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

A mechanism 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 −7 M ☉ 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 H 2 , 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 H 2 . 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 H 2 O should be detectable in other carbon-rich objects.