We present the results of a detailed chemical kinetic model of the expanding circumstellar envelope of a cool oxygen-rich star. Parent molecules injected into the envelope are photodissociated by the external interstellar radiation field and neutral-neutral reactions between radicals, in particular OH, determine the chemical composition of the envelope. We show that the recent detection of HCN in several oxygen-rich envelopes can only be understood if a source of carbon other than CO, most probably methane, is injected into the envelope with an abundance much greater than that predicted by LTE chemistry. We suggest several observational consequences of our model.
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Nejad et al. (1988) studied this question.