CIRCUMSTELLAR PHOTOCHEMISTRY

The cooling flows or winds from evolved stars are ideal for the formation of molecules and dust. The main location of molecular synthesis is the outer circumstellar envelope, where UV radiation from the interstellar medium penetrates the envelope and, by photodissociating parent molecules, produces the high-energy radicals and ions that activate gas-phase neutral and ion-molecule chemistry. After introducing relevant observational results and theoretical ideas, the salient aspects of the photochemical model are described. The primary application is to the nearby C star, IRC + 10216, where 50 or more circumstellar molecules have been detected. Recent interferometer maps, with resolution approaching 1′′, provide the means to verify the main ideas of the model and to indicate directions for its improvement.