818 research outputs found
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
Statistical Modeling of Spatial Extremes
The areal modeling of the extremes of a natural process such as rainfall or
temperature is important in environmental statistics; for example,
understanding extreme areal rainfall is crucial in flood protection. This
article reviews recent progress in the statistical modeling of spatial
extremes, starting with sketches of the necessary elements of extreme value
statistics and geostatistics. The main types of statistical models thus far
proposed, based on latent variables, on copulas and on spatial max-stable
processes, are described and then are compared by application to a data set on
rainfall in Switzerland. Whereas latent variable modeling allows a better fit
to marginal distributions, it fits the joint distributions of extremes poorly,
so appropriately-chosen copula or max-stable models seem essential for
successful spatial modeling of extremes.Comment: Published in at http://dx.doi.org/10.1214/11-STS376 the Statistical
Science (http://www.imstat.org/sts/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Rejoinder to "Statistical Modeling of Spatial Extremes"
Rejoinder to "Statistical Modeling of Spatial Extremes" by A. C. Davison, S.
A. Padoan and M. Ribatet [arXiv:1208.3378].Comment: Published in at http://dx.doi.org/10.1214/12-STS376REJ the
Statistical Science (http://www.imstat.org/sts/) by the Institute of
Mathematical Statistics (http://www.imstat.org
Accuracy of core mass estimates in simulated observations of dust emission
We study the reliability of mass estimates obtained for molecular cloud cores
using sub-millimetre and infrared dust emission. We use magnetohydrodynamic
simulations and radiative transfer to produce synthetic observations with
spatial resolution and noise levels typical of Herschel surveys. We estimate
dust colour temperatures using different pairs of intensities, calculate column
densities and compare the estimated masses with the true values. We compare
these results to the case when all five Herschel wavelengths are available. We
investigate the effects of spatial variations of dust properties and the
influence of embedded heating sources. Wrong assumptions of dust opacity and
its spectral index beta can cause significant systematic errors in mass
estimates. These are mainly multiplicative and leave the slope of the mass
spectrum intact, unless cores with very high optical depth are included.
Temperature variations bias colour temperature estimates and, in quiescent
cores with optical depths higher than for normal stable cores, masses can be
underestimated by up to one order of magnitude. When heated by internal
radiation sources the observations recover the true mass spectra. The shape,
although not the position, of the mass spectrum is reliable against
observational errors and biases introduced in the analysis. This changes only
if the cores have optical depths much higher than expected for basic
hydrostatic equilibrium conditions. Observations underestimate the value of
beta whenever there are temperature variations along the line of sight. A bias
can also be observed when the true beta varies with wavelength. Internal
heating sources produce an inverse correlation between colour temperature and
beta that may be difficult to separate from any intrinsic beta(T) relation of
the dust grains. This suggests caution when interpreting the observed mass
spectra and the spectral indices.Comment: Revised version, 17 pages, 17 figures, submitted to A&
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
The Density Variance Mach Number Relation in the Taurus Molecular Cloud
Supersonic turbulence in molecular clouds is a key agent in generating
density enhancements that may subsequently go on to form stars. The stronger
the turbulence - the higher the Mach number - the more extreme the density
fluctuations are expected to be. Numerical models predict an increase in
density variance with rms Mach number of the form: sigma^{2}_{rho/rho_{0}} =
b^{2}M^{2}, where b is a numerically-estimated parameter, and this prediction
forms the basis of a large number of analytic models of star formation. We
provide an estimate of the parameter b from 13CO J=1-0 spectral line imaging
observations and extinction mapping of the Taurus molecular cloud, using a
recently developed technique that needs information contained solely in the
projected column density field to calculate sigma^{2}_{rho/rho_{0}}. We find b
~ 0.48, which is consistent with typical numerical estimates, and is
characteristic of turbulent driving that includes a mixture of solenoidal and
compressive modes. More conservatively, we constrain b to lie in the range
0.3-0.8, depending on the influence of sub-resolution structure and the role of
diffuse atomic material in the column density budget. We also report a break in
the Taurus column density power spectrum at a scale of ~1pc, and find that the
break is associated with anisotropy in the power spectrum. The break is
observed in both 13CO and dust extinction power spectra, which, remarkably, are
effectively identical despite detailed spatial differences between the 13CO and
dust extinction maps. [ abridged ]Comment: 8 pages, 9 figures. Accepted for publication in A&
Angular Momentum Transfer and Lack of Fragmentation in Self-Gravitating Accretion Flows
Rapid inflows associated with early galaxy formation lead to the accumulation
of self-gravitating gas in the centers of proto-galaxies. Such gas
accumulations are prone to non-axisymmetric instabilities, as in the well-known
Maclaurin sequence of rotating ellipsoids, which are accompanied by a
catastrophic loss of angular momentum (J). Self-gravitating gas is also
intuitively associated with star formation. However, recent simulations of the
infall process display highly turbulent continuous flows. We propose that
J-transfer, which enables the inflow, also suppresses fragmentation.
