106 research outputs found
Stellar Velocity Distribution in Galactic Disks
We present numerical studies of the properties of the stellar velocity
distribution in galactic disks which have developed a saturated, two-armed
spiral structure. In previous papers we used the Boltzmann moment equations
(BME) up to second order for our studies of the velocity structure in
self-gravitating stellar disks. A key assumption of our BME approach is the
zero-heat flux approximation, i.e. the neglection of third order velocity
terms. We tested this assumption by performing test particle simulations for
stars in a disk galaxy subject to a rotating spiral perturbation. As a result
we corroborated qualitatively the complex velocity structure found in the BME
approach. It turned out that an equilibrium configuration in velocity space is
only slowly established on a typical timescale of 5 Gyrs or more. Since many
dynamical processes in galaxies (like the growth of spirals or bars)act on
shorter timescales, pure equilibrium models might not be fully appropriate for
a detailed comparison with observations like the local Galactic velocity
distribution. Third order velocity moments were typically small and
uncorrelated over almost all of the disk with the exception of the 4:1
resonance region (UHR). Near the UHR (normalized) fourth and fifth order
velocity moments are still of the same order as the second and third order
terms. Thus, at the UHR higher order terms are not negligible.Comment: 8 pages, 3 figures, to appear in the Proceedings of "Chaos in
Astronomy", Athens, G. Contopoulos & P.A. Patsis (eds.
Improving the thin-disk models of circumstellar disk evolution. The 2+1-dimensional model
Circumstellar disks of gas and dust are naturally formed from contracting
pre-stellar molecular cores during the star formation process. To study various
dynamical and chemical processes that take place in circumstellar disks prior
to their dissipation and transition to debris disks, the appropriate numerical
models capable of studying the long-term disk chemodynamical evolution are
required. We present a new 2+1-dimensional numerical hydrodynamics model of
circumstellar disk evolution, in which the thin-disk model is complemented with
the procedure for calculating the vertical distributions of gas volume density
and temperature in the disk. The reconstruction of the disk vertical structure
is performed at every time step via the solution of the time-dependent
radiative transfer equations coupled to the equation of the vertical
hydrostatic equilibrium. We perform a detailed comparison between circumstellar
disks produced with our previous 2D model and with the improved 2+1D approach.
The structure and evolution of resulting disks, including the differences in
temperatures, densities, disk masses and protostellar accretion rates, are
discussed in detail. The new 2+1D model yields systematically colder disks,
while the in-falling parental clouds are warmer. Both effects act to increase
the strength of disk gravitational instability and, as a result, the number of
gravitationally bound fragments that form in the disk via gravitational
fragmentation as compared to the purely 2D thin-disk simulations with a
simplified thermal balance calculation.Comment: Accepted for publication in Astronomy & Astrophysic
Lifetime of the embedded phase of low-mass star formation and the envelope depletion rates
Motivated by a considerable scatter in the observationally inferred lifetimes
of the embedded phase of star formation, we study the duration of the Class 0
and Class I phases in upper-mass brown dwarfs and low-mass stars using
numerical hydrodynamics simulations of the gravitational collapse of a large
sample of cloud cores. We resolve the formation of a star/disk/envelope system
and extend our numerical simulations to the late accretion phase when the
envelope is nearly totally depleted of matter. We adopted a classification
scheme of Andre et al. and calculate the lifetimes of the Class 0 and Class I
phases (\tau_C0 and \tau_CI, respectively) based on the mass remaining in the
envelope. When cloud cores with various rotation rates, masses, and sizes (but
identical otherwise) are considered, our modeling reveals a sub-linear
correlation between the Class 0 lifetimes and stellar masses in the Class 0
phase with the least-squares fit exponent m=0.8 \pm 0.05. The corresponding
correlation between the Class I lifetimes and stellar masses in the Class I is
super-linear with m=1.2 \pm 0.05. If a wider sample of cloud cores is
considered, which includes possible variations in the initial gas temperature,
cloud core truncation radii, density enhancement amplitudes, initial gas
density and angular velocity profiles, and magnetic fields, then the
corresponding exponents may decrease by as much as 0.3. The duration of the
Class I phase is found to be longer than that of the Class~0 phase in most
models, with a mean ratio \tau_CI / \tau_C0 \approx 1.5--2. A notable exception
are YSOs that form from cloud cores with large initial density enhancements, in
which case \tau_C0 may be greater than \tau_CI. Moreover, the upper-mass (>=
1.0 Msun) cloud cores with frozen-in magnetic fields and high cloud core
rotation rates may have the \tau_CI / \tau_C0 ratios as large as 3.0--4.0.
