545 research outputs found
Turbulent Fragmentation and Star Formation
We review the main results from recent numerical simulations of turbulent
fragmentation and star formation. Specifically, we discuss the observed scaling
relationships, the ``quiescent'' (subsonic) nature of many star-forming cores,
their energy balance, their synthesized polarized dust emission, the ages of
stars associated with the molecular gas from which they have formed, the mass
spectra of clumps, and the density and column density probability distribution
function of the gas. We then give a critical discussion on recent attempts to
explain and/or predict the star formation efficiency and the stellar initial
mass function from the statistical nature of turbulent fields. Finally, it
appears that turbulent fragmentation alone cannot account for the final stages
of fragmentation: although the turbulent velocity field is able to produce
filaments, the spatial distribution of cores in such filaments is better
explained in terms of gravitational fragmentation.Comment: 14 pages, 1 ps figure. Refered invited review, to appear in "Magnetic
Fields and Star Formation: Theory versus Observations", eds. A.I. Gomez de
Castro et al. (Kluwer), in pres
On the gravitational content of molecular clouds and their cores
(Abridged) The gravitational term for clouds and cores entering in the virial
theorem is usually assumed to be equal to the gravitational energy, since the
contribution to the gravitational force from the mass distribution outside the
volume of integration is assumed to be negligible. Such approximation may not
be valid in the presence of an important external net potential. In the present
work we analyze the effect of an external gravitational field on the
gravitational budget of a density structure. Our cases under analysis are (a) a
giant molecular cloud (GMC) with different aspect ratios embedded within a
galactic net potential, and (b) a molecular cloud core embedded within the
gravitational potential of its parent molecular cloud. We find that for
roundish GMCs, the tidal tearing due to the shear in the plane of the galaxy is
compensated by the tidal compression in the z direction. The influence of the
external effective potential on the total gravitational budget of these clouds
is relatively small, although not necessarily negligible. However, for more
filamentary GMCs, the external effective potential can be dominant and can even
overwhelm self-gravity, regardless of whether its main effect on the cloud is
to disrupt it or compress it. This may explain the presence of some GMCs with
few or no signs of massive star formation, such as the Taurus or the
Maddalena's clouds. In the case of dense cores embedded in their parent
molecular cloud, we found that the gravitational content due to the external
field may be more important than the gravitational energy of the cores
themselves. This effect works in the same direction as the gravitational
energy, i.e., favoring the collapse of cores. We speculate on the implications
of these results for star formation models.Comment: Accepted for publication in MNRA
Molecular Cloud Turbulence And The Star Formation Efficiency: Enlarging the Scope
We summarize recent numerical results on the control of the star formation
efficiency (SFE), addressing the effects of turbulence and the magnetic field
strength. In closed-box numerical simulations, the effect of the turbulent Mach
number \Ms depends on whether the turbulence is driven or decaying: In driven
regimes, increasing \Ms decreases the SFE, while in decaying regimes the
converse is true. The efficiencies in non-magnetic cases for realistic Mach
numbers \Ms \sim 10 are somewhat too high compared to observed values.
Including the magnetic field can bring the SFE down to levels consistent with
observations, but the intensity of the magnetic field necessary to accomplish
this depends again on whether the turbulence is driven or decaying. In this
kind of simulations, a lifetime of the molecular cloud (MC) needs to be
assumed. Further progress requires determining the true nature of the
turbulence driving and the lifetimes of the clouds. Simulations of MC formation
by large-scale compressions in the warm neutral medium (WNM) show that the
clouds' initial turbulence is produced by the accumulation process that forms
them, and that the turbulence is driven for as long as this process lasts,
producing realistic velocity dispersions and also thermal pressures in excess
of the mean WNM value. In simulations including self-gravity, but neglecting
the magnetic field and stellar energy feedback, the clouds never reach an
equilibrium state, but rather evolve secularly, increasing their mass and
gravitational energy until they engage in generalized gravitational collapse.
However, local collapse events begin midways through this process, and produce
enough stellar objetcs to disperse the cloud or at least halt its collapse
before the latter is completed.Comment: 8 pages, 2 postscript figures, invited talk in IAU Symposium 237,
"Triggered Star Formation in a Turbulent ISM", 14-18 August, Prague, Czech
Republic, eds. B. Elmegreen & J. Palous. Abstract abridge
Gravitational Collapse and Filament Formation: Comparison with the Pipe Nebula
Recent models of molecular cloud formation and evolution suggest that such
clouds are dynamic and generally exhibit gravitational collapse. We present a
simple analytic model of global collapse onto a filament and compare this with
our numerical simulations of the flow-driven formation of an isolated molecular
cloud to illustrate the supersonic motions and infall ram pressures expected in
models of gravity-driven cloud evolution. We apply our results to observations
of the Pipe Nebula, an especially suitable object for our purposes as its low
star formation activity implies insignifcant perturbations from stellar
feedback. We show that our collapsing cloud model can explain the magnitude of
the velocity dispersions seen in the CO filamentary structure by Onishi
et al. and the ram pressures required by Lada et al. to confine the lower-mass
cores in the Pipe nebula. We further conjecture that higher-resolution
simulations will show small velocity dispersions in the densest core gas, as
observed, but which are infall motions and not supporting turbulence. Our
results point out the inevitability of ram pressures as boundary conditions for
molecular cloud filaments, and the possibility that especially lower-mass cores
still can be accreting mass at significant rates, as suggested by observations.Comment: 10 pages, 4 figures, accepted by Ap
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