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.