Low-mass Stars and the 3He 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).