30 research outputs found
Collapse of Primordial Filamentary Clouds under Far-Ultraviolet Radiation
Collapse and fragmentation of primordial filamentary clouds under isotropic
dissociation radiation is investigated with one-dimensional hydrodynamical
calculations. We investigate the effect of dissociation photon on the
filamentary clouds with calculating non-equilibrium chemical reactions. With
the external radiation assumed to turn on when the filamentary cloud forms, the
filamentary cloud with low initial density ()
suffers photodissociation of hydrogen molecules. In such a case, since main
coolant is lost, temperature increases adiabatically enough to suppress
collapse. As a result, the filamentary cloud fragments into very massive clouds
(). On the other hand, the evolution of the filamentary
clouds with high initial density () is hardly
affected by the external radiation. This is because the filamentary cloud with
high initial density shields itself from the external radiation. It is found
that the external radiation increases fragment mass. This result is consistent
with previous results with one-zone models. It is also found that fragment mass
decreases owing to the external dissociation radiation in the case with
sufficiently large line mass.Comment: 26 pages, 15 figures, accepted by PAS
Dust-cooling--induced Fragmentation of Low-metallicity Clouds
Dynamical collapse and fragmentation of low-metallicity cloud cores is
studied using three-dimensional hydrodynamical calculations, with particular
attention devoted whether the cores fragment in the dust-cooling phase or not.
The cores become elongated in this phase, being unstable to non-spherical
perturbation due to the sudden temperature decrease. In the metallicity range
of 10^{-6}-10^{-5}Z_sun, cores with an initial axis ratio >2 reach a critical
value of the axis ratio (>30) and fragment into multiple small clumps. This
provides a possible mechanism to produce low-mass stars in ultra-metal-poor
environments.Comment: 4 pages, 3 figures, ApJ Letters 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
Critical phenomena in Newtonian gravity
We investigate the stability of self-similar solutions for a gravitationally
collapsing isothermal sphere in Newtonian gravity by means of a normal mode
analysis. It is found that the Hunter series of solutions are highly unstable,
while neither the Larson-Penston solution nor the homogeneous collapse one have
an analytic unstable mode. Since the homogeneous collapse solution is known to
suffer the kink instability, the present result and recent numerical
simulations strongly support a proposition that the Larson-Penston solution
will be realized in astrophysical situations. It is also found that the Hunter
(A) solution has a single unstable mode, which implies that it is a critical
solution associated with some critical phenomena which are analogous to those
in general relativity. The critical exponent is calculated as
. In contrast to the general relativistic case, the order
parameter will be the collapsed mass. In order to obtain a complete picture of
the Newtonian critical phenomena, full numerical simulations will be needed.Comment: 25 pages, 7 figures, accepted for publication in Physical Review
No Go Theorem for Kinematic Self-Similarity with A Polytropic Equation of State
We have investigated spherically symmetric spacetimes which contain a perfect
fluid obeying the polytropic equation of state and admit a kinematic
self-similar vector of the second kind which is neither parallel nor orthogonal
to the fluid flow. We have assumed two kinds of polytropic equations of state
and shown in general relativity that such spacetimes must be vacuum.Comment: 5 pages, no figures. Revtex. One word added to the title. Final
version to appear in Physical Review D as a Brief Repor
Criticality and convergence in Newtonian collapse
We study through numerical simulation the spherical collapse of isothermal
gas in Newtonian gravity. We observe a critical behavior which occurs at the
threshold of gravitational instability leading to core formation. For a given
initial density profile, we find a critical temperature, which is of the same
order as the virial temperature of the initial configuration. For the exact
critical temperature, the collapse converges to a self-similar form, the first
member in Hunter's family of self-similar solutions. For a temperature close to
the critical value, the collapse first approaches this critical solution. Later
on, in the supercritical case, the collapse converges to another self-similar
solution, which is called the Larson-Penston solution. In the subcritical case,
the gas bounces and disperses to infinity. We find two scaling laws: one for
the collapsed mass in the supercritical case and the other for the maximum
density reached before dispersal in the subcritical case. The value of the
critical exponent is measured to be in the supercritical case,
