The local abundance of³He: A confrontation between theory and observation

Determinations of the ³He 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 ³He in the solar system and in the interstellar medium today suggest that the 3He 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 ³He. We consider the possibility that the solar 3He 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 2 may be evidenced by several isotopic anomalies. Local destruction of ³He by a similar amount could serve to help reconcile the expected increase in the ³He 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 ³He destruction. We also explore the implications of both alternative models of Galactic chemical evolution and the stellar yields for ³He in low-mass stars, which can explain the history of the ³He concentration in the Galaxy.