105 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
Six Myths on the Virial Theorem for Interstellar Clouds
It has been paid little or no attention to the implications that turbulent
fragmentation has on the validity of at least six common assumptions on the
Virial Theorem (VT), which are: (i) the only role of turbulent motions within a
cloud is to provide support against collapse, (ii) the surface terms are
negligible compared to the volumetric ones, (iii) the gravitational term is a
binding source for the clouds, (iv) the sign of the second-time derivative of
the moment of inertia determines whether the cloud is contracting or expanding,
(v) interstellar clouds are in Virial Equilibrium (VE), and (vi) Larson's
(1981) relations are the observational proof that clouds are in VE.
Interstellar clouds cannot fulfill these assumptions, however, because
turbulent fragmentation will induce flux of mass, moment and energy between the
clouds and their environment, and will favor local collapse while may disrupt
the clouds within a dynamical timescale. It is argued that, although the
observational and numerical evidence suggests that interstellar clouds are not
in VE, the so-called ``Virial Mass'' estimations, which actually should be
called ``energy-equipartition mass'' estimations, are good order-of magnitude
estimations of the actual mass of the clouds just because observational surveys
will tend to detect interstellar clouds appearing to be close to energy
equipartition. However, since clouds are actually out of VE, as suggested by
asymmetrical line profiles, they should be transient entities. These results
are compatible with observationally-based estimations for rapid star formation.
, and call into question the models for the star formation efficiency based on
clouds being in VE.Comment: Accepted by MNRAS. 9 pages, no figure
The Role of Gravity in Producing Power-Law Mass Functions
Numerical simulations of star formation have found that a power-law mass
function can develop at high masses. In a previous paper, we employed
isothermal simulations which created large numbers of sinks over a large range
in masses to show that the power law exponent of the mass function, , asymptotically and accurately approaches
Simple analytic models show that such a power law can develop if the mass
accretion rate , as in Bondi-Hoyle accretion; however, the
sink mass accretion rates in the simulations show significant departures from
this relation. In this paper we show that the expected accretion rate
dependence is more closely realized provided the gravitating mass is taken to
be the sum of the sink mass and the mass in the near environment. This
reconciles the observed mass functions with the accretion rate dependencies,
and demonstrates that power-law upper mass functions are essentially the result
of gravitational focusing, a mechanism present in, for example, the competitive
accretion model.Comment: 11 pages, 10 figures, accepted by Ap
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