1,442 research outputs found
Semi-analytical homologous solutions of the gravo-magnetic contraction
We propose an extension of the semi-analytical solutions derived by Lin et
al. (1965) describing the two-dimensional homologous collapse of a
self-gravitating rotating cloud having uniform density and spheroidal shape,
which includes magnetic field (with important restrictions) and thermal
pressure. The evolution of the cloud is reduced to three time dependent
ordinary equations allowing to conduct a quick and preliminary investigation of
the cloud dynamics during the precollapse phase, for a wide range of
parameters. We apply our model to the collapse of a rotating and magnetized
oblate and prolate isothermal core. Hydrodynamical numerical simulations are
performed and comparison with the semi-analytical solutions is discussed. Under
the assumption that all cores are similar, an apparent cloud axis ratio
distribution is calculated from the sequence of successive evolutionary states
for each of a large set of initial conditions. The comparison with the
observational distribution of the starless dense cores belonging to the catalog
of Jijina et al. (1999) shows a good agreement for the rotating and initially
prolate cores (aspect ratio ) permeated by an helical magnetic
field (G for a density of cm).Comment: accepted for publication in A&
Theory of Feedback in Clusters and Molecular Cloud Turbulence
I review recent numerical and analytical work on the feedback from both low-
and high-mass cluster stars into their gasoeus environment. The main
conclusions are that i) outflow driving appears capable of maintaing the
turbulence in parsec-sized clumps and retarding their collapse from the
free-fall rate, although there exist regions within molecular clouds, and even
some examples of whole clouds, which are not actively forming stars, yet are
just as turbulent, so that a more universal turbulence-driving mechanism is
needed; ii) outflow-driven turbulence exhibits specific spectral features that
can be tested observationally; iii) feedback plays an important role in
reducing the star formation rate; iv) nevertheless, numerical simulations
suggest that feedback cannot completely prevent a net contracting motion of
clouds and clumps. Therefore, an appealing source for driving the turbulence
everywhere in GMCs is the accretion from the environment, at all scales. In
this case, feedback's most important role may be to prevent a fraction of the
gas nearest to newly formed stars from actually reaching them, thus reducing
the star formation efficiency.Comment: 8 pages, no figures. Invited review for IAU symposium 270
'Computational star formation', Ed., J. Alves, B. Elmegreen, J. Girart, V.
Trimbl
On the origin of non self-gravitating filaments in the ISM
{Filaments are ubiquitous in the interstellar medium as recently emphasized
by Herschel, yet their physical origin remains elusive} {It is therefore
important to understand the physics of molecular clouds to investigate how
filaments form and what is the role played by various processes such as
turbulence and magnetic field.} {We use ideal MHD simulations to study the
formation of clumps in various conditions including different magnetization and
Mach numbers as well as two completely different setup. We then perform several
analysis to compute the shape of the clumps and their link to velocities and
forces using various approaches.} {We find that on average, clumps in MHD
simulations are more filamentary that clumps in hydrodynamical simulations.
Detailed analyses reveal that the filaments are in general preferentially
aligned with the strain which means that these structures simply result from
the strech induced by turbulence. Moreover filaments tend to be confined by the
Lorentz force which therefore lead them to survive longer in magnetized flows.
We show that they have in all simulations a typical thickness equal to a few
grid cells suggesting that they are primarily associated to the energy
dissipation within the flow. We estimate the order of magnitude of the
dissipation length associated to the ion-neutral friction and conclude that in
well UV shielded regions it is of the order of 0.1 pc and therefore could
possibly set the typical size of non self-gravitating filaments.} {Filaments
are ubiquitous because they are the results of the very generic turbulent
strain and because magnetic field help to keep them coherent. We suggest that
energy dissipation is playing a determinant role in their formation.}Comment: 18 pages, to be published in A&
Theories of the initial mass function
We review the various theories which have been proposed along the years to
explain the origin of the stellar initial mass function. We pay particular
attention to four models, namely the competitive accretion and the theories
based respectively on stopped accretion, MHD shocks and turbulent dispersion.
