A Q condition for long-range propagating star formation

Abstract

Collapse conditions for large expanding shells and rings in a disk galaxy are derived and shown to lead to a condition for star formation that is similar to the condition Q < 1 for spontaneous instabilities. This result implies that both spontaneous and stimulated star formation are sensitive to Q, and that the observation of a critical surface density for star formation that is based on Q does not necessarily imply that star formation results from large-scale quiescent instabilities. The results also suggest that in regions with high gas densities and high rotation rates, such as starburst galaxy nuclei, the normal balance between stimulated and spontaneous star formation mechanisms could shift to give a higher proportion of stars forming in shells and other swept-up debris, and less in giant cloud complexes containing the local Jeans mass.