219 research outputs found
Statistics of Core Lifetimes in Numerical Simulations of Turbulent, Magnetically Supercritical Molecular Clouds
We present measurements of the mean dense core lifetimes in numerical
simulations of magnetically supercritical, turbulent, isothermal molecular
clouds, in order to compare with observational determinations. "Prestellar"
lifetimes (given as a function of the mean density within the cores, which in
turn is determined by the density threshold n_thr used to define them) are
consistent with observationally reported values, ranging from a few to several
free-fall times. We also present estimates of the fraction of cores in the
"prestellar", "stellar'', and "failed" (those cores that redisperse back into
the environment) stages as a function of n_thr. The number ratios are measured
indirectly in the simulations due to their resolution limitations. Our approach
contains one free parameter, the lifetime of a protostellar object t_yso (Class
0 + Class I stages), which is outside the realm of the simulations. Assuming a
value t_yso = 0.46 Myr, we obtain number ratios of starless to stellar cores
ranging from 4-5 at n_thr = 1.5 x 10^4 cm^-3 to 1 at n_thr = 1.2 x 10^5 cm^-3,
again in good agreement with observational determinations. We also find that
the mass in the failed cores is comparable to that in stellar cores at n_thr =
1.5 x 10^4 cm^-3, but becomes negligible at n_thr = 1.2 x 10^5 cm^-3, in
agreement with recent observational suggestions that at the latter densities
the cores are in general gravitationally dominated. We conclude by noting that
the timescale for core contraction and collapse is virtually the same in the
subcritical, ambipolar diffusion-mediated model of star formation, in the model
of star formation in turbulent supercritical clouds, and in a model
intermediate between the previous two, for currently accepted values of the
clouds' magnetic criticality.Comment: 25 pages, 8 figures, ApJ accepted. Fig.1 animation is at
http://www.astrosmo.unam.mx/~e.vazquez/turbulence/movies/Galvan_etal07/Galvan_etal07.htm
Larson's third law and the universality of molecular cloud structure
Larson (1981) first noted a scaling relation between masses and sizes in
molecular clouds that implies that these objects have approximately constant
column densities. This original claim, based upon millimeter observations of
carbon monoxide lines, has been challenged by many theorists, arguing that the
apparent constant column density observed is merely the result of the limited
dynamic range of observations, and that in reality clouds have column density
variations over two orders of magnitudes. In this letter we investigate a set
of nearby molecular clouds with near-infrared excess methods, which guarantee
very large dynamic ranges and robust column density measurements, to test the
validity of Larson's third law. We verify that different clouds have almost
identical average column densities above a given extinction threshold; this
holds regardless of the extinction threshold, but the actual average surface
mass density is a function of the specific threshold used. We show that a
second version of Larson's third law, involving the mass-radius relation for
single clouds and cores, does not hold in our sample, indicating that
individual clouds are not objects that can be described by constant column
density. Our results instead indicate that molecular clouds are characterized
by a universal structure. Finally we point out that this universal structure
can be linked to the log-normal nature of cloud column density distributions.Comment: 5 pages, 4 figures, A&A in press (letter
Formation and Collapse of Quiescent Cloud Cores Induced by Dynamic Compressions
(Abridged) We present numerical hydrodynamical simulations of the formation,
evolution and gravitational collapse of isothermal molecular cloud cores. A
compressive wave is set up in a constant sub-Jeans density distribution of
radius r = 1 pc. As the wave travels through the simulation grid, a
shock-bounded spherical shell is formed. The inner shock of this shell reaches
and bounces off the center, leaving behind a central core with an initially
almost uniform density distribution, surrounded by an envelope consisting of
the material in the shock-bounded shell, with a power-law density profile that
at late times approaches a logarithmic slope of -2 even in non-collapsing
cases. The resulting density structure resembles a quiescent core of radius <
0.1 pc, with a Bonnor-Ebert-like (BE-like) profile, although it has significant
dynamical differences: it is initially non-self-gravitating and confined by the
ram pressure of the infalling material, and consequently, growing continuously
in mass and size. With the appropriate parameters, the core mass eventually
reaches an effective Jeans mass, at which time the core begins to collapse.
Thus, there is necessarily a time delay between the appearance of the core and
the onset of its collapse, but this is not due to the dissipation of its
internal turbulence as it is often believed. These results suggest that
pre-stellar cores may approximate Bonnor-Ebert structures which are however of
variable mass and may or may not experience gravitational collapse, in
qualitative agreement with the large observed frequency of cores with BE-like
profiles.Comment: Accepted for publication in ApJ. Associated mpeg files can be found
in http://www.astrosmo.unam.mx/~g.gomez/publica.htm
Physical vs. Observational Properties of Clouds in Turbulent Molecular Cloud Models
We examine how well the physical properties of clumps in turbulent molecular
clouds can be determined by measurements of observed clump structures. We
compare simulated observations of three-dimensional numerical models of
isothermal, magnetized, supersonic turbulence to the actual physical structure
of the models. We determine how changing the parameters of turbulence changes
the structure of the simulations. Stronger driving produces greater density
fluctuations, and longer wavelength driving produces larger structures.
