30 research outputs found
Models of the formation of the planets in the 47 UMa system
Formation of planets in the 47 UMa system is followed in an evolving
protoplanetary disk composed of gas and solids. The evolution of the disk is
calculated from an early stage, when all solids, assumed to be high-temperature
silicates, are in the dust form, to the stage when most solids are locked in
planetesimals. The simulation of planetary evolution starts with a solid embryo
of ~1 Earth mass, and proceeds according to the core accretion -- gas capture
model. Orbital parameters are kept constant, and it is assumed that the
environment of each planet is not perturbed by the second planet. It is found
that conditions suitable for both planets to form within several Myr are easily
created, and maintained throughout the formation time, in disks with . In such disks, a planet of 2.6 Jupiter masses (the minimum for
the inner planet of the 47 UMa system) may be formed at 2.1 AU from the star in
\~3 Myr, while a planet of 0.89 Jupiter masses (the minimum for the outer
planet) may be formed at 3.95 AU from the star in about the same time. The
formation of planets is possible as a result of a significant enhancement of
the surface density of solids between 1.0 and 4.0 AU, which results from the
evolution of a disk with an initially uniform gas-to-dust ratio of 167 and an
initial radius of 40 AU.Comment: Accepted for publication in A&A. 10 pages, 10 figure
Interaction of massive stars with their surroundings
Due to their short lifetimes but their enormous energy release in all stages
of their lives massive stars are the major engines for the comic matter
circuit. They affect not only their close environment but are also responsible
to drive mass flows on galactic scales. Recent 2D models of radiation-driven
and wind-blown HII regions are summarized which explore the impact of massive
stars to the interstellar medium but find surprisingly small energy transfer
efficiencies while an observable Carbon self-enrichment in the Wolf-Rayet phase
is detected in the warm ionized gas. Finally, the focus is set on
state-of-the-art modelling of HII regions and its present weaknesses with
respect to uncertainties and simplifications but on a perspective of the
requested art of their modelling in the 21st century.Comment: 7 pages, 3 fig.s, to be published in IAU Symp. No. 252, "The art of
modelling stars in the 21st century", L. Deng & K.L. Chang (eds.), 2008,
invited tal
Formation of giant planets around stars with various masses
We examine the predictions of the core accretion - gas capture model
concerning the efficiency of planet formation around stars with various masses.
First, we follow the evolution of gas and solids from the moment when all
solids are in the form of small grains to the stage when most of them are in
the form of planetesimals. We show that the surface density of the planetesimal
swarm tends to be higher around less massive stars. Then, we derive the minimum
surface density of the planetesimal swarm required for the formation of a giant
planet both in a numerical and in an approximate analytical approach. We
combine these results by calculating a set of representative disk models
characterized by different masses, sizes, and metallicities, and by estimating
their capability of forming giant planets. Our results show that the set of
protoplanetary disks capable of giant planet formation is larger for less
massive stars. Provided that the distribution of initial disk parameters does
not depend too strongly on the mass of the central star, we predict that the
percentage of stars with giant planets should increase with decreasing stellar
mass. Furthermore, we identify the radial redistribution of solids during the
formation of planetesimal swarms as the key element in explaining these
effects.Comment: Accepted for publication in A&A. 9 pages, 9 figure
Formation of giant planets in disks with different metallicities
We present the first results from simulations of processes leading to planet
formation in protoplanetary disks with different metallicities. For a given
metallicity, we construct a two-dimensional grid of disk models with different
initial masses and radii (, ). For each disk, we follow the evolution
of gas and solids from an early evolutionary stage, when all solids are in the
form of small dust grains, to the stage when most solids have condensed into
planetesimals. Then, based on the core accretion - gas capture scenario, we
estimate the planet-bearing capability of the environment defined by the final
planetesimal swarm and the still evolving gaseous component of the disk. We
define the probability of planet-formation, , as the normalized fractional
area in the (, ) plane populated by disks that have formed
planets inside 5 AU. With such a definition, and under the assumption that the
population of planets discovered at 5 AU is not significantly
contaminated by planets that have migrated from 5 AU, our results agree
fairly well with the observed dependence between the probability that a star
harbors a planet and the star's metal content. The agreement holds for the disk
viscosity parameter ranging from to , and it
becomes much poorer when the redistribution of solids relative to the gas is
not allowed for during the evolution of model disks.Comment: Accepted for publication in A&A. 6 pages, 6 figure
The anomalous accretion disk of the Cataclysmic Variable RW Sextantis
Synthetic spectra covering the wavelength range 900\AA~to 3000\AA~provide an
accurate fit, established by a analysis, to a combined
observed spectrum of RW Sextantis. Two separately calibrated distances to the
system establish the synthetic spectrum comparison on an absolute flux basis
but with two alternative scaling factors, requiring alternative values of
for final models. Based on comparisons for a range of
values, the observed spectrum does not follow the standard model. Rather than
the exponent 0.25 in the expression for the radial temperature profile, a value
close to 0.125 produces a synthetic spectrum with an accurate fit to the
combined spectrum. A study of time-series spectra shows that a proposed
warped or tilted disk is not supported by the data; an alternative proposal is
that an observed non-axisymmetric wind results from an interaction with the
mass transfer stream debris.Comment: 56 pages, 15 figures, 11 tables. Accepted for The Astrophysical
Journa
Reconstructing Cosmic Peculiar Velocities from the Mildly Nonlinear Density Field
We present a numerical study of the cosmic density vs. velocity divergence
relation (DVDR) in the mildly non-linear regime. We approximate the dark matter
as a non-relativistic pressureless fluid, and solve its equations of motion on
a grid fixed in comoving coordinates. Unlike N-body schemes, this method yields
directly the volume-averaged velocity field. The results of our simulations are
compared with the predictions of the third-order perturbation theory (3PT) for
the DVDR. We investigate both the mean `forward' relation (density in terms of
velocity divergence) and the mean `inverse' relation (velocity divergence in
terms of density), with emphasis on the latter. On scales larger than about 20
megaparsecs, our code recovers the predictions of 3PT remarkably well,
significantly better than recent N-body simulations. On scales of a few
megaparsecs, the DVDR predicted by 3PT differs slightly from the simulated one.