In each case, we derive the main assumptions and calculations that support each
theory and stress their respective successes and failures or difficulties.Comment: Invited review for IAU symposium 270 'Computational star formation',
Ed., J. Alves, B. Elmegreen, J. Girart, V. Trimbl
Analytical star formation rate from gravoturbulent fragmentation
We present an analytical determination of the star formation rate (SFR) in
molecular clouds, based on a time-dependent extension of our analytical theory
of the stellar initial mass function (IMF). The theory yields SFR's in good
agreement with observations, suggesting that turbulence {\it is} the dominant,
initial process responsible for star formation. In contrast to previous SFR
theories, the present one does not invoke an ad-hoc density threshold for star
formation; instead, the SFR {\it continuously} increases with gas density,
naturally yielding two different characteristic regimes, thus two different
slopes in the SFR vs gas density relationship, in agreement with observational
determinations. Besides the complete SFR derivation, we also provide a
simplified expression, which reproduces reasonably well the complete
calculations and can easily be used for quick determinations of SFR's in cloud
environments. A key property at the heart of both our complete and simplified
theory is that the SFR involves a {\it density-dependent dynamical time},
characteristic of each collapsing (prestellar) overdense region in the cloud,
instead of one single mean or critical freefall timescale. Unfortunately, the
SFR also depends on some ill determined parameters, such as the core-to-star
mass conversion efficiency and the crossing timescale. Although we provide
estimates for these parameters, their uncertainty hampers a precise
quantitative determination of the SFR, within less than a factor of a few.Comment: accepted for publication in ApJ
Analytical theory for the initial mass function: III time dependence and star formation rate
The present paper extends our previous theory of the stellar initial mass
function (IMF) by including the time-dependence, and by including the impact of
magnetic field. The predicted mass spectra are similar to the time independent
ones with slightly shallower slopes at large masses and peak locations shifted
toward smaller masses by a factor of a few. Assuming that star-forming clumps
follow Larson type relations, we obtain core mass functions in good agreement
with the observationally derived IMF, in particular when taking into account
the thermodynamics of the gas. The time-dependent theory directly yields an
analytical expression for the star formation rate (SFR) at cloud scales. The
SFR values agree well with the observational determinations of various Galactic
molecular clouds. Furthermore, we show that the SFR does not simply depend
linearly on density, as sometimes claimed in the literature, but depends also
strongly on the clump mass/size, which yields the observed scatter. We stress,
however, that {\it any} SFR theory depends, explicitly or implicitly, on very
uncertain assumptions like clump boundaries or the mass of the most massive
stars that can form in a given clump, making the final determinations uncertain
by a factor of a few. Finally, we derive a fully time-dependent model for the
IMF by considering a clump, or a distribution of clumps accreting at a constant
rate and thus whose physical properties evolve with time. In spite of its
simplicity, this model reproduces reasonably well various features observed in
numerical simulations of converging flows. Based on this general theory, we
present a paradigm for star formation and the IMF.Comment: accepted for publication in Ap
Simulations of magnetized multiphase galactic disk regulated by supernovae explosions
What exactly controls star formation in the Galaxy remains controversial. In
particular, the role of feedback and magnetic field are still partially
understood. We investigate the role played by supernovae feedback and magnetic
field onto the star formation and the structure of the Galactic disk. We
perform numerical simulations of the turbulent, magnetized, self-gravitating,
multi-phase, supernovae regulated ISM within a 1 kpc stratified box. We
implemented various schemes for the supernovae. This goes from a random
distribution at a fixed rate to distributions for which the supernovae are
spatially and temporally correlated to the formation of stars. To study the
influence of magnetic field on star formation, we perform both hydrodynamical
and magneto-hydrodynamical simulations. We find that supernovae feedback has a
drastic influence on the galactic evolution. The supernovae distribution is
playing a very significant role. When the supernovae are not correlated with
star formation events, they do not modify significantly the very high star
formation rate obtained without feedback. When the supernovae follow the
accretion, the star formation rate can be reduced by a factor up to 30.
Magnetic field is also playing a significant role. It reduces the star
formation rate by a factor up to 2-3 and reduces the number of collapse sites
by a factor of about 2. The exact correlation between the supernovae and the
dense gas appears to have significant consequences on the galactic disk
evolution and the star formation. This implies that small scale studies are
necessary to understand and quantify the feedback efficiency. Magnetic field
does influence the star formation at galactic scales by reducing the star
formation rate and the number of star formation sites.Comment: to be published in A&
Star formation: statistical measure of the correlation between the prestellar core mass function and the stellar initial mass function
We present a simple statistical analysis of recent numerical simulations
exploring the correlation between the core mass function obtained from the
fragmentation of a molecular cloud and the stellar mass function which forms
from these collapsing cores. Our analysis shows that the distributions of bound
cores and sink particles obtained in the simulations are consistent with the
sinks being formed predominantly from their parent core mass reservoir, with a
statistical dispersion of the order of one third of the core mass. Such a
characteristic dispersion suggests that the stellar initial mass function is
relatively tightly correlated to the parent core mass function, leading to two
similar distributions, as observed. This in turn argues in favor of the IMF
being essentially determined at the early stages of core formation and being
only weakly affected by the various environmental factors beyond the initial
core mass reservoir, at least in the mass range explored in the present study.
Accordingly, the final IMF of a star forming region should be determined
reasonably accurately, statistically speaking, from the initial core mass
function, provided some uniform efficiency factor. The calculations also show
that these statistical fluctuations, due e.g. to variations among the core
properties, broaden the low-mass tail of the IMF compared with the parent CMF,
providing an explanation for the fact that this latter appears to underestimate
the number of "pre brown dwarf" cores compared with the observationally-derived
brown dwarf IMF.Comment: To appear in ApJ Letter
Turbulent molecular clouds
Stars form within molecular clouds but our understanding of this fundamental
process remains hampered by the complexity of the physics that drives their
evolution. We review our observational and theoretical knowledge of molecular
clouds trying to confront the two approaches wherever possible. After a broad
presentation of the cold interstellar medium and molecular clouds, we emphasize
the dynamical processes with special focus to turbulence and its impact on
cloud evolution. We then review our knowledge of the velocity, density and
magnetic fields. We end by openings towards new chemistry models and the links
between molecular cloud structure and star--formation rates.Comment: To be published in AARv, 58 pages, 13 figures (higher resolution
figures will be available on line
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