Theories and implications of hierarchical fragmentation

Abstract

Interstellar cloud complexes contain multiply nested substructure covering about four orders of magnitude in size. Various theoretical ideas for the origin of this structure from the interplay of gravitational instability, rotation, magnetic fields, and turbulence are reviewed. Available numerical collapse calculations, limited to one fragmentation stage by lack of spatial resolution, do suggest the likelihood of repeated fragmentation, at least for non-magnetic models, but do not support the predictions of fragmentation scenarios based on linear stability analysis or the virial theorem as applied to stationary initial states. Specifically, the relevance of the thermal or magnetic Jeans mass to fragmentation is questioned. A number of simplified examples are used to show how the rate, efficiency, and mass spectrum of star formation, as well as protostellar angular momentum and other properties, may depend sensitively on the hierarchical fragmentation structure. Some of the implications of recent results concerning the fractal nature of cloud shapes are briefly summarized.