372 research outputs found
On type-I migration near opacity transitions. A generalized Lindblad torque formula for planetary population synthesis
We give an expression for the Lindblad torque acting on a low-mass planet
embedded in a protoplanetary disk that is valid even at locations where the
surface density or temperature profile cannot be approximated by a power law,
such as an opacity transition. At such locations, the Lindblad torque is known
to suffer strong deviation from its standard value, with potentially important
implications for type I migration, but the full treatment of the tidal
interaction is cumbersome and not well suited to models of planetary population
synthesis. The expression that we propose retains the simplicity of the
standard Lindblad torque formula and gives results that accurately reproduce
those of numerical simulations, even at locations where the disk temperature
undergoes abrupt changes. Our study is conducted by means of customized
numerical simulations in the low-mass regime, in locally isothermal disks, and
compared to linear torque estimates obtained by summing fully analytic torque
estimates at each Lindblad resonance. The functional dependence of our modified
Lindblad torque expression is suggested by an estimate of the shift of the
Lindblad resonances that mostly contribute to the torque, in a disk with sharp
gradients of temperature or surface density, while the numerical coefficients
of the new terms are adjusted to seek agreement with numerics. As side results,
we find that the vortensity related corotation torque undergoes a boost at an
opacity transition that can counteract migration, and we find evidence from
numerical simulations that the linear corotation torque has a non-negligible
dependency upon the temperature gradient, in a locally isothermal disk.Comment: Appeared in special issue of "Celestial Mechanics and Dynamical
Astronomy" on Extrasolar Planetary System
Secular interactions between inclined planets and a gaseous disk
In a planetary system, a secular particle resonance occurs at a location
where the precession rate of a test particle (e.g. an asteroid) matches the
frequency of one of the precessional modes of the planetary system. We
investigate the secular interactions of a system of mutually inclined planets
with a gaseous protostellar disk that may contain a secular nodal particle
resonance. We determine the normal modes of some mutually inclined planet-disk
systems. The planets and disk interact gravitationally, and the disk is
internally subject to the effects of gas pressure, self-gravity, and turbulent
viscosity. The behavior of the disk at a secular resonance is radically
different from that of a particle, owing mainly to the effects of gas pressure.
The resonance is typically broadened by gas pressure to the extent that global
effects, including large-scale warps, dominate. The standard resonant torque
formula is invalid in this regime. Secular interactions cause a decay of the
inclination at a rate that depends on the disk properties, including its mass,
turbulent viscosity, and sound speed. For a Jupiter-mass planet embedded within
a minimum-mass solar nebula having typical parameters, dissipation within the
disk is sufficient to stabilize the system against tilt growth caused by
mean-motion resonances.Comment: 30 pages, 6 figures, to be published in The Astrophysical Journa
Predictive validity of WXYfm and SAIN,LIM food nutrient profiling models in the Whitehall II cohort
Background: Nutrient profiling (NP) aims to identify healthier food options according to the content of selected ‘positive’ nutrients e.g. fibre, protein, and ‘negative’ nutrients e.g. sodium, saturated fat. The British and French food safety agencies developed the WXYfm and SAIN,LIM models, respectively. Their predictive validity in relation to chronic disease has yet to be demonstrated. Aim: To test the hypothesis that ‘healthy’ diets as defined by NP have predictive validity. Methods: Between 1991-93, 7,251 participants of the Whitehall II study completed a 127-item food frequency questionnaire (FFQ). WXYfm and SAIN,LIM scores for each FFQ-item were used to derive energy-weighted aggregate diet scores (AS) for each participant and NP model. Validity was assessed against baseline factors including dietary quality indices. Prospective associations were examined with incident CHD, diabetes and cancer, and all-cause mortality (318, 754, 251, and 524 events, respectively—median follow-up time was approximately 17 years). Results: AS were weakly associated with dietary quality indices. Cox modelling identified U-shaped associations (p quadratic trend <.05) between both AS and all outcomes except diabetes. Participants with middle AS had slightly reduced risk; SAIN,LIM estimates were significant for CHD and all-cause mortality. Dietary misreporting, particularly of energy-dense foods with high ‘negative’ nutrient content, was associated with BMI, hypertension and other risk factors, and explained much of the unexpected U-shaped AS-outcome associations. Alternative AS less sensitive to dietary misreporting confirmed the potential of NP as a public health tool. In particular, the WXYfm ‘positive’ nutrients predicted risk reduction for all outcomes. Conclusions: Predictive validity of the NP approach was partly established. The prospective effects of AS on chronic disease outcomes were confounded by the association between vascular risk and energy misreporting. Further predictive validity studies of NP methods ideally require food-based dietary assessment (e.g. diet diaries, 24h recalls) with less reporting bias
Halting Type I planet migration in non-isothermal disks
Aims: We investigate the effect of including a proper energy balance on the
interaction of a low-mass planet with a protoplanetary disk. Methods: We use a
three-dimensional version of the RODEO method to perform hydrodynamical
simulations including the energy equation. Radiation is included in the
flux-limited diffusion approach. Results: The sign of the torque depends
