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

The results from a program for determining the dredge-up properties of thermally pulsing asymptotic giant branch (AGB) stars of low mass are analyzed. It is found that the dredge up to the surface of freshly made isotopes, such as C-12, does not continue throughout a star's AGB lifetime but rather has a beginning and an end. It is only in stars of small initial metallicity and of sufficiently small mass that semi-convection plays a role in the production of C-13, which can act as a source of neutrons from s-process-like isotope production. It is also found that the principal reason for the 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. When dredge up first begins, the minimum core mass increases with increasing metallicity and with decreasing total stellar mass. Dredge up occurs only above a critical value of mixing length over scale height. Even though model stars of larger than solar metallicity can dredge up carbon, it is believed that they probably do not become carbon stars; this is in agreement with the known lack of carbon stars in regions of high metallicity, such as the galactic bulge.