PHOTOCHEMISTRY IN THE INNER LAYERS OF CLUMPY CIRCUMSTELLAR ENVELOPES: FORMATION OF WATER IN C-RICH OBJECTS AND OF C-BEARING MOLECULES IN O-RICH OBJECTS

摘要

Amechanism based on the penetration of interstellar ultraviolet photons into the inner layers of clumpy circumstellar envelopes (CSEs) around asymptotic giant branch stars is proposed to explain the non-equilibrium chemistry observed in such objects. We show through a simple modeling approach that in CSEs with a certain degree of clumpiness or with moderately low mass loss rates (a few 10?7M yr?1) a photochemistry can take place in the warm and dense inner layers, inducing important changes in the chemical composition. In carbon-rich objects water vapor and ammonia would be formed with abundances of 10?8–10?6 relative to H2, while in oxygen-rich envelopes ammonia and carbon-bearing molecules such as HCN and CS would form with abundances of 10?9–10?7 relative to H2. The proposed mechanism would explain the recent observation of warm water vapor in the carbon-rich envelope IRC+10216 with the Herschel Space Observatory and predict that H2O should be detectable in other carbon-rich objects.