The Conditions for Dredge-Up of Carbon During the Helium Shell Flash and the Production of Carbon Stars

Abstract

Asymptotic giant branch (AGB) evolution, including helium shell flashing, has been studied in stars of low mass (M ≤ 3 M_Ө) with envelope helium abundance Y = 0.3 and metal abundances Z = 0.001, 0.01 and 0.02. It was found that reducing the envelope metal abundance in an AGB star of given total mass lowered the luminosity at which carbon stars were formed by the dredge-up of carbon at the helium shell flash. A lower mass limit for single carbon stars is found for each Z value by a simple envelope integration technique. The present results provide estimates of the core mass at the onset of dredge-up as a function of AGB mass and Z, as required for calculations such as those of Iben & Truran (1978) and Renzini and Voli (1980) which consider enhancement of stellar surface abundances during stellar evolution and subsequent heavy metal enrichment of the interstellar medium via stellar mass loss.