EVOLUTION OF SPIRAL GALAXIES .1. HALO-DISK CONNECTION FOR THE EVOLUTION OF THE SOLAR NEIGHBORHOOD

This paper is the first of a series dealing with the evolution of spiral galaxies and in particular of the Milky Way. Here was present a complex model for the chemical evolution of the solar neighborhood, containing several improvements with respect to previous models. The solar neighborhood is subdivided into halo and disk regions and their individual but interconnected evolution is computed in detail. In particular, the evolution of the abundances of 15 chemical species (H, D, He-3, He-4, C-12, C-13, O-16, N-14, Ne-20, Mg-24, Si-28, S-32, Ca-40, Fe-56, and neutron-rich isotopes synthesized from C-12, C-13, N-14, and O-16) is predicted both for the halo and the disk. Detailed stellar nucleosynthesis as well as stellar lifetimes are taken into account as a consequence of relaxing the instantaneous recycling approximation. A new initial mass function, constant in space and time, has also been adopted. Only one phase for the gas (diffuse) is considered in the halo evolution, whereas diffuse gas and gas clouds are taken into account in the disk evolution. The star formation rate in the disk is therefore assumed to be a two-step process, in the sense that first gas clouds form out of diffuse gas and then stars form out of gas clouds. Cloud-cloud collisions and stimulated processes are considered as the main causes for star formation the disk. It should be noted that this model, at variance with the majority of the existing models, computes the evolution of iron abundance in great detail, so that comparison with observations is really meaningful. Our results show that the majority of observational constraints for the solar neighborhood are reproduced by the model, within the large uncertainties present in the theory and in observations. The evolution of the relative abundances as functions of iron abundance, which is the most common indicator of metallicity in stars, has been predicted for halo and disk stars. The distributions of stars as functions of metallicity are also predicted for halo and disk. The predicted distribution of disk stars with respect to the observational data is not as good as other predictions, and further improvements are suggested. Considering the contemporaneous evolution of two coupled zones, we find an overlap of halo and disk stars in the metallicity range -1.6 to -1.O. Finally, good agreement is obtained for current supernova rates, star formation rate, and remnant, gas, and cloud masses when compared with the available data.