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 $R \sim M^{0.5}$, 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, $\vec{\nabla}\rho$, and the magnetic field, $\vec{B}$, in molecular clouds (MCs). For that purpose, we construct an expression for the time evolution of the angle, $\phi$, between $\vec{\nabla}\rho$ and $\vec{B}$ based on the transport equations of MHD turbulence. Using this expression, we find that the configuration where $\vec{\nabla}\rho$ and $\vec{B}$ are mostly parallel, $\cos\phi=1$, and where $\vec{\nabla}\rho$ and $\vec{B}$ are mostly perpendicular, $\cos\phi=0$, 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, $N_H$, structures and the projected magnetic field orientation, $\hat{B}_\perp$, reported in observations. Additionally, we find that departures from the $\cos\phi=0$ configurations are related to convergent flows, quantified by the divergence of the velocity field, $\vec{\nabla}\cdot\vec{v}$, in the presence of a relatively strong magnetic field. This would explain the observed change in relative orientation between $N_H$-structures and $\hat{B}_\perp$ towards MCs, from mostly parallel at low $N_H$ to mostly perpendicular at the highest $N_H$, 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 $N_H$ and $\hat{B}_\perp$ being mostly parallel at low $N_H$ to mostly perpendicular at the highest $N_H$, 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