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Astron. Astrophys. 341, L47-L50 (1999)

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1. Introduction

Carbon-bearing molecules have been identified in the outer envelopes of many oxygen-rich (O-rich), late-type stars at millimeter and microwave wavelengths (Deguchi & Goldsmith 1985, Lindqvist et al. 1988, Omont et al. 1993, Bujarrabal et al. 1994) and include HCN, CS, OCS, HNC and CN. Because HNC is typically formed in the outer envelopes of AGB stars from ion-molecule chemistry triggered by the penetration of cosmic rays and ultra-violet radiation, it was first thought that the observed carbon species could be produced in the outer wind of oxygen stars. Photo-chemical models of several oxygen Miras by Willacy & Millar (1997, hereafter WM97) tried to reproduce the observations and relied on the injection of methane, CH4, ammonia, NH3, silicon sulfide, SiS and hydrogen sulfide, H2S, to generate a carbon-rich chemistry at large radii. These input molecular abundances are questionable since there exist no observational or theoretical evidences for the formation of these species in the inner and intermediate envelopes of O-rich Miras. The model succeeded in reproducing the observed values of certain species but failed to reproduce some molecular abundances, in particular that of hydrogen cyanide, HCN, which is known to be a "parent" molecule in carbon stars and forms in the inner regions of the winds (Willacy & Cherchneff 1998, hereafter WC98).

The recent ISO detection of several infrared (IR) emission bands of CO2 in various O-rich Miras and semi-regular stars by Justtanont et al. (1998) suggests that this carbon species has to form in the deep layers of the stellar winds in order to excite the IR transitions observed. In this letter, we investigate the non-equilibrium chemistry of the inner regions close to the stellar photosphere of the oxygen Mira IK Tau (NML Tau) and raise the following questions: (a) Can some of these carbon molecules form in the inner parts of the wind and be ejected as "parent" species?; (b) Can methane and ammonia form in the inner wind of O-rich Mira stars?

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© European Southern Observatory (ESO) 1999

Online publication: December 4, 1998