Galactic chemical evolution: abundance gradients of individual elements

Models for chemical evolution of the Galaxy are presented, where the most recent ideas on stellar nucleosynthesis and supernova (SN) progenitors are taken into account. It is assumed that the disc forms by accretion of primordial material coming from outside with a time-scale which is a function of the galactocentric distance. The possibility of forming the disc out of enriched halo gas is also studied in a simple way. The evolution of the abundances of several single elements (C, N, O, Ne, Mg, Si, S and Fe) is predicted as well as that of gas mass, SN rates (type I and II) and star formation rate, both in the solar neighborhood and in the whole disc, under different assumptions for the star formation rate. Abundance gradients along the galactic disc are well reproduced and a possible interpretation for the differences among gradients of different elements is given in terms of nucleosynthesis and stellar lifetimes. This interpretation can represent a key for understanding gradients also in external galaxies. In particular, a natural explanation is suggested for the positive gradient of S/O, found in M101 and M33. Possible mechanisms of disc formation are discussed and the conclusion is reached that, in order to reproduce the majority of the observational constraints, the hypothesis of the Galactic disc formed out of primordial gas coming from outside the Galaxy should be favoured, at least in these kinds of models.