Low-mass stars and the³He problem

Standard chemical evolution models predict an overabundance of ³He from low-mass stars (in the range of 1-3 M_⊙) with respect to the measured solar and present-day abundances and hence indicate a possible problem with the yields of ³He in these stars. Because ³He is one of the nuclei produced in big bang nucleosynthesis (BBN), it is noted that Galactic and stellar evolution uncertainties necessarily relax constraints based on ³He. We incorporate into chemical evolution models that include outflow the new yields for ³He of Boothroyd & Malaney (1995), which predict that low-mass stars are net destroyers of ³He. Since these yields do not account for the high ³He/H ratio observed in some planetary nebulae, we also consider the possibility that some fraction of stars in the 1-3 M⊙ range do not destroy their ³He in their post-main-sequence phase. We also consider the possibility that the gas expelled by stars in these mass ranges does not mix with the ISM instantaneously thus delaying the ³He produced in these stars, according to standard yields, from reaching the ISM. In general, we find that the Galactic D and ³He abundances can be fitted regardless of whether the primordial D/H value is high (2 × 10-⁴) or low (2.5 × 10^-5).