14C in AGB stars : the case of IRC+10216

Abstract

. Observations and stellar evolution computations have been dedicated to the search for 14C in the archetype of AGB stars, IRC+10216/CW Leo. Contrary to Wright’s (1994) tentative detection, we see no trace of 14CO in this star down toa3σ limit R = 13CO/14CO≥ 1400. The new limit is consistent with our stellar evolution computations, which indicate that R should be > 4000. Comparison of the C, N, O, Mg, Si, and S isotopic ratios observed in this source with the calculated ones reinforces our earlier conclusion that CW Leo had a main sequence mass 3 M < MZAMS < 5 M (see Guelin ´ et al. 1995). While searching for the 14CO 2–1 line, we detected the (1ν3) 936 − 835 line of vibrationally excited SiC2. The latter line pertains to the first vibrational level of the ν3 bending state and its upper level has an energy of 290 K, lower than the temperature of the dust forming shell. The line has a pyramidal shape and shows acceleration steps, which we interpreted in terms of dust formation.