7Li creation and 3He destruction in low mass stars

Abstract

In low mass stars (< 2.5 M ⊙), the 12C 13C ratio on the upper red giant branch (RGB) is lower than expected from first dredge-up in standard stellar models; this has been attributed to “extra mixing” induced by rotation, and resulting partial CNO processing of material in the stellar envelope. A simple circulation model of such extra mixing has been constructed; it is shown that 7Li-rich low-mass RGB stars can result. Also, considerable 3He destruction is predicted, counteracting the enrichment from first dredgeup. An estimate of the final 3He abundance as a function of stellar mass and metallicity indicates that stars are net destroyers of 3He. A slight increase is thereby permitted in the inferred upper bound on the primordial (D+ 3He) H ratio, yielding a slight reduction in the lower bound on the cosmic baryon density Ω b from big bang nucleosynthesis calculations.