which agrees well with the predicted value . These critical
properties are quite similar to those observed in the collapse of a radiation
fluid in general relativity. We study the response of the system to temperature
fluctuation and discuss astrophysical implications for the insterstellar medium
structure and for the star formation process. Newtonian critical behavior is
important not only because it provides a simple model for general relativity
but also because it is relevant for astrophysical systems such as molecular
clouds.Comment: 15 pages, 8 figures, accepted for publication in PRD, figures 1 and 3
at lower resolution than in journal version, typos correcte
Convergence to a self-similar solution in general relativistic gravitational collapse
We study the spherical collapse of a perfect fluid with an equation of state
by full general relativistic numerical simulations. For 0, it has been known that there exists a general relativistic counterpart
of the Larson-Penston self-similar Newtonian solution. The numerical
simulations strongly suggest that, in the neighborhood of the center, generic
collapse converges to this solution in an approach to a singularity and that
self-similar solutions other than this solution, including a ``critical
solution'' in the black hole critical behavior, are relevant only when the
parameters which parametrize initial data are fine-tuned. This result is
supported by a mode analysis on the pertinent self-similar solutions. Since a
naked singularity forms in the general relativistic Larson-Penston solution for
0, this will be the most serious known counterexample against
cosmic censorship. It also provides strong evidence for the self-similarity
hypothesis in general relativistic gravitational collapse. The direct
consequence is that critical phenomena will be observed in the collapse of
isothermal gas in Newton gravity, and the critical exponent will be
given by , though the order parameter cannot be the black
hole mass.Comment: 22 pages, 15 figures, accepted for publication in Physical Review D,
reference added, typos correcte
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Gravitating discs around black holes
Fluid discs and tori around black holes are discussed within different
approaches and with the emphasis on the role of disc gravity. First reviewed
are the prospects of investigating the gravitational field of a black
hole--disc system by analytical solutions of stationary, axially symmetric
Einstein's equations. Then, more detailed considerations are focused to middle
and outer parts of extended disc-like configurations where relativistic effects
are small and the Newtonian description is adequate.
Within general relativity, only a static case has been analysed in detail.
Results are often very inspiring, however, simplifying assumptions must be
imposed: ad hoc profiles of the disc density are commonly assumed and the
effects of frame-dragging and completely lacking. Astrophysical discs (e.g.
accretion discs in active galactic nuclei) typically extend far beyond the
relativistic domain and are fairly diluted. However, self-gravity is still
essential for their structure and evolution, as well as for their radiation
emission and the impact on the environment around. For example, a nuclear star
cluster in a galactic centre may bear various imprints of mutual star--disc
interactions, which can be recognised in observational properties, such as the
relation between the central mass and stellar velocity dispersion.Comment: Accepted for publication in CQG; high-resolution figures will be
available from http://www.iop.org/EJ/journal/CQ
The stellar content of the Hamburg/ESO survey. V. The metallicity distribution function of the Galactic halo
We determine the metallicity distribution function (MDF) of the Galactic halo
by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO
objective-prism survey (HES). The sample was corrected for minor biases
introduced by the strategy for spectroscopic follow-up observations of the
metal-poor candidates, namely "best and brightest stars first". [...] We
determined the selection function of the HES, which must be taken into account
for a proper comparison between the HES MDF with MDFs of other stellar
populations or those predicted by models of Galactic chemical evolution. The
latter show a reasonable agreement with the overall shape of the HES MDF for
[Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical
metallicity for low-mass star formation of Z_cr = 10^{-3.4} * Z_Sun reproduces
the sharp drop at [Fe/H] ~-3.6 present in the HES MDF. [...] A comparison of
the MDF of Galactic globular clusters and of dSph satellites to the Galaxy
shows qualitative agreement with the halo MDF, derived from the HES, once the
selection function of the latter is included. However, statistical tests show
that the differences between these are still highly significant. [ABSTRACT
ABRIDGED]Comment: Accepted for publication in A&