Inefficient J loss by the gas leads to decay of turbulence, triggering global
instabilities and renewed turbulence driving. Flow regulated in this way is
stable against fragmentation, whilst staying close to the instability threshold
for bar formation -- thick self-gravitating disks are prone to global
instabilities before they become unstable locally. On smaller scales, the
fraction of gravitationally unstable matter swept up by shocks in such a flow
is a small and decreasing function of the Mach number. We conclude
counterintuitively that gas able to cool down to a small fraction of its virial
temperature will not fragment as it collapses. This provides a venue for
supermassive black holes to form via direct infall, without the intermediary
stage of forming a star cluster. Some black holes could have formed or grown in
massive halos at low redshifts. Thus the fragmentation is intimately related to
J redistribution within the system: it is less dependent on the molecular and
metal cooling but is conditioned by the ability of the flow to develop virial,
supersonic turbulence.Comment: 5 pp., 1 figures, to be published by the Astrophysical Journal
Letters. Minor corrections following the referee repor
A method for reconstructing the variance of a 3D physical field from 2D observations: Application to turbulence in the ISM
We introduce and test an expression for calculating the variance of a
physical field in three dimensions using only information contained in the
two-dimensional projection of the field. The method is general but assumes
statistical isotropy. To test the method we apply it to numerical simulations
of hydrodynamic and magnetohydrodynamic turbulence in molecular clouds, and
demonstrate that it can recover the 3D normalised density variance with ~10%
accuracy if the assumption of isotropy is valid. We show that the assumption of
isotropy breaks down at low sonic Mach number if the turbulence is
sub-Alfvenic. Theoretical predictions suggest that the 3D density variance
should increase proportionally to the square of the Mach number of the
turbulence. Application of our method will allow this prediction to be tested
observationally and therefore constrain a large body of analytic models of star
formation that rely on it.Comment: 8 pages, 9 figures, accepted for publication in MNRA
Structure Function Scaling of a 2MASS Extinction Map of Taurus
We compute the structure function scaling of a 2MASS extinction map of the
Taurus molecular cloud complex. The scaling exponents of the structure
functions of the extinction map follow the Boldyrev's velocity structure
function scaling of supersonic turbulence. This confirms our previous result
based on a spectral map of 13CO J=1-0 covering the same region and suggests
that supersonic turbulence is important in the fragmentation of this
star--forming cloud.Comment: submitted to Ap
On the interaction of a thin, supersonic shell with a molecular cloud
Molecular clouds (MCs) are stellar nurseries, however, formation of stars
within MCs depends on the ambient physical conditions. MCs, over a free-fall
time are exposed to numerous dynamical phenomena, of which, the interaction
with a thin, dense shell of gas is but one. Below we present results from
self-gravitating, 3-D smoothed particle hydrodynamics ({\small SPH})
simulations of the problem; seven realisations of the problem have been
performed by varying the precollision density within the cloud, the nature of
the post-collision shock, and the spatial resolution in the computational
domain. Irrespective of the type of shock, a complex network of dense
filaments, seeded by numerical noise, readily appears in the shocked cloud.
Segregation of the dense and rarefied gas phases also manifests itself in a
bimodal distribution of gas density. We demonstrate that the power-spectrum for
rarefied gas is Kolomogorov like, while that for the denser gas is considerably
steeper. As a corollary to the main problem, we also look into the possibly
degenerative effect of the {\small SPH} artificial viscosity on the impact of
the incident shell. It is observed that stronger viscosity leads to greater
post-shock dissipation, that strongly decelerates the incident shock-front and
promotes formation of contiguous structure, albeit on a much longer timescale.
We conclude that too much viscosity is likely to enhance the proclivity towards
gravitational boundedness of structure, leading to unphysical fragmentation.On
the other hand, insufficient resolution appears to suppress fragmentation.
Convergence of results is tested at both extremes, first by repeating the test
case with more than a million particles and then with only half the number of
particles in the original test case.Comment: 15 pages, 15 figures, and 1 Table; To appear in Monthly Notices to
the RA
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