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

TL;DR

This paper proposes a photochemical mechanism driven by UV photons penetrating clumpy circumstellar envelopes, explaining the formation of water and other molecules in both carbon-rich and oxygen-rich AGB star environments, aligning with recent observations.

Contribution

It introduces a simple model showing how clumpiness enables inner envelope photochemistry, explaining molecular abundances not accounted for by equilibrium chemistry.

Findings

Water vapor and ammonia form in C-rich envelopes with abundances of 10^(-8) to 10^(-6).

Ammonia and C-bearing molecules form in O-rich envelopes with abundances of 10^(-9) to 10^(-7).

The mechanism explains observed warm water vapor in IRC +10216 and predicts its presence in other C-rich objects.

Abstract

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.