Galactic He-4 and heavy elements enrichment by stellar nucleosynthesis

The contribution to the galactic abundance of He and heavy elements by stellar nucleosynthesis is calculated as a function of time, taking into account present knowledge about stellar and galactic evolution. A model is used which distinguishes the phase of the contracting halo from the subsequent history of the disk. Various uncertainties involved both in stellar and in galactic evolutionary theory are discussed. The primordial abundance of He-4 is treated as a free parameter, ranging from 0 up to 0.4. It is found that stellar activity provides a significant contribution to cosmic He-4, though not sufficient to explain the observed abundance. The best agreement with observational data is obtained by starting with a primordial abundance of 0.20 to 0.23, which is consistent with the 'big-bang theory' predictions and with recent observational estimates. The contribution to the abundance of heavy elements depends on the last stellar stages and the final explosion mechanism.