Low‐Mass Stars and the3He Problem

The prediction of standard chemical evolution models of higher abundances ofHe3 at the solar and present-day epochs than are observed indicates a possibleproblem with the yield of He3 for stars in the range of 1-3 solar masses.Because He3 is one of the nuclei produced in Big Bang Nucleosynthesis (BBN), itis noted that galactic and stellar evolution uncertainties necessarily relaxconstraints based on He3. We incorporate into chemical evolution models whichinclude outflow, the new yields for He3 of Boothroyd & Malaney (1995) whichpredict that low mass stars are net destroyers of He3. Since these yields donot account for the high \he3/H ratio observed in some planetary nebulae, wealso consider the possibility that some fraction of stars in the 1 - 3 solarmass range do not destroy their He3 in theirpost main-sequence phase. We alsoconsider the possibility that the gas expelled by stars in these mass rangesdoes not mix with the ISM instantaneously thus delaying the He3 produced inthese stars, according to standard yields, from reaching the ISM. In general,we find that the Galactic D and He3 abundances can be fit regardless of whetherthe primordial D/H value is high (2 x 10^{-4}) or low (2.5 x 10^{-5}).Comment: 20 pages, latex, 9 ps figure