The Local Abundance of $^3$He: A Confrontation Between Theory and Observation

Abstract

Determinations of the \he3 concentrations in Galactic matter serve to impose interesting and important constraints both on cosmological models and on models of Galactic chemical evolution. At present, observations of \he3 in the solar system and in the interstellar medium today suggest that the \he3 abundance has not increased significantly over the history of the Galaxy, while theoretical models of Galactic chemical evolution (utilizing current nucleosynthesis yields from stellar evolution and supernova models) predict a rather substantial increase in \he3. We consider the possibility that the solar \he3 abundance may have been affected by stellar processing in the solar neighborhood prior to the formation of the solar system. Such a discrepancy between solar abundances and average galactic abundances by as much as a factor of two, may be evidenced by several isotopic anomalies. Local destruction of \he3 by a similar amount could serve to help to reconcile the expected increase in the \he3 abundance predicted by models of galactic chemical evolution. We find however, that the production of heavier elements, such as oxygen, places a strong constraint on the degree of \he3 destruction. We also explore the implications of both alternative models of Galactic chemical evolution and the stellar yields for \he3 in low mass stars, which can explain the history of the \he3 concentration in the Galaxy.