15,211 research outputs found
Formation of the First Stars by Accretion
The process of star formation from metal-free gas is investigated by
following the evolution of accreting protostars with emphasis on the properties
of massive objects. The main aim is to establish the physical processes that
determine the upper mass limit of the first stars. Although the consensus is
that massive stars were commonly formed in the first cosmic structures, our
calculations show that their actual formation depends sensitively on the mass
accretion rate and its time variation. Even in the rather idealized case in
which star formation is mainly determined by dot{M}acc, the characteristic mass
scale of the first stars is rather uncertain. We find that there is a critical
mass accretion rate dot{M}crit = 4 10^{-3} Msun/yr that separates solutions
with dot{M}acc> 100 Msun can form,
provided there is sufficient matter in the parent clouds, from others
(dot{M}acc > dot{M}crit) where the maximum mass limit decreases as dot{M}acc
increases. In the latter case, the protostellar luminosity reaches the
Eddington limit before the onset of hydrogen burning at the center via the
CN-cycle. This phase is followed by a rapid and dramatic expansion of the
radius, possibly leading to reversal of the accretion flow when the stellar
mass is about 100Msun. (abridged)Comment: 34 pages, 12 figures. ApJ, in pres
Thermal and Fragmentation Properties of Star-forming Clouds in Low-metallicity Environments
The thermal and chemical evolution of star-forming clouds is studied for
different gas metallicities, Z, using the model of Omukai (2000), updated to
include deuterium chemistry and the effects of cosmic microwave background
(CMB) radiation. HD-line cooling dominates the thermal balance of clouds when Z
\~ 10^{-5}-10^{-3} Z_sun and density ~10^{5} cm^{-3}. Early on, CMB radiation
prevents the gas temperature to fall below T_CMB, although this hardly alters
the cloud thermal evolution in low-metallicity gas. From the derived
temperature evolution, we assess cloud/core fragmentation as a function of
metallicity from linear perturbation theory, which requires that the core
elongation E := (b-a)/a > E_NL ~ 1, where a (b) is the short (long) core axis
length. The fragment mass is given by the thermal Jeans mass at E = E_NL. Given
these assumptions and the initial (gaussian) distribution of E we compute the
fragment mass distribution as a function of metallicity. We find that: (i) For
Z=0, all fragments are very massive, > 10^{3}M_sun, consistently with previous
studies; (ii) for Z>10^{-6} Z_sun a few clumps go through an additional high
density (> 10^{10} cm^{-3}) fragmentation phase driven by dust-cooling, leading
to low-mass fragments; (iii) The mass fraction in low-mass fragments is
initially very small, but at Z ~ 10^{-5}Z_sun it becomes dominant and continues
to grow as Z is increased; (iv) as a result of the two fragmentation modes, a
bimodal mass distribution emerges in 0.01 0.1Z_sun,
the two peaks merge into a singly-peaked mass function which might be regarded
as the precursor of the ordinary Salpeter-like IMF.Comment: 38 pages, 16 figures, ApJ in pres
Dynamics of Dense Cores in the Perseus Molecular Cloud
We survey the kinematics of over one hundred and fifty candidate (and
potentially star-forming) dense cores in the Perseus molecular cloud with
pointed N2H+(1-0) and simultaneous C18O(2-1) observations. Our detection rate
of N2H+ is 62%, rising to 84% for JCMT SCUBA-selected targets. In agreement
with previous observations, we find that the dense N2H+ targets tend to display
nearly thermal linewidths, particularly those which appear to be starless
(using Spitzer data), indicating turbulent support on the small scales of
molecular clouds is minimal. For those N2H+ targets which have an associated
SCUBA dense core, we find their internal motions are more than sufficient to
provide support against the gravitational force on the cores. Comparison of the
N2H+ integrated intensity and SCUBA flux reveals fractional N2H+ abundances
between 10^-10 and 10^-9. We demonstrate that the relative motion of the dense
N2H+ gas and the surrounding C18O gas is less than the sound speed in the vast
majority of cases (~90%). The point-to-point motions we observe within larger
extinction regions appear to be insufficient to provide support against
gravity, although we sparsely sample these regions.Comment: 49 pages, 20 figures. Accepted for publication in the Astrophysical
Journa
Population polygons of tektite specific gravity for various localities in australasia
