GALACTIC EVOLUTION OF D AND HE-3

The evolution of D and He-3 is considered in the framework of a consistent model for the chemical evolution of the Galaxy. The destruction of D in the course of galactic evolution is found to be modest, of the order of a factor similar to 2; the good agreement found with the observed abundance of D in the presolar material and in the interstellar medium makes possible to derive narrow constraints on the primordial D, or, equivalently, on the allowed range of the baryon-to-photon ratio ? according to standard big-bang nucleosynthesis. Conversely, the evolution of He-3 remains one of the major problems in the field of chemical galactic evolution: results based on the predictions of updated stellar models are shown to lead to an overproduction of He-3 by a factor similar to 5-7 at the time of formation of the Sun, and by a factor similar to 5-20 with respect to measured abundances in galactic H II regions. Several possibilities to reduce this discrepancy are presented and discussed quantitatively; none of them is found to produce the desired result, with the exception of a hypothetical low-energy resonance in the cross section of the He-3(He-3, 2p)He-4 reaction, advocated also as a possible solution to the solar neutrino puzzle. Otherwise, the observed underabundance of He-3 in the presolar material and in H II regions must be considered as evidencing local inhomogeneities in the interstellar medium, in which He-3 is strongly depleted by some unknown process.