In particular, approximating the inverse DVDR by a third-order polynomial turns
out to be a poor fit. We propose a simple analytical description of the inverse
relation, which works well for mildly non-linear scales.Comment: 9 pages, 7 figures (9 ps files), mn.st
An alternative look at the snowline in protoplanetary disks
We have calculated an evolution of protoplanetary disk from an extensive set
of initial conditions using a time-dependent model capable of simultaneously
keeping track of the global evolution of gas and water-ice. A number of
simplifications and idealizations allows for an embodiment of gas-particle
coupling, coagulation, sedimentation, and evaporation/condensation processes.
We have shown that, when the evolution of ice is explicitly included, the
location of the snowline has to be calculated directly as the inner edge of the
region where ice is present and not as the radius where disk's temperature
equals the evaporation temperature of water-ice. The final location of the
snowline is set by an interplay between all involved processes and is farther
from the star than implied by the location of the evaporation temperature
radius. The evolution process naturally leads to an order of magnitude
enhancement in surface density of icy material.Comment: Accepted for publication in A&A. 8 pages, 4 figure
Evolution of gaseous disk viscosity driven by supernova explosion. II. Structure and emissions from star-forming galaxies at high redshift
(Abridged) High redshift galaxies are undergoing intensive evolution of
dynamical structure and morphologies. We incorporate the feedback into the
dynamical equations through mass dropout and angular momentum transportation
driven by the SNexp-excited turbulent viscosity. We numerically solve the
equations and show that there can be intensive evolution of structure of the
gaseous disk. Secular evolution of the disk shows interesting characteristics
that are 1) high viscosity excited by SNexp can efficiently transport the gas
from 10kpc to kpc forming a stellar disk whereas a stellar ring forms
for the case with low viscosity; 2) starbursts trigger SMBH activity with a lag
yr depending on star formation rates, prompting the joint evolution
of SMBHs and bulges; 3) the velocity dispersion is as high as \sim 100~\kms
in the gaseous disk. In order to compare the present models with the observed
dynamical structure and images, we use the incident continuum from the simple
stellar synthesis (GALAXEV) and CLOUDY to calculate emission line ratios of
H, H, \OIII and \NII, and H brightness of gas
photoionized by young massive stars formed on the disks. The models can produce
the main features of emission from star forming galaxies and the observed
relation between turbulent velocity and the H brightness. We
successfully apply the present model to BX 389 and BX 482 observed in SINS
high sample, which are bulge and disk-dominated, respectively. High
viscosity excited by SNexp is able to efficiently transport the gas into a
bulge to maintain high star formation rates, or, to form a stellar ring close
enough to the bulge so that it immigrates into the bulge of its host galaxy.
This leads to a fast growing bulge. Implications and future work of the present
models have been extensively discussed for galaxy formation.Comment: Accepted by ApJ; 22 page in emulateapj, 16 color figure
2-D models of layered protoplanetary discs: I. The ring instability
In this work we use the radiation hydrodynamic code TRAMP to perform a
two-dimensional axially symmetric model of the layered disc. Using this model
we follow the accumulation of mass in the dead zone due to the radially varying
accretion rate. We found a new type of instability which causes the dead zone
to split into rings. This "ring instability" works due to the positive feedback
between the thickness of the dead zone and the mass accumulation rate.
We give an analytical description of this instability, taking into account
non-zero thickness of the dead zone and deviations from the Keplerian
rotational velocity. The analytical model agrees reasonably well with results
of numerical simulations. Finally, we speculate about the possible role of the
ring instability in protoplanetary discs and in the formation of planets.Comment: 9 pages, 5 figures, accepted for publication in MNRA