A new picture for the Galaxy formation

The observational constraints are of fundamental importance to build a real istic chemical evolution model. With respect to these constraints the last years have been of crucial importance and, in the case of the Milky Way, th e new observational data required a revision of the previous chemical evolu tion models (see Chiappini et al, 1997 - CMG97) and Pagel and Tautvaisiene, 1995 - PT95). The results obtained by CMG97 from a careful comparison betw een model predictions and observational constraints strongly suggest that t he previously adopted picture for the Galaxy formation in which the gas she d from the halo was the main contributor to the thin disk formation, is not valid anymore. With our detailed chemical evolution model we are able to p ut some constraints on the IMF variation and on the Deuterium primordial va lue.