(Abdridged).Comment: Accepted for publication by The Astrophysical Journa
A Hybrid Scenario for the Formation of Brown Dwarfs and Very Low Mass Stars
We present a calculation of protostellar disk formation and evolution in
which gaseous clumps (essentially, the first Larson cores formed via disk
fragmentation) are ejected from the disk during the early stage of evolution.
This is a universal process related to the phenomenon of ejection in multiple
systems of point masses. However, it occurs in our model entirely due to the
interaction of compact, gravitationally-bound gaseous clumps and is free from
the smoothing-length uncertainty that is characteristic of models using sink
particles. Clumps that survive ejection span a mass range of 0.08--0.35
, and have ejection velocities km s, which are
several times greater than the escape speed. We suggest that, upon contraction,
these clumps can form substellar or low-mass stellar objects with notable
disks, or even close-separation very-low-mass binaries. In this hybrid
scenario, allowing for ejection of clumps rather than finished
protostars/proto--brown-dwarfs, disk formation and the low velocity dispersion
of low-mass objects are naturally explained, while it is also consistent with
the observation of isolated low-mass clumps that are ejection products. We
conclude that clump ejection and the formation of isolated low mass stellar and
substellar objects is a common occurrence, with important implications for
understanding the initial mass function, the brown dwarf desert, and the
formation of stars in all environments and epochs.Comment: 20 pages, 6 figures, to appear in The Astrophysical Journa
Stellar hydrodynamical modeling of dwarf galaxies: simulation methodology, tests, and first results
Cosmological simulations still lack numerical resolution or physical
processes to simulate dwarf galaxies in sufficient details. Accurate numerical
simulations of individual dwarf galaxies are thus still in demand. We aim at
(i) studying in detail the coupling between stars and gas in a galaxy,
exploiting the so-called stellar hydrodynamical approach, and (ii) studying the
chemo-dynamical evolution of individual galaxies starting from
self-consistently calculated initial gas distributions. We present a novel
chemo-dynamical code in which the dynamics of gas is computed using the usual
hydrodynamics equations, while the dynamics of stars is described by the
stellar hydrodynamics approach, which solves for the first three moments of the
collisionless Boltzmann equation. The feedback from stellar winds and dying
stars is followed in detail. In particular, a novel and detailed approach has
been developed to trace the aging of various stellar populations, which enables
an accurate calculation of the stellar feedback depending on the stellar age.
We build initial equilibrium models of dwarf galaxies that take gas
self-gravity into account and present different levels of rotational support.
Models with high rotational support develop prominent bipolar outflows; a
newly-born stellar population in these models is preferentially concentrated to
the galactic midplane. Models with little rotational support blow away a large
fraction of the gas and the resulting stellar distribution is extended and
diffuse. The stellar dynamics turns out to be a crucial aspect of galaxy
evolution. If we artificially suppress stellar dynamics, supernova explosions
occur in a medium heated and diluted by the previous activity of stellar winds,
thus artificially enhancing the stellar feedback (abridged).Comment: 22 pages, 19 figures, accepted for publication in Astronomy &
Astrophysic
The Ejection of Low Mass Clumps During Star Formation
Modeling of the self-consistent formation and evolution of disks as a result
of prestellar core collapse reveals an intense early phase of recurrent
gravitational instability and clump formation. These clumps generally migrate
inward due to gravitational interaction with trailing spiral arms, and can be
absorbed into the central object. However, in situations of multiple clump
formation, gravitational scattering of clumps can result in the ejection of a
low mass clump. These clumps can then give rise to free-floating low mass
stars, brown dwarfs, or even giant planets. Detailed modeling of this process
in the context of present-day star formation reveals that these clumps start
out essentially as Larson first cores and grow subsequently by accretion. In
the context of Pop III star formation, preliminary indications are that the
disk clumps may also be of low mass. This mechanism of clump formation and
possible ejection provides a channel for the formation of low mass objects in
the first generation of stars.Comment: 4 pages, 2 figures, to appear in proceedings of First Stars IV
meeting (Kyoto, Japan; 2012
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