LOW-MASS STARS AND THE HE-3 PROBLEM

Standard chemical evolution models predict an overabundance of He-3 fr om low-mass stars (in the range of 1-3 M.) with respect to the measure d solar and present-day abundances and hence indicate a possible probl em with the yields of He-3 in these stars. Because 3He is one of the n uclei produced in big bang nucleosynthesis (BBN), it is noted that Gal actic and stellar evolution uncertainties necessarily relax constraint s based on He-3. We incorporate into chemical evolution models that in clude outflow the new yields for He-3 of Boothroyd & Malaney (1995), w hich predict that low-mass stars are net destroyers of He-3. Since the se yields do not account for the high He-3/H ratio observed in some pl anetary nebulae, we also consider the possibility that some fraction o f stars in the 1-3 M. range do not destroy their He-3 in their post-ma in-sequence phase. We also consider the possibility that the gas expel led by stars in these mass ranges does not mix with the ISM instantane ously thus delaying the He-3 produced in these stars, according to sta ndard yields, from reaching the ISM. In general, we find that the Gala ctic D and He-3 abundances can be fitted regardless of whether the pri mordial D/H value is high (2 x 10(-4)) (2.5 x 10(-5)).