Excitation conditions in molecular clouds

Abstract

Interstellar radio spectroscopy over the last 12 years has shown that molecular and atomic transitions can be used as a powerful tool for probing the physical conditions and kinematics of the interstellar gas. By using different molecular transitions we have learned to investigate in quite some detail the molecular clouds, from their tenuous outer surroundings to their dense interiors. In effect molecular transitions are now being used to investigate interstellar gas ranging in temperature from 10K in cold dark clouds to 3000K in molecular shock fronts and circumstellar shells. Astronomical objects which can be investigated in molecular lines include quiescent dark clouds, young protostellar objects and molecular envelopes of evolved stars. Since molecular spectroscopy both in the radio and infrared region allows the properties of this wide range of objects to be derived with a fair amount of certainty, one may hope to understand their morphology and evolutionary status.