Comparison of specific gravity of tektites from australia, asia, texas, and czechoslovaki
Dark cloud cores and gravitational decoupling from turbulent flows
We test the hypothesis that the starless cores may be gravitationally bound
clouds supported largely by thermal pressure by comparing observed molecular
line spectra to theoretical spectra produced by a simulation that includes
hydrodynamics, radiative cooling, variable molecular abundance, and radiative
transfer in a simple one-dimensional model. The results suggest that the
starless cores can be divided into two categories: stable starless cores that
are in approximate equilibrium and will not evolve to form protostars, and
unstable pre-stellar cores that are proceeding toward gravitational collapse
and the formation of protostars. The starless cores might be formed from the
interstellar medium as objects at the lower end of the inertial cascade of
interstellar turbulence. Additionally, we identify a thermal instability in the
starless cores. Under par ticular conditions of density and mass, a core may be
unstable to expansion if the density is just above the critical density for the
collisional coupling of the gas and dust so that as the core expands the
gas-dust coupling that cools the gas is reduced and the gas warms, further
driving the expansion.Comment: Submitted to Ap
Evolution of Angular Momentum Distribution during Star Formation
If the angular momentum of the molecular cloud core were conserved during the
star formation process, a new-born star would rotate much faster than its
fission speed. This constitutes the angular momentum problem of new-born stars.
In this paper, the angular momentum transfer in the contraction of a rotating
magnetized cloud is studied with axisymmetric MHD simulations. Owing to the
large dynamic range covered by the nested-grid method, the structure of the
cloud in the range from 10 AU to 0.1 pc is explored. First, the cloud
experiences a run-away collapse, and a disk forms perpendicularly to the
magnetic field, in which the central density increases greatly in a finite
time-scale. In this phase, the specific angular momentum j of the disk
decreases to of the initial cloud. After the central density of
the disk exceeds , the infall on to the central
object develops. In this accretion stage, the rotation motion and thus the
toroidal magnetic field drive the outflow. The angular momentum of the central
object is transferred efficiently by the outflow as well as the effect of the
magnetic stress. In 7000 yr from the core formation, the specific angular
momentum of the central decreases a factor of 10^{-4} from the
initial value (i.e. from to ).Comment: 15 pages, 2 figures, Astrophysical Journal Letters in pres
Lattice Boltzmann simulations of a viscoelastic shear-thinning fluid
We present a hybrid lattice Boltzmann algorithm for the simulation of flow
glass-forming fluids, characterized by slow structural relaxation, at the level
of the Navier-Stokes equation. The fluid is described in terms of a nonlinear
integral constitutive equation, relating the stress tensor locally to the
history of flow. As an application, we present results for an integral
nonlinear Maxwell model that combines the effects of (linear) viscoelasticity
and (nonlinear) shear thinning. We discuss the transient dynamics of
velocities, shear stresses, and normal stress differences in planar
pressure-driven channel flow, after switching on (startup) and off (cessation)
of the driving pressure. This transient dynamics depends nontrivially on the
channel width due to an interplay between hydrodynamic momentum diffusion and
slow structural relaxation
Defect-Mediated Emulsification in Two Dimensions
We consider two dimensional dispersions of droplets of isotropic phase in a
liquid with an XY-like order parameter, tilt, nematic, and hexatic symmetries
being included. Strong anchoring boundary conditions are assumed. Textures for
a single droplet and a pair of droplets are calculated and a universal
droplet-droplet pair potential is obtained. The interaction of dispersed
droplets via the ordered phase is attractive at large distances and repulsive
at short distances, which results in a well defined preferred separation for
two droplets and topological stabilization of the emulsion. This interaction
also drives self-assembly into chains. Preferred separations and energy
barriers to coalescence are calculated, and effects of thermal fluctuations and
film thickness are discussed.Comment: revtex4, 13 pages, 12 figure
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