471 research outputs found
The Two States of Star Forming Clouds
We examine the effects of self-gravity and magnetic fields on supersonic
turbulence in isothermal molecular clouds with high resolution simulations and
adaptive mesh refinement. These simulations use large root grids (512^3) to
capture turbulence and four levels of refinement to capture high density, for
an effective resolution of 8,196^3. Three Mach 9 simulations are performed, two
super-Alfv\'enic and one trans-Alfv\'enic. We find that gravity splits the
clouds into two populations, one low density turbulent state and one high
density collapsing state. The low density state exhibits properties similar to
non-self-gravitating in this regime, and we examine the effects of varied
magnetic field strength on statistical properties: the density probability
distribution function is approximately lognormal; velocity power spectral
slopes decrease with field strength; alignment between velocity and magnetic
field increases with field; the magnetic field probability distribution can be
fit to a stretched exponential. The high density state is characterized by
self-similar spheres; the density PDF is a power-law; collapse rate decreases
with increasing mean field; density power spectra have positive slopes,
P({\rho},k) \propto k; thermal-to-magnetic pressure ratios are unity for all
simulations; dynamic-to-magnetic pressure ratios are larger than unity for all
simulations; magnetic field distribution is a power-law. The high Alfv\'en Mach
numbers in collapsing regions explain recent observations of magnetic influence
decreasing with density. We also find that the high density state is found in
filaments formed by converging flows, consistent with recent Herschel
observations. Possible modifications to existing star formation theories are
explored.Comment: 19 pages, 20 figure
On the Fundamental Properties of Dynamically Hot Galaxies
A two-component isothermal equilibrium model is applied to reproduce basic
structural properties of dynamically hot stellar systems immersed in their
massive dark haloes. The origin of the fundamental plane relation for giant
ellipticals is naturally explained as a consequence of dynamical equilibrium in
the context of the model. The existence of two galactic families displaying
different behaviour in the luminosity--surface-brightness diagram is shown to
be a result of a smooth transition from dwarfs, dominated by dark matter near
the centre, to giants dominated by the luminous stellar component. The
comparison of empirical scaling relations with model predictions suggests that
probably a unique dissipative process was operating during the violent stage of
development of stellar systems in the dark haloes, and the depth of the
potential well controlled the observed luminosity of the resulting galaxies.
The interpretation also provides some restrictions on the properties of dark
haloes implied by the fundamental scaling laws.Comment: 9 pages, 7 PostScript figures, uses MNRaS LaTeX style file; accepted
for publication in MNRaS (Aug 1996
Pop. III stars from turbulent fragmentation at redshift ~ 11
We report results from a cosmological simulation with non-equilibrium
chemistry of 21 species, including H2, HD, and LiH molecular cooling. Starting
from cosmological initial conditions, we focus on the evolution of the central
1.8 Kpc region of a 3 x 10^7 Msun halo. The crossing of a few 10^6 Msun halos
and the gas accretion through larger scale filaments generate a turbulent
environment within this region. Due to the short cooling time caused by the
non-equilibrium formation of H2, the supersonic turbulence results in a very
fragmented mass distribution, where dense, gravitationally unstable clumps
emerge from a complex network of dense filaments. At z=10.87, we find
approximately 25 well defined, gravitationally unstable clumps, with masses of
4 x 10^3-9 x 10^5 Msun, temperatures of approximately 300K, and cooling times
much shorter than the free-fall time. Only the initial phase of the collapse of
individual clumps is spatially resolved in the simulation. Depending on the
density reached in the collapse, the estimated average Bonnor-Ebert masses are
in the range 200-800 Msun. We speculate that each clump may further fragment
into a cluster of stars with a characteristic mass in the neighborhood of 50
Msun. This process at z ~ 11 may represent the dominant mode of Pop. III star
formation, causing a rapid chemical enrichment of the protogalactic
environment
Interstellar Turbulence and Star Formation
We provide a brief overview of recent advances and outstanding issues in
simulations of interstellar turbulence, including isothermal models for
interior structure of molecular clouds and larger-scale multiphase models
designed to simulate the formation of molecular clouds. We show how
self-organization in highly compressible magnetized turbulence in the
multiphase ISM can be exploited in simple numerical models to generate
realistic initial conditions for star formation.Comment: 8 pages, 5 color figures; submitted to Proceedings of IAU Symposium
270 "Computational Star Formation" held in Barcelona, May 31 - June 4, 201
The Power Spectrum of Turbulence in NGC 1333: Outflows or Large-Scale Driving?
