Dynamics of Molecular Clouds

TL;DR

This paper extends a model of molecular cloud fragmentation to include external pressure effects, predicting observable properties like velocity scaling, HI presence, and dissipation coefficients, aligning with recent observations.

Contribution

It introduces a scale-dependent critical mass influenced by external pressure and links it to observable phenomena in molecular clouds without assuming specific pressure sources.

Findings

External pressure determines fragment confinement and collapse.

Predicted power-law relation between velocity dispersion and size.

HI at fragment edges with specific column densities and velocities.

Abstract

We further develop the model of molecular cloud fragmentation introduced in Field, Blackman and Keto (2007; FBK). We show that external pressure acting on fragments establishes a scale-dependent critical mass. Fragments with masses less than the critical value are confined largely by pressure, while those with masses greater than or equal to the critical value collapse under self gravitation. Both types of fragments are commonly observed. Without specifying the source of the external pressure, and without assuming any other scaling relations, we predict the power - law index in the relation between the rms velocity of supersonic motions and the size of fragments . We then investigate the possibility that the external pressure is due to the kinetic energy of H atoms released by photodissociation of hydrogen molecules in the fragment. This can account approximately for the observed values of external pressure and two additional observations: the value of the scaling coefficient in the power law mentioned above, and the observation of outflowing atomic hydrogen around molecular clouds. A further prediction is HI at fragment edges with column densities of order 1E20 per sq. cm and velocities of a few km/s that should be detectable with high resolution 21 cm observations. Finally, we predict the magnitude of the coefficient of dissipation in the observed supersonic flows.