A Q-CONDITION FOR LONG-RANGE PROPAGATING STAR-FORMATION

Collapse conditions for large expanding shells and rings in a disk gal axy are derived and shown to lead to a condition for star formation th at is similar to the condition Q < 1 for spontaneous instabilities. Th is result implies that both spontaneous and stimulated star formation are sensitive to Q, and that the observation of a critical surface den sity 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 h igh rotation rates, such as starburst galaxy nuclei, the normal balanc e between stimulated and spontaneous star formation mechanisms could s hift to give a higher proportion of stars forming in shells and other swept-up debris, and less in giant cloud complexes containing the loca l Jeans mass.