sensitively on the ability of the disk to radiate away the energy generated in
the immediate surroundings of the planet. In the case of high opacity,
corresponding to the dense inner regions of protoplanetary disks, migration is
directed \emph{outward}, instead of the usual inward migration that was found
in locally isothermal disks. For low values of the opacity we recover inward
migration, and we show that torques originating in the coorbital region are
responsible for the change in migration direction.Comment: 4 pages, 5 figures, accepted for A&A letter
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 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 Parker Instability in 3-D: Corrugations and Superclouds Along the Carina-Sagittarius Arm
Here we present three-dimensional MHD models for the Parker instability in a
thick magnetized disk, including the presence of a spiral arm. The -field is
assumed parallel to the arm, and the model results are applied to the optical
segment of the Carina-Sagittarius arm. The characteristic features of the
undular and interchange modes are clearly apparent in the simulations. The
undular mode creates large gas concentrations distributed along the arm. This
results in a clear arm/inter-arm difference: the instability triggers the
formation of large interstellar clouds inside the arms, but generates only
small structures with slight density enhancements in the inter-arm regions. The
resulting clouds are distributed in an antisymmetric way with respect to the
midplane, creating an azimuthal corrugation along the arm. For conditions
similar to those of the optical segment of the Carina-Sagittarius arm, it has a
wavelength of about 2.4 kpc. This structuring can explain the origin of both HI
superclouds and the azimuthal corrugations in spiral arms. The wavelength
matches the corrugation length derived with the young stellar groups located in
the optical segment of the Carina-Sagittarius arm. Keywords: Galaxy: kinematics
and dynamics -- Galaxy: structure -- Instabilities -- ISM: clouds -- ISM:
magnetic fields -- ISM: structure -- MHDComment: 29 pages, 12 figures, Latex, Accepted by the Astrophysical Journa
Numerical simulations of the type III migration:I. Disc model and convergence tests
We investigate the fast (type III) migration regime of high-mass protoplanets
orbiting in protoplanetary disks. This type of migration is dominated by
corotational torques. We study the details of flow structure in the planet's
vicinity, the dependence of migration rate on the adopted disc model, and the
numerical convergence of models (independence of certain numerical parameters
such as gravitational softening). We use two-dimensional hydrodynamical
simulations with adaptive mesh refinement,based on the FLASH code with improved
time-stepping scheme. We perform global disk simulations with sufficient
resolution close to the planet, which is allowed to freely move throughout the
grid. We employ a new type of equation of state in which the gas temperature
depends on both the distance to the star and planet, and a simplified
correction for self-gravity of the circumplanetary gas. We find that the
migration rate in the type III migration regime depends strongly on the gas
dynamics inside the Hill sphere (Roche lobe of the planet) which, in turn, is
sensitive to the aspect ratio of the circumplanetary disc. Furthermore,
corrections due to the gas self-gravity are necessary to reduce numerical
artifacts that act against rapid planet migration. Reliable numerical studies
of Type III migration thus require consideration of both the thermal andthe
self-gravity corrections, as well as a sufficient spatial resolution and the
calculation of disk-planet attraction both inside and outside the Hill sphere.
With this proviso, we find Type III migration to be a robust mode of migration,
astrophysically promising because of a speed much faster than in the previously
studied modes of migration.Comment: 17 pages, 15 figures, submitted to MNRAS. Comments welcom
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
On the evolution of eccentric and inclined protoplanets embedded in protoplanetary disks
Young planets embedded in their protoplanetary disk interact gravitationally
with it leading to energy and angular momentum exchange. This interaction
determines the evolution of the planet through changes to the orbital
parameters. We investigate changes in the orbital elements of a 20 Earth--mass
planet due to the torques from the disk. We focus on the non-linear evolution
of initially non-vanishing eccentricity and/or inclination . We treat
the disk as a two- or three-dimensional viscous fluid and perform
hydrodynamical simulations with an embedded planet. We find rapid exponential
decay of the planet orbital eccentricity and inclination for small initial
values of and , in agreement with linear theory. For larger values of the decay time increases and the decay rate scales as , consistent with existing theoretical models. For large inclinations
( > 6 deg) the inclination decay rate shows an identical scaling . We find an interesting dependence of the migration on the
eccentricity. In a disk with aspect ratio the migration rate is
enhanced for small non-zero eccentricities (), while for larger values
we see a significant reduction by a factor of . We find no indication
for a reversal of the migration for large , although the torque experienced
by the planet becomes positive when . This inward migration is
caused by the persisting energy loss of the planet.
For non gap forming planets, eccentricity and inclination damping occurs on a
time scale that is very much shorter than the migration time scale. The results
of non linear hydrodynamic simulations are in very good agreement with linear
theory for small and .Comment: accepted for Astronomy & Astrophysics, 16 pages, 16 figures,
animations under:
http://www.tat.physik.uni-tuebingen.de/~kley/publ/paper/eccp.htm
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