204 research outputs found
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&
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&
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
Our Knowledge of High-Mass Star Formation at the Dawn of Herschel
We review the theories and observations of high-mass star formation
emphasizing the differences with those of low-mass star formation. We hereafter
describe the progress expected to be achieved with Herschel, thanks notably to
Key Programmes dedicated to the earliest phases of high-mass star formation.Comment: 16 page
Formation of proto-cluster: a virialized structure from gravo-turbulent collapse II. A two-dimensional analytical model for rotating and accreting system
Most stars are born in the gaseous proto-cluster environment. The knowledge
of this intermediate stage gives more accurate constraints on star formation
characteristics. We demonstrate that a virialized globally supported structure,
in which star formation happens, is formed out of a collapsing molecular cloud,
and derive a mapping from the parent cloud parameters to the proto-cluster to
predict its properties, with a view to confront analytical calculations with
observations and simulations. The virial theorem is decomposed into two
dimensions to account for the rotation and the flattened geometry. Equilibrium
is found by balancing rotation, turbulence and self-gravity, while turbulence
is maintained by accretion driving and dissipates in one crossing time. The
angular momentum and the accretion rate of the proto-cluster are estimated from
the parent cloud properties. The two-dimensional virial model predicts the size
and velocity dispersion given the mass of the proto-cluster and that of the
parent cloud. The gaseous proto-clusters lie on a sequence of equilibrium with
the trend , with limited variations depending on the
evolutionary stage, the parent cloud, and the parameters not well known like
turbulence driving efficiency by accretion and the turbulence anisotropy. The
model reproduces successfully observations and simulation results. The
properties of proto-clusters follow universal relations and they can be derived
from that of the parent cloud. Using simple estimates to infer the peak
position of the core mass function (CMF) we find a weak dependence on the
cluster mass suggesting that the physical conditions inside proto-clusters may
contribute to set a CMF, and by extension an IMF, that looks independent of the
environment
What are we learning from the relative orientation between density structures and the magnetic field in molecular clouds?
We investigate the conditions of ideal magnetohydrodynamic (MHD) turbulence
responsible for the relative orientation between density structures,
characterized by their gradient, , and the magnetic field,
, in molecular clouds (MCs). For that purpose, we construct an
expression for the time evolution of the angle, , between
and based on the transport equations of MHD
turbulence. Using this expression, we find that the configuration where
and are mostly parallel, , and where
and are mostly perpendicular, ,
constitute attractors, that is, the system tends to evolve towards either of
these configurations and they are more represented than others. This fact would
explain the predominant alignment or anti-alignment between column density,
, structures and the projected magnetic field orientation,
, reported in observations. Additionally, we find that
departures from the configurations are related to convergent
flows, quantified by the divergence of the velocity field,
, in the presence of a relatively strong magnetic
field. This would explain the observed change in relative orientation between
-structures and towards MCs, from mostly parallel at low
to mostly perpendicular at the highest , as the result of the
gravitational collapse and/or convergence of flows. Finally, we show that the
density threshold that marks the observed change in relative orientation
towards MCs, from and being mostly parallel at low
to mostly perpendicular at the highest , is related to the magnetic field
strength and constitutes a crucial piece of information for determining the
role of the magnetic field in the dynamics of MCs.Comment: 10 pages, 8 figures. Submitted to 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
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