378 research outputs found
On the coorbital corotation torque in a viscous disk and its impact on planetary migration
We evaluate the coorbital corotation torque on a planet on a fixed circular
orbit embedded in a viscous protoplanetary disk, for the case of a steady flow
in the planet frame. This torque can be evaluated just from the flow properties
at the separatrix between the librating (horseshoe) and circulating
streamlines. A stationary solution is searched for the flow in the librating
region. When used to evaluate the torque exerted by the circulating material of
the outer and inner disk on the trapped material of the librating region, this
solution leads to an expression of the coorbital corotation torque in agreement
with previous estimates. An analytical expression is given for the corotation
torque as a function of viscosity. Lastly, we show that additional terms in the
torque expression can play a crucial role. In particular, they introduce a
coupling with the disk density profile perturbation (the `dip' which surrounds
the planet) and add to the corotation torque a small, positive fraction of the
one-sided Lindblad torque. As a consequence, the migration could well be
directed outwards in very thin disks (aspect ratio smaller than a few percent).
This 2D analysis is especially relevant for mildly embedded protoplanets
(sub-Saturn sized objects).Comment: 10 pages, 2 figure
The Lopsidedness of Present-Day Galaxies: Results from the Sloan Digital Sky Survey
Large-scale asymmetries in the stellar mass distribution in galaxies are
believed to trace non-equilibrium situations in the luminous and/or dark matter
component. These may arise in the aftermath of events like mergers, accretion,
and tidal interactions. These events are key in the evolution of galaxies. In
this paper we quantify the large-scale lopsidedness of light distributions in
25155 galaxies at z < 0.06 from the Sloan Digital Sky Survey Data Release 4
using the m = 1 azimuthal Fourier mode. We show that the lopsided distribution
of light is primarily due to a corresponding lopsidedness in the stellar mass
distribution. Observational effects, such as seeing, Poisson noise, and
inclination, introduce only small errors in lopsidedness for the majority of
this sample. We find that lopsidedness correlates strongly with other basic
galaxy structural parameters: galaxies with low concentration, stellar mass,
and stellar surface mass density tend to be lopsided, while galaxies with high
concentration, mass, and density are not. We find that the strongest and most
fundamental relationship between lopsidedness and the other structural
parameters is with the surface mass density. We also find, in agreement with
previous studies, that lopsidedness tends to increase with radius. Both these
results may be understood as a consequence of several factors. The outer
regions of galaxies and low-density galaxies are more susceptible to tidal
perturbations, and they also have longer dynamical times (so lopsidedness will
last longer). They are also more likely to be affected by any underlying
asymmetries in the dark matter halo.Comment: 42 pages, 13 figures, 3 tables, accepted to Ap
Dynamics of pebbles in the vicinity of a growing planetary embryo: hydro-dynamical simulations
Understanding the growth of the cores of giant planets is a difficult
problem. Recently, Lambrechts and Johansen (2012; LJ12) proposed a new model in
which the cores grow by the accretion of pebble-size objects, as the latter
drift towards the star due to gas drag. Here, we investigate the dynamics of
pebble-size objects in the vicinity of planetary embryos of 1 and 5 Earth
masses and the resulting accretion rates. We use hydrodynamical simulations, in
which the embryo influences the dynamics of the gas and the pebbles suffer gas
drag according to the local gas density and velocities. The pebble dynamics in
the vicinity of the planetary embryo is non-trivial, and it changes
significantly with the pebble size. Nevertheless, the accretion rate of the
embryo that we measure is within an order of magnitude of the rate estimated in
LJ12 and tends to their value with increasing pebble-size. We conclude that the
model by LJ12 has the potential to explain the rapid growth of giant planet
cores. The actual accretion rates however, depend on the surface density of
pebble size objects in the disk, which is unknown to date.Comment: In press in Astronomy and Astrophysic
Recent developments in planet migration theory
Planetary migration is the process by which a forming planet undergoes a
drift of its semi-major axis caused by the tidal interaction with its parent
protoplanetary disc. One of the key quantities to assess the migration of
embedded planets is the tidal torque between the disc and planet, which has two
components: the Lindblad torque and the corotation torque. We review the latest
results on both torque components for planets on circular orbits, with a
special emphasis on the various processes that give rise to additional, large
components of the corotation torque, and those contributing to the saturation
of this torque. These additional components of the corotation torque could help
address the shortcomings that have recently been exposed by models of planet
population syntheses. We also review recent results concerning the migration of
giant planets that carve gaps in the disc (type II migration) and the migration
of sub-giant planets that open partial gaps in massive discs (type III
migration).Comment: 52 pages, 18 figures. Review article to be published in "Tidal
effects in Astronomy and Astrophysics", Lecture Notes in Physic
On disc driven inward migration of resonantly coupled planets with application to the system around GJ876
We consider two protoplanets gravitationally interacting with each other and
a protoplanetary disc. The two planets orbit interior to a tidally maintained
disc cavity while the disc interaction indices inward migration. When the
migration is slow enough, the more rapidly migrating outer protoplanet
approaches and becomes locked in a 2:1 commensurability with the inner one.
This is maintained in subsequent evolution. We study this evolution using a
simple anaytic model, full hydrodynamic 2D simulations of the disc planet
system and longer time N body integrations incorporating simple prescriptions
for the effect of the disc on the planet orbits. The eccentricity of the
protoplanets are found to be determined by the migration rate induced in the
outer planet orbit by the external disc. We apply our results to the recently
discovered resonant planets around GJ876. Simulation shows that a disc with
parameters expected for protoplanetary discs causes trapping in the 2:1
commensurability when the planets orbit in an inner cavity and that
eccentricities in the observed range may be obtained.Comment: 8 pages, 5 figures, submitted to A&A on 30/03/200
The Importance of Disk Structure in Stalling Type I Migration
As planets form they tidally interact with their natal disks. Though the
tidal perturbation induced by Earth and super-Earth mass planets is generally
too weak to significantly modify the structure of the disk, the interaction is
potentially strong enough to cause the planets to undergo rapid type I
migration. This physical process may provide a source of short-period
super-Earths, though it may also pose a challenge to the emergence and
retention of cores on long-period orbits with sufficient mass to evolve into
gas giants. Previous numerical simulations have shown that the type I migration
rate sensitively depends upon the circumstellar disk's properties, particularly
the temperature and surface density gradients. Here, we derive these structure
parameters for 1) a self-consistent viscous-disk model based on a constant
\alpha-prescription, 2) an irradiated disk model that takes into account
heating due to the absorption of stellar photons, and 3) a layered-accretion
disk model with variable \alpha-parameter. We show that in the inner
viscously-heated regions of typical protostellar disks, the horseshoe and
corotation torques of super-Earths can exceed their differential Lindblad
torque and cause them to undergo outward migration. However, the temperature
profile due to passive stellar irradiation causes type I migration to be
inwards throughout much of the disk. For disks in which there is outwards
migration, we show that location and the mass range of the "planet traps"
depends on some uncertain assumptions adopted for these disk models. Competing
physical effects may lead to dispersion in super-Earths' mass-period
distribution.Comment: 12 pages, Submitted to Ap
Double Bars, Inner Disks, and Nuclear Rings in Early-Type Disk Galaxies
We present results from an imaging survey of an unbiased sample of
thirty-eight early-type (S0--Sa), low-inclination, optically barred galaxies in
the field. Our goal was to find and characterize central stellar and gaseous
structures: secondary bars, inner disks, and nuclear rings. Bars inside bars
are surprisingly common: at least one quarter of the sample galaxies (possibly
as many as 40%) are double-barred, with no preference for Hubble type or the
strength of the primary bar. A typical secondary bar is ~12% of the size of its
primary bar and 240--750 pc in radius. We see no significant effect of
secondary bars on nuclear activity. We also find kiloparsec-scale inner disks
in at least 20% of our sample, almost exclusively in S0 galaxies. These disks
are on average 20% the size of their host bar, and show a wider range of
relative sizes than do secondary bars. Nuclear rings are present in about a
third of our sample. Most are dusty, sites of current or recent star formation,
or both; such rings are preferentially found in Sa galaxies. Three S0 galaxies
(15% of the S0's) appear to have purely stellar nuclear rings, with no evidence
for dust or recent star formation. The fact that these central stellar
structures are so common indicates that the inner regions of early-type barred
galaxies typically contain dynamically cool and disklike structures. This is
especially true for S0 galaxies, where secondary bars, inner disks, and/or
stellar nuclear rings are present at least two thirds of the time. (abridged)Comment: LaTeX, 15 pages, 7 EPS figures; to appear in The Astronomical Journal
(July 2002
Toward a Deterministic Model of Planetary Formation VI: Dynamical Interaction and Coagulation of Multiple Rocky Embryos and Super-Earth Systems around Solar Type Stars
Radial velocity and transit surveys indicate that solar-type stars bear
super-Earths, with mass and period up to ~ 20 M_E and a few months, are more
common than those with Jupiter-mass gas giants. In many cases, these
super-Earths are members of multiple-planet systems in which their mutual
dynamical interaction has influenced their formation and evolution. In this
paper, we modify an existing numerical population synthesis scheme to take into
account protoplanetary embryos' interaction with their evolving natal gaseous
disk, as well as their close scatterings and resonant interaction with each
other. We show that it is possible for a group of compact embryos to emerge
interior to the ice line, grow, migrate, and congregate into closely-packed
convoys which stall in the proximity of their host stars. After the disk-gas
depletion, they undergo orbit crossing, close scattering, and giant impacts to
form multiple rocky Earths or super-Earths in non-resonant orbits around ~
0.1AU with moderate eccentricities of ~0.01-0.1. We suggest that most
refractory super-Earths with period in the range of a few days to weeks may
have formed through this process. These super-Earths differ from Neptune-like
ice giants by their compact sizes and lack of a substantial gaseous envelope.Comment: 37 pages, 10 figures, accepted for publication in Ap
Type I planet migration in nearly laminar disks - long term behavior
We carry out 2-D high resolution numerical simulations of type I planet
migration with different disk viscosities. We find that the planet migration is
strongly dependent on disk viscosities. Two kinds of density wave damping
mechanisms are discussed. Accordingly, the angular momentum transport can be
either viscosity dominated or shock dominated, depending on the disk
viscosities. The long term migration behavior is different as well. Influences
of the Rossby vortex instability on planet migration are also discussed. In
addition, we investigate very weak shock generation in inviscid disks by small
mass planets and compare the results with prior analytic results.Comment: Accepted for publication in Ap
On the formation of hot Neptunes and super-Earths
The discovery of short-period Neptune-mass objects, now including the
remarkable system HD69830 (Lovis et al. 2006) with three Neptune analogues,
raises difficult questions about current formation models which may require a
global treatment of the protoplanetary disc. Several formation scenarios have
been proposed, where most combine the canonical oligarchic picture of core
accretion with type I migration (e.g. Terquem & Papaloizou 2007) and planetary
atmosphere physics (e.g. Alibert et al. 2006). To date, published studies have
considered only a small number of progenitors at late times. This leaves
unaddressed important questions about the global viability of the models. We
seek to determine whether the most natural model -- namely, taking the
canonical oligarchic picture of core accretion and introducing type I migration
-- can succeed in forming objects of 10 Earth masses and more in the innermost
parts of the disc. This problem is investigated using both traditional
semianalytic methods for modelling oligarchic growth as well as a new parallel
multi-zone N-body code designed specifically for treating planetary formation
problems with large dynamic range (McNeil & Nelson 2009). We find that it is
extremely difficult for oligarchic tidal migration models to reproduce the
observed distribution. Even under many variations of the typical parameters, we
form no objects of mass greater than 8 Earth masses. By comparison, it is
relatively straightforward to form icy super-Earths. We conclude that either
the initial conditions of the protoplanetary discs in short-period Neptune
systems were substantially different from the standard disc models we used, or
there is important physics yet to be understood.Comment: 19 pages, 18 figures. Final version accepted to MNRAS 30 September
200
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