The Earliest Phases of Galaxy Evolution

In this paper we study the very early phases of the evolution of our Galaxyby means of a chemical evolution model which reproduces most of theobservational constraints in the solar vicinity and in the disk. We haverestricted our analysis to the solar neighborhood and present the predictedabundances of several elements (C, N, O, Mg, Si, S, Ca, Fe) over an extendedrange of metallicities $[Fe/H] = -4.0$ to $[Fe/H] = 0.0$ compared to previousmodels. We adopted the most recent yield calculations for massive stars takenfrom different authors (Woosley & Weaver 1995 and Thielemann et al. 1996) andcompared the results with a very large sample of data, one of the largest everused to this purpose. These data have been analysed with a new and powerfulstatistical method which allows us to quantify the observational spread inmeasured elemental abundances and obtain a more meaningful comparison with thepredictions from our chemical evolution model. Our analysis shows that the``plateau'' observed for the [