3,201 research outputs found
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
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
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
Towards deterministic subspace identification for autonomous nonlinear systems
The problem of identifying deterministic autonomous linear and nonlinear systems is studied. A specific version of the theory of deterministic subspace identification for discrete-time autonomous linear systems is developed in continuous time. By combining the subspace approach to linear identification and the differential-geometric approach to nonlinear control systems, a novel identification framework for continuous-time autonomous nonlinear systems is developed
Mass and Magnetic distributions in Self Gravitating Super Alfvenic Turbulence with AMR
In this work, we present the mass and magnetic distributions found in a
recent Adaptive Mesh Refinement (AMR) MHD simulation of supersonic, \sa, self
gravitating turbulence. Powerlaw tails are found in both volume density and
magnetic field probability density functions, with and . A power law is also found between
magnetic field strength and density, with , throughout
the collapsing gas. The mass distribution of gravitationally bound cores is
shown to be in excellent agreement with recent observation of prestellar cores.
The mass to flux distribution of cores is also found to be in excellent
agreement with recent Zeeman splitting measurements.Comment: 9 pages, 10 figures (3 color). Submitted to the Astrophysical Journa
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
The observable prestellar phase of the IMF
The observed similarities between the mass function of prestellar cores (CMF)
and the stellar initial mass function (IMF) have led to the suggestion that the
IMF is already largely determined in the gas phase. However, theoretical
arguments show that the CMF may differ significantly from the IMF. In this
Letter, we study the relation between the CMF and the IMF, as predicted by the
IMF model of Padoan and Nordlund. We show that 1) the observed mass of
prestellar cores is on average a few times smaller than that of the stellar
systems they generate; 2) the CMF rises monotonically with decreasing mass,
with a noticeable change in slope at approximately 3-5 solar masses, depending
on mean density; 3) the selection of cores with masses larger than half their
Bonnor-Ebert mass yields a CMF approximately consistent with the system IMF,
rescaled in mass by the same factor as our model IMF, and therefore suitable to
estimate the local efficiency of star formation, and to study the dependence of
the IMF peak on cloud properties; 4) only one in five pre-brown-dwarf core
candidates is a true progenitor to a brown dwarf.Comment: ApJ Letters, accepte
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