INTERSTELLAR SO+

Because of its promise as a unique tracer of dissociative shocks (Turner 1992), we have undertaken an extensive survey of interstellar SO+, comprising four transitions and 28 sources of varied morphological types. SO+ is widespread and quite abundant, (being easily detected in 23 of the 28 objects). Radiative transfer analysis has been done on all sources, using cross sections calculated for the general case of a 2pi molecule. SO+ can be formed by ion-molecule chemistry in either cold or warm clouds, and by dissociative (but not nondissociative) shocks. Our analysis involves carefully selected physical conditions derived from other species where possible. We find that cold ion-molecule chemistry models fit observations of SO+ in cold clouds quite well. Warm-cloud ion-molecule chemistry is not indicated in general. Dissociative-shock chemistry (Neufeld & Dalgarno 1989) is strongly indicated for several energetic star-forming regions but in general such models cannot be uniquely distinguished from ion-molecule chemistry. Simple planar shock geometries generally cannot fit both N(SO+) and X(SO+) dissociative shock models; an ensemble of randomly directed shocks is required. Without observations of high spatial resolution, a distinction between ion-molecule and dissociative shock chemistries for SO+ will remain difficult for many interstellar sources, but where shocks can be established, they must be dissociative.