Low‐Mass Stars and the3He Problem

Standard chemical evolution models predict an overabundance of 3 He 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 3 He in these stars. Because 3 He 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 3 He. We incorporate into chemical evolution models that include outflow the new yields for 3 He of Boothroyd & Malaney (1995), which predict that low-mass stars are net destroyers of 3 He. Since these yields do not account for the high 3 He/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 3 He 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 3 He produced in these stars, according to standard yields, from reaching the ISM. In general, we find that the Galactic D and 3 He abundances can be fitted regardless of whether the primordial D/H value is high (2 × 10 -4 ) or low (2.5 × 10 -5 ).