1997A&A...324..237W

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 C₂H₄ can be abundant as can CH₃CN and CH₃OH. 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 CH₃⁺ leads to other carbon-bearing species such as H₂CS, SiCH₂, H₂CN and CCl. Sulphur is an important source of electrons in some regions of the envelope. Its main effect is on the abundance of CH₃⁺ and molecules formed from this radical e.g. CH₃OH. 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.