The local abundance of He-3: A confrontation between theory and observation

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