Magnetic fields have a less pronounced effect on structure, and one that is not
monotonic with field strength. Aligned structures are seen only with
low-density tracers, and when the intensity of the field is large. Comparing
different regions with the same tracers (or the same region with different
tracers) can give information about physical conditions: different density
tracers can help determine the size of the density fluctuations and thus the
strength of the driving. Nevertheless, velocity superposition of multiple
physical clumps can fully obscure the physical properties of those clumps, and
short wavelength driving worsens this effect. Regarding Larson's relationships,
we confirm previous claims that the mean density-size relationship is an
artifact of the observations; and the velocity dispersion-size relationship, is
consistent with observations. Regarding the mass spectra, we show that, when we
look for clumps with high enough resolution, they converge to a log-normal
function, rather than the power-law obtained in the literature.Comment: ApJ accepted. 14 gif figures. PS file available at
ftp://ftp.astrosmo.unam.mx/pub/j.ballesteros/Papers
On the Effects of Projection on Morphology
We study the effects of projection of three-dimensional (3D) data onto the
plane of the sky by means of numerical simulations of turbulence in the
interstellar medium including the magnetic field, parameterized cooling and
diffuse and stellar heating, self-gravity and rotation. We compare the
physical-space density and velocity distributions with their representation in
position-position-velocity (PPV) space (``channel maps''), noting that the
latter can be interpreted in two ways: either as maps of the column density's
spatial distribution (at a given line-of-sight (LOS) velocity), or as maps of
the spatial distribution of a given value of the LOS velocity (weighted by
density). This ambivalence appears related to the fact that the spatial and PPV
representations of the data give significantly different views. First, the
morphology in the channel maps more closely resembles that of the spatial
distribution of the LOS velocity component than that of the density field, as
measured by pixel-to-pixel correlations between images. Second, the channel
maps contain more small-scale structure than 3D slices of the density and
velocity fields, a fact evident both in subjective appearance and in the power
spectra of the images. This effect may be due to a pseudo-random sampling
(along the LOS) of the gas contributing to the structure in a channel map: the
positions sampled along the LOS (chosen by their LOS velocity) may vary
significantly from one position in the channel map to the next.Comment: 6 figures. To appear in the March 20th volume in Ap
Cooling, Gravity and Geometry: Flow-driven Massive Core Formation
We study numerically the formation of molecular clouds in large-scale
colliding flows including self-gravity. The models emphasize the competition
between the effects of gravity on global and local scales in an isolated cloud.
Global gravity builds up large-scale filaments, while local gravity --
triggered by a combination of strong thermal and dynamical instabilities --
causes cores to form. The dynamical instabilities give rise to a local focusing
of the colliding flows, facilitating the rapid formation of massive
protostellar cores of a few 100 M. The forming clouds do not reach an
equilibrium state, though the motions within the clouds appear comparable to
``virial''. The self-similar core mass distributions derived from models with
and without self-gravity indicate that the core mass distribution is set very
early on during the cloud formation process, predominantly by a combination of
thermal and dynamical instabilities rather than by self-gravity.Comment: 13 pages, 12 figures, accepted by Ap
Unveiling two expanding stellar groups formed through violent relaxation in The Lagoon Nebula Cluster
The current kinematic state of young stellar clusters can give clues on their
actual dynamical state and origin. In this contribution, we use Gaia DR3 data
of the Lagoon Nebula Cluster (LNC) to show that the cluster is composed of two
expanding groups, likely formed from different molecular cloud clumps. We find
no evidence of massive stars having larger velocity dispersion than low-mass
stars or being spatially segregated across the LNC, as a whole, or within the
Primary group. However, the Secondary group, with 1/5th of the stars, exhibits
intriguing features. On the one hand, it shows a bipolar nature, with an aspect
ratio of 3:1. In addition, the massive stars in this group exhibit larger
velocity dispersion than the low-mass stars, although they are not concentrated
towards the center of the group. This suggests that this group may have
undergone dynamical relaxation, first, and some explosive event afterward.
However, further observations and numerical work have to be performed to
confirm this hypothesis. The results of this work suggest that, although
stellar clusters may form by the global and hierarchical collapse of their
parent clump, still some dynamical relaxation may take place.Comment: Accepted for publication in MNRA
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