Super–Alfvénic Turbulent Fragmentation in Molecular Clouds

Summary

The dynamics of molecular clouds are often described in terms of magneto–hydro–dynamic (MHD) waves, in order to explain the super–sonic line widths and the fact that molecular clouds do not seem to be efficiently fragmenting into stars on a free–fall time–scale. In this work we discuss an alternative scenario, where the dynamics of molecular clouds are super–Alfvénic, due to a lower magnetic field strength than usually assumed (or inferred from observations).

Molecular clouds are modeled here as random MHD super–sonic flows, using numerical solutions of the three–dimensional MHD equations. A Monte Carlo non-LTE radiative transfer code is used to calculate synthetic spectra from the molecular cloud models.

The comparison with observational data shows that the super–Alfvénic model we discuss provides a natural description of the dynamics of molecular clouds, while the traditional equipartition model encounters several difficulties.

Introduction

Molecular clouds (MCs) are recognized to be the sites of present day star formation in our galaxy. The description of their dynamics is an essential ingredient for the theory of star formation.

A lot of work has been devoted to understand i) how super-sonic random motions in MCs can persist for at least a few dynamical times and ii) why MCs do not collapse, or fragment gravitationally into stars, on a free–fall time–scale. The magnetic field has been advocated as the solution for both problems. Magneto–hydrodynamic (MHD) waves were believed to dissipate at a significantly lower rate then super–Alfvénic and super–sonic random motions.