Abundance gradients and their evolution in the Milky Way disk

Based on a simple, but fairly successful, model of the chemical evolution o f the Milky Way disk, we study the evolution of the abundances of the eleme nts He, C, N, O, Ne, Mg, Al, Si, S, Ar and Fe. We use metallicity dependent yields for massive stars with and without mass loss. We find that most obs erved abundance profiles are correctly reproduced by massive star yields, b ut C and N require supplementary sources. We argue that massive, mass losin g stars can totally account for the abundance profile of C, while intermedi ate mass stars are the main source of N; in both cases, some conflict with corresponding data on extragalactic HII regions arises, at least if current observations in the Galaxy are taken at face value. The observed behaviour of Al is marginally compatible with current massive star yields, which pro bably overestimate the "odd-even" effect. We also find that the adopted "in side-out" formation scheme for the Milky Way disk produces abundance profil es steeper in the past. The corresponding abundance scatter is smaller in t he inner disk than in the outer regions for a given interval of Galactic ag e.