A Super‐Alfvenic Model of Dark Clouds

Abstract

Supersonic random motions are observed in dark clouds and are traditionally interpreted as Alfvén waves, but the possibility that these motions are super-Alfvénic has not been ruled out. In this work we report the results of numerical experiments in two opposite regimes: A ~ 1 and A ≫ 1, where A is the initial Alfvénic Mach number—the ratio of the rms velocity to the Alfvén speed. Our results show that models with A ≫ 1 are consistent with the observed properties of molecular clouds that we have tested (statistics of extinction measurements, distribution of integrated antenna temperature, Zeeman-splitting measurements of magnetic field strength, line width versus integrated antenna temperature of molecular emission-line spectra, statistical B - n relation, and scatter in that relation), while models with A ~ 1 have properties that are in conflict with the observations. We find that both the density and the magnetic field in molecular clouds may be very intermittent. The statistical distributions of the magnetic field and gas density are related by a power law, with an index that decreases with time in experiments with decaying turbulence. After about one dynamical time it stabilizes at B ∝ n 0.4 . Magnetically dominated cores form early in the evolution, while later on the intermittency in the density field wins out, and also cores with a weak field can be generated by mass accretion along magnetic field lines.