Carbon star formation and neutron-rich isotope formation in low-mass asymptotic giant branch stars

A preliminary analysis of the results of a very time-consuming program for the determination of dredge-up properties of thermally pulsing asymptotic giant branch (AGB) stars of low mass shows that: the dredge up to the surface of freshly made isotopes such as 12C does not continue throughout a star's AGB lifetime, but has a beginning and an end; only in stars of small initial metallicity and of sufficiently small mass does semicondvection play a role in the production of 13C, which can act as a source of neutrons for s-process-like isotope production; the primary reason for dredge-up properties of low-mass stars is the fact that the strength of thermal pulses reaches values that are an order of magnitude larger than in high-mass AGB stars; the minimum core mass when dredge up first begins increases with increasing metallicity and with decreasing total stellar mass; only above the critical value of mixing length over scale height does dredge up occur at all; although model stars of larger than solar metallicity can dredge up carbon, they probably do not become carbon stars, in agreement with the known lack of carbon stars in regions of high metallicity such as the galactic bulge. It is postulated that, in stars of moderate initial mass which can survive wind mass loss long enough to develop a core mass larger than those studied here, a second dredge-up phase may occur, and this may account for the abundances of Nb and Tc in stars such as R CMi and CY Cyg