Chemical evolution of the galactic halo. II - Enrichment in primary elements

The differences in the relative abundances of primary elements between the galactic halo and the disk are interpreted using a numerical model for the chemical evolution of the halo. A number of hypotheses for the yields from massive stars taken from current literature are combined with available data on the evolution of low and intermediate masses and with recent suggestions on the contribution from type-I supernovae. A number of models of the metal enrichment from stars of different masses are constructed, with particular attention to the evolution of metal-poor stars. The model predictions are then compared with the solar system composition, using the instantaneous recycling approximation, and the chemical composition of the galactic halo, using a previously described numerical model. The results obtained using the various considered hypotheses are presented and a possible interpretation of the dichotomy of the two abundance scales for globular clusters is discussed.