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A mechanism based on the penetration of interstellar ultraviolet photons intothe inner layers of clumpy circumstellar envelopes around AGB stars is proposedto explain the non-equilibrium chemistry observed in such objects. We showthrough a simple modelling approach that in circumstellar envelopes with acertain degree of clumpiness or with moderately low mass loss rates (a few10^(-7) solar masses per year) a photochemistry can take place in the warm anddense inner layers inducing important changes in the chemical composition. Incarbon-rich objects water vapor and ammonia would be formed with abundances of10^(-8) - 10(^-6) relative to H2, while in oxygen-rich envelopes ammonia andcarbon-bearing molecules such as HCN and CS would form with abundances of10^(-9) - 10^(-7) relative to H2. The proposed mechanism would explain therecent observation of warm water vapor in the carbon-rich envelope IRC +10216with the Herschel Space Observatory, and predict that H2O should be detectablein other carbon-rich objects.

the astrophysical journal. letters

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