What's The Problem With ^3He?

Abstract

We consider the galactic evolutionary history of \he3 in models which deplete deuterium by as much as a factor of 2 to $\sim$ 15 from its primordial value to its present day observed value in the ISM. We show that when \he3 production in low mass stars (1 -- 3 $M_\odot$) is included over the history of the galaxy, \he3 is greatly over-produced and exceeds the inferred solar values and the abundances determined in galactic \hii 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 \he3 production in low mass stars; 2) the observations of the \he3 abundance; or 3) an observational bias towards regions of depleted \he3. Since \he3 observations in planetary nebula support the calculated \he3 production in low mass stars, option (1) is unlikely. We will argue for option (3) since the \he3 interstellar observations are indeed made in regions dominated by massive stars in which \he3 is destroyed. In conclusion, we note that the problem with \he3 seems to be galactic and not cosmological.