The influence of dust condensation on the molecular abundances of the carbon star circumstellar envelope

Abstract

Abstract Calculations of chemical equilibrium in the circumstellar envelope of carbon star IRC+10216 are performed. In order to account for the amount of C and Si lost by gas phase due to C and SiC dust condensation the variations of concentration of C and Si in this phase were used and the problem of chemical equilibrium in the gas phase was solved. Our calculations lead to the conclusion that if C/O changes from 2 to 1.1 the abundances of such C-bearing molecules as HCN, CH4 and C2H2 change by about two orders and, if dust formation stops when C/O ≤ 1.1, the assumption of chemical equilibrium can explain the observable abundances of C4H2, CH4, HCN, and HC3N in the inner envelope of the star. The SiC dust formation in the envelope can explain the observable deficiency of some silicon-containing molecules, namely SiO and SiC2 when Si depletion is about 70%. The upper limit of SiC2 abundance given by Gensheimer et al. (1995) is in accordance with our equilibrium calculations. The calculated abundances of s...