Is the turbulence in cluster-forming regions internally driven by stellar
outflows or the consequence of a large-scale turbulent cascade? We address this
question by studying the turbulent energy spectrum in NGC 1333. Using synthetic
13CO maps computed with a snapshot of a supersonic turbulence simulation, we
show that the VCS method of Lazarian and Pogosyan provides an accurate estimate
of the turbulent energy spectrum. We then apply this method to the 13CO map of
NGC 1333 from the COMPLETE database. We find the turbulent energy spectrum is a
power law, E(k) k^-beta, in the range of scales 0.06 pc < ell < 1.5 pc, with
slope beta=1.85\pm 0.04. The estimated energy injection scale of stellar
outflows in NGC 1333 is ell_inj 0.3 pc, well resolved by the observations.
There is no evidence of the flattening of the energy spectrum above the scale
ell_inj predicted by outflow-driven simulations and analytical models. The
power spectrum of integrated intensity is also a nearly perfect power law in
the range of scales 0.16 pc < ell < 7.9 pc, with no feature above ell_inj. We
conclude that the observed turbulence in NGC 1333 does not appear to be driven
primarily by stellar outflows.Comment: Submitted to APJ Letters on September 22, 2009 - Accepted on November
18, 200
Convective cores in galactic cooling flows
We use hydrodynamic simulations with adaptive grid refinement to study the
dependence of hot gas flows in X-ray luminous giant elliptical galaxies on the
efficiency of heat supply to the gas. We consider a number of potential heating
mechanisms including Type Ia supernovae and sporadic nuclear activity of a
central supermassive black hole. As a starting point for this research we use
an equilibrium hydrostatic recycling model (Kritsuk 1996). We show that a
compact cooling inflow develops, if the heating is slightly insufficient to
counterbalance radiative cooling of the hot gas in the central few kiloparsecs.
An excessive heating in the centre, instead, drives a convectively unstable
outflow. We model the onset of the instability and a quasi-steady convective
regime in the core of the galaxy in two-dimensions assuming axial symmetry.
Provided the power of net energy supply in the core is not too high, the
convection remains subsonic. The convective pattern is dominated by buoyancy
driven large-scale mushroom-like structures. Unlike in the case of a cooling
inflow, the X-ray surface brightness of an (on average) isentropic convective
core does not display a sharp maximum at the centre. A hybrid model, which
combines a subsonic peripheral cooling inflow with an inner convective core,
appears to be stable. We also discuss observational implications of these
results.Comment: 14 pages, LaTeX requires mn.sty, 12 postscript figures including 3
colour figures, MNRAS accepted; mpeg movies available from
http://www.mpa-garching.mpg.de/Hydro/CoolHyd/coolhyd.htm
The density variance -- Mach number relation in supersonic, isothermal turbulence
We examine the relation between the density variance and the mean-square Mach
number in supersonic, isothermal turbulence, assumed in several recent analytic
models of the star formation process. From a series of calculations of
supersonic, hydrodynamic turbulence driven using purely solenoidal Fourier
modes, we find that the `standard' relationship between the variance in the log
of density and the Mach number squared, i.e., sigma^2_(ln rho/rhobar)=ln (1+b^2
M^2), with b = 1/3 is a good fit to the numerical results in the supersonic
regime up to at least Mach 20, similar to previous determinations at lower Mach
numbers. While direct measurements of the variance in linear density are found
to be severely underestimated by finite resolution effects, it is possible to
infer the linear density variance via the assumption of log-normality in the
Probability Distribution Function. The inferred relationship with Mach number,
consistent with sigma_(rho/rhobar) ~ b M with b=1/3, is, however, significantly
shallower than observational determinations of the relationship in the Taurus
Molecular Cloud and IC5146 (both consistent with b~ 0.5), implying that
additional physics such as gravity is important in these clouds and/or that
turbulent driving in the ISM contains a significant compressive component.
Magnetic fields are not found to change this picture significantly, in general
reducing the measured variances and thus worsening the discrepancy with
observations.Comment: 5 pages, 4 figures, emulateapj. v2: accepted to ApJL, minor changes
onl
A Simple Law of Star Formation
We show that supersonic MHD turbulence yields a star formation rate (SFR) as
low as observed in molecular clouds (MCs), for characteristic values of the
free-fall time divided by the dynamical time, , the
alfv\'{e}nic Mach number, , and the sonic Mach number, . Using a very large set of deep adaptive-mesh-refinement
simulations, we quantify the dependence of the SFR per free-fall time,
, on the above parameters. Our main results are: i)
decreases exponentially with increasing , but is insensitive to changes in , for constant values
of and . ii) Decreasing values of
(stronger magnetic fields) reduce , but
only to a point, beyond which increases with a further
decrease of . iii) For values of
characteristic of star-forming regions, varies with by less than a factor of two. We propose a simple star-formation
law, based on the empirical fit to the minimum , and
depending only on : . Because it only
depends on the mean gas density and rms velocity, this law is straightforward
to implement in simulations and analytical models of galaxy formation and
evolution.Comment: ApJ Letters - in pres
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