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Astron. Astrophys. 324, 237-248 (1997)


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Chemistry in oxygen-rich circumstellar envelopes

K. Willacy and T.J. Millar

Department of Physics, UMIST, PO Box 88, Manchester, UK

Received 9 December 1996 / Accepted 27 January 1997

Abstract

We have considered the chemistry occuring in the circumstellar envelope surrounding an oxygen-rich AGB star and have specifically modelled 4 sources; R Dor, TX Cam, OH231.8+4.2 and IK Tau. Methane has been assumed to be a parent molecule and the resulting carbon chemistry is investigated. We find that carbon chain molecules up to C2 H4 can be abundant as can CH3 CN and CH3 OH. Our model extends previous work by including the chemistry of silicon, chlorine and phosphorus. The presence of CH4 as a parent and hence its daughter species CH3 and CH [FORMULA] leads to other carbon-bearing species such as H2 CS, SiCH2, H2 CN and CCl.

Sulphur is an important source of electrons in some regions of the envelope. Its main effect is on the abundance of CH [FORMULA] and molecules formed from this radical e.g. CH3 OH.

Where possible we have compared our results to observations and in general there is good agreement. A notable exception is the case of OCS in OH231.8+4.2 whose calculated abundance is roughly 104 times lower than observed.

Key words: stars: abundances – stars: circumstellar matter – stars: late type

Send offprint requests to: T. J. Millar

© European Southern Observatory (ESO) 1997

Online publication: May 26, 1998

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