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 around AGB stars is proposed
to explain the non-equilibrium chemistry observed in such objects. We show
through a simple modelling approach that in circumstellar envelopes with a
certain degree of clumpiness or with moderately low mass loss rates (a few
10^(-7) solar masses per year) 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.Comment: 5 pages, 3 figures; accepted for publication in ApJ Letter
