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

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.

5 pages, 3 figures; accepted for publication in ApJ Letters

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• Water in IRC+10216: a genuine formation process by shock-induced chemistry in the inner wind
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• The peculiar distribution of CH3CN in IRC+10216 seen by ALMA
• The Role of Internal Photons on the Chemistry of the Circumstellar Envelopes of AGB Stars
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• Dust-gas chemistry in AGB outflows: Chemical modelling of dust-gas chemistry within AGB outflows I. Effect on the gas-phase chemistry
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• Late Stages of Stellar Evolution - Herschel's contributions
• Constraints on the H2O formation mechanism in the wind of carbon-rich AGB stars
• Investigating Anomalous Photochemistry in the Inner Wind of IRC+10216 Through Interferometric Observations of HC${}_3$N
• Herschel/HIFI observations of IRC+10216: water vapor in the inner envelope of a carbon-rich AGB star
• Circumstellar ammonia in oxygen-rich evolved stars
• Carbon chemistry in IRC+10216: Infrared detection of diacetylene
• The maser emitting structure and time variability of the SiS lines J=14-13 and 15-14 in IRC+10216
• Herschel/HIFI observations of the circumstellar ammonia lines in IRC+10216
• Chemical modelling of dust-gas chemistry within AGB outflows -- II. Effect of the dust-grain size distribution
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• Chemistry in AGB stars: successes and challenges
• H12CN and H13CN excitation analysis in the circumstellar outflow of R Scl
• The Role of Ultraviolet Photons in Circumstellar Astrochemistry
• Observational study of hydrocarbons in the bright photodissociation region of Messier 8
• Herschel/HIFI search for H2-17O and H2-18O in IRC+10216: constraints on models for the origin of water vapor
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• High-angular-resolution ALMA imaging of the inhomogeneous dynamical atmosphere of the asymptotic giant branch star W Hya
• Exploring circumstellar chemistry in X-ray emitting AGB stars
• IRC+10216 mass loss properties through the study of $\lambda$3mm emission: Large spatial scale distribution of SiO, SiS, and CS
• The vertical and adiabatic ionization energies of silicon carbide clusters, (SiC)${}_n$, with n=1-12