explain the reason for this. We present a
model of chiral perturbation theory that is capable of describing the data
beyond the Thouless energy and that has a common range of applicability with
chiral random matrix theory.Comment: 8 pages, RevTeX, 15 .eps figure
Random Matrix Theory and Chiral Logarithms
Recently, the contributions of chiral logarithms predicted by quenched chiral
perturbation theory have been extracted from lattice calculations of hadron
masses. We argue that a detailed comparison of random matrix theory and lattice
calculations allows for a precise determination of such corrections. We
estimate the relative size of the m*log(m), m, and m^2 corrections to the
chiral condensate for quenched SU(2).Comment: LaTeX (elsart.cls), 9 pages, 6 .eps figures, added reference, altered
discussion of Eq.(9
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