We consider the galactic evolutionary history of 3He in models which deplete deuterium by as much as a factor of 2 to ∼15 from its primordial value to its present-day observed value in the interstellar medium (ISM). We show that when 3He production in low-mass stars (1-3 M⊙) is included over the history of the galaxy, 3He is greatly overproduced and exceeds the inferred solar values and the abundances determined in galactic H II regions. Furthermore, the ISM abundances show a disturbing dispersion which is difficult to understand from the point of view of standard chemical evolution models. In principle, resolution of the problem may lie in either (1) the calculated 3He production in low-mass stars; (2) the observations of the 3He abundance; or (3) an observational bias toward regions of depleted 3He. Since 3He observations in planetary nebula support the calculated 3He production in low-mass stars, option (1) is unlikely. We will argue for option (3) since the 3He interstellar observations are indeed made in regions dominated by massive stars in which 3He is destroyed. In conclusion, we note that the problem with 3He seems to be galactic and not cosmological.
- Galaxy: evolution
- ISM: abundances
- Nuclear reactions, nucleosynthesis, abundances