Galactic Evolution Of D And 3He Including Stellar Production Of 3He
Abstract
New stellar models which track the production and destruction of 3He (and D) have been evolved for a range of stellar masses (0.65≤ M/M≤ 100), metallicities (0.01 ≤ Z/Z ≤ 1) and initial (main sequence) 3He mass fractions (10-5 ≤ X3,MS ≤ 10-3). Armed with the 3He yields from these stellar models we have followed the evolution of D and 3He using a variety of chemical evolution models with and without infall of primordial or processed material. Production of new 3He by the lower mass stars overwhelms any reasonable primordial contributions and leads to predicted abundances in the presolar nebula and/or the present interstellar medium in excess of the observationally inferred values. This result, which obtains even for zero primordial D and 3He, and was anticipated by Rood, Steigman \& Tinsley (1976), is insensitive to the choice of chemical evolution model; it is driven by the large 3He yields from low mass stars. In an attempt to ameliorate this problem we have considered a number of non-standard models in which the yields from low mass stars have been modified. Although several of these non-standard models may be consistent with the 3He data, they may be inconsistent with observations of 12C/13C, 18O and, most seriously, the super-3He rich planetary nebulae (Rood, Bania \& Wilson 1992). Even using the most extreme of these non-standard models (Hogan 1995), we obtain a generous upper bound to pre-galactic 3He: X3P ≤ 3.2 ×10-5 which, nonetheless, leads to a stringent lower bound to the universal density of nucleons.
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