CHEMISTRY IN OXYGEN-RICH CIRCUMSTELLAR ENVELOPES

We have considered the chemistry occuring in the circumstellar envelop e surrounding an oxygen-rich AGE star and have specifically modelled 4 sources; R Dor, TX Cam, OH231.8+4.2 and IK Tau. Methane has been assu med to be a parent molecule and the resulting carbon chemistry is inve stigated. We find that carbon chain molecules up to C2H4 can be abunda nt as can CH3CN and CH3OH. Our model extends previous work by includin g the chemistry of silicon, chlorine and phosphorus. The presence of C H4 as a parent and hence its daughter species CH3 and CH3+ leads to ot her carbon-bearing species such as H2CS, SiCH2, H2CN and CCl. Sulphur is an important source of electrons in some regions of the envelope. I ts main effect is on the abundance of CH3+ and molecules formed from t his radical e.g. CH3OH. Where possible we have compared our results to observations and in general there is good agreement. A notable except ion is the case of OCS in OH231.8+4.2 whose calculated abundance is ro ughly 10(4) times lower than observed.