351 research outputs found
Effects of Turbulence, Eccentricity Damping, and Migration Rate on the Capture of Planets into Mean Motion Resonance
Pairs of migrating extrasolar planets often lock into mean motion resonance
as they drift inward. This paper studies the convergent migration of giant
planets (driven by a circumstellar disk) and determines the probability that
they are captured into mean motion resonance. The probability that such planets
enter resonance depends on the type of resonance, the migration rate, the
eccentricity damping rate, and the amplitude of the turbulent fluctuations.
This problem is studied both through direct integrations of the full 3-body
problem, and via semi-analytic model equations. In general, the probability of
resonance decreases with increasing migration rate, and with increasing levels
of turbulence, but increases with eccentricity damping. Previous work has shown
that the distributions of orbital elements (eccentricity and semimajor axis)
for observed extrasolar planets can be reproduced by migration models with
multiple planets. However, these results depend on resonance locking, and this
study shows that entry into -- and maintenance of -- mean motion resonance
depends sensitively on migration rate, eccentricity damping, and turbulence.Comment: 43 pages including 14 figures; accepted for publication in The
Astrophysical Journa
Origin and Detectability of coorbital planets from radial velocity data
We analyze the possibilities of detection of hypothetical exoplanets in
coorbital motion from synthetic radial velocity (RV) signals, taking into
account different types of stable planar configurations, orbital eccentricities
and mass ratios. For each nominal solution corresponding to small-amplitude
oscillations around the periodic solution, we generate a series of synthetic RV
curves mimicking the stellar motion around the barycenter of the system. We
then fit the data sets obtained assuming three possible different orbital
architectures: (a) two planets in coorbital motion, (b) two planets in a 2/1
mean-motion resonance, and (c) a single planet. We compare the resulting
residuals and the estimated orbital parameters.
For synthetic data sets covering only a few orbital periods, we find that the
discrete radial velocity signal generated by a coorbital configuration could be
easily confused with other configurations/systems, and in many cases the best
orbital fit corresponds to either a single planet or two bodies in a 2/1
resonance. However, most of the incorrect identifications are associated to
dynamically unstable solutions.
We also compare the orbital parameters obtained with two different fitting
strategies: a simultaneous fit of two planets and a nested multi-Keplerian
model. We find that the nested models can yield incorrect orbital
configurations (sometimes close to fictitious mean-motion resonances) that are
nevertheless dynamically stable and with orbital eccentricities lower than the
correct nominal solutions.
Finally, we discuss plausible mechanisms for the formation of coorbital
configurations, by the interaction between two giant planets and an inner
cavity in the gas disk. For equal mass planets, both Lagrangian and
anti-Lagrangian configurations can be obtained from same initial condition
depending on final time of integration.Comment: 14 pages, 16 figures.2012. MNRAS, 421, 35
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
The dynamical role of the circumplanetary disc in planetary migration
Numerical simulations of planets embedded in protoplanetary gaseous discs are
a precious tool for studying the planetary migration ; however, some
approximations have to be made. Most often, the selfgravity of the gas is
neglected. In that case, it is not clear in the literature how the material
inside the Roche lobe of the planet should be taken into account. Here, we want
to address this issue by studying the influence of various methods so far used
by different authors on the migration rate.
We performed high-resolution numerical simulations of giant planets embedded
in discs. We compared the migration rates with and without gas selfgravity,
testing various ways of taking the circum-planetary disc (CPD) into account.
Different methods lead to significantly different migration rates. Adding the
mass of the CPD to the perturbing mass of the planet accelerates the migration.
Excluding a part of the Hill sphere is a very touchy parameter that may lead to
an artificial suppression of the type III, runaway migration. In fact, the CPD
is smaller than the Hill sphere. We recommend excluding no more than a 0.6 Hill
radius and using a smooth filter. Alternatively, the CPD can be given the
acceleration felt by the planet from the rest of the protoplanetary disc.
The gas inside the Roche lobe of the planet should be very carefully taken
into account in numerical simulations without any selfgravity of the gas. The
entire Hill sphere should not be excluded. The method used should be explicitly
given. However, no method is equivalent to computing the full selfgravity of
the gas.Comment: 15 pages, 19 figures (most in color), in press in Astronomy and
Astrophysic
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
A torque formula for non-isothermal Type I planetary migration - I. Unsaturated horseshoe drag
We study the torque on low-mass planets embedded in protoplanetary discs in
the two-dimensional approximation, incorporating non-isothermal effects. We
couple linear estimates of the Lindblad (or wave) torque to a simple, but
non-linear, model of adiabatic corotation torques (or horseshoe drag),
resulting in a simple formula that governs Type I migration in non-isothermal
discs. This formula should apply in optically thick regions of the disc, where
viscous and thermal diffusion act to keep the horseshoe drag unsaturated. We
check this formula against numerical hydrodynamical simulations, using three
independent numerical methods, and find good agreement.Comment: 17 pages, 17 figures, accepted for publication in MNRA
Disk Planet Interactions and Early Evolution in Young Planetary Systems
We study and review disk protoplanet interactions using local shearing box
simulations. These suffer the disadvantage of having potential artefacts
arising from periodic boundary conditions but the advantage, when compared to
global simulations, of being able to capture much of the dynamics close to the
protoplanet at high resolution for low computational cost. Cases with and
without self sustained MHD turbulence are considered. The conditions for gap
formation and the transition from type I migration are investigated and found
to depend on whether the single parameter M_p R^3/(M_* H^3), with M_p, M_*, R
and H being the protoplanet mass, the central mass, the orbital radius and the
disk semi-thickness respectively exceeds a number of order unity. We also
investigate the coorbital torques experienced by a moving protoplanet in an
inviscid disk. This is done by demonstrating the equivalence of the problem for
a moving protoplanet to one where the protoplanet is in a fixed orbit which the
disk material flows through radially as a result of the action of an
appropriate external torque. For sustainable coorbital torques to be realized a
quasi steady state must be realized in which the planet migrates through the
disk without accreting significant mass. In that case although there is
sensitivity to computational parameters, in agreement with earlier work by
Masset & Papaloizou (2003) based on global simulations, the coorbital torques
are proportional to the migration speed and result in a positive feedback on
the migration, enhancing it and potentially leading to a runaway. This could
lead to a fast migration for protoplanets in the Saturn mass range in massive
disks and may be relevant to the mass period correlation for extrasolar planets
which gives a preponderance of sub Jovian masses at short orbital period.Comment: To appear in Celestial Mechanics and Dynamical Astronomy (with higher
resolution figures
Do low surface brightness galaxies have dense disks?
The disk masses of four low surface brightness galaxies (LSB) were estimated
using marginal gravitational stability criterion and the stellar velocity
dispersion data which were taken from Pizzella et al., 2008 [1]. The
constructed mass models appear to be close to the models of maximal disk. The
results show that the disks of LSB galaxies may be significantly more massive
than it is usually accepted from their brightnesses. In this case their surface
densities and masses appear to be rather typical for normal spirals. Otherwise,
unlike the disks of many spiral galaxies, the LSB disks are dynamically
overheated.Comment: 14 pages, 10 figures, submitted to Astronomy Report
Ferromagnetism and large negative magnetoresistance in Pb doped Bi-Sr-Co-O misfit-layer compound
Ferromagnetism and accompanying large negative magnetoresistance in
Pb-substituted Bi-Sr-Co-O misfit-layer compound are investigated in detail.
Recent structural analysis of (Bi,Pb)SrCoO, which has
been believed to be a Co analogue of
BiSrCaCuO, revealed that it has a more complex
structure including a CoO hexagonal layer [T. Yamamoto {\it et al.}, Jpn.
J. Appl. Phys. {\bf 39} (2000) L747]. Pb substitution for Bi not only
introduces holes into the conducting CoO layers but also creates a
certain amount of localized spins. Ferromagnetic transition appears at =
3.2 K with small spontaneous magnetization along the axis, and around the
transition temperature large and anisotropic negative magnetoresistance was
observed. This compound is the first example which shows ferromagnetic
long-range order in a two-dimensional metallic hexagnonal CoO layer.Comment: 8 pages including eps figures. To be published in J. Phys. Soc. Jp
Metal insulator transition in TlSr2CoO5 from orbital degeneracy and spin disproportionation
To describe the metal insulator transition in the new oxide TlSr2CoO5 we
investigate its electronic structure by LDA and model Hartree-Fock
calculations. Within LDA we find a homogeneous metallic and ferromagnetic
ground state, but when including the Coulomb interaction more explicitly within
the Hartree-Fock approximation, we find an insulating state of lower energy
with both spin and orbital order. We also interpret our results in terms of a
simple model.Comment: 8 pages, 9 figure
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