C-14 in AGB stars: The case of IRC+10216

Observations and stellar evolution computations have been dedicated to the search for C-14 in the archetype of AGE stars, IRC+10216/CW Lee. Contrary to Wright's (1994) tentative detection, we see no trace of (CO)-C-14 in this star down to a 3 sigma limit R = (CO)-C-13/(CO)-C-14 greater than or equal to 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 3M. < M(ZAMS) < 5M. (See Guelin et al. 1995). While searching for the (CO)-C-14 2-1 line, we detected the (1 nu 3) 9(36) - 8(35) line of vibrationally excited SiC2. The latter line pertains to the first vibrational level of the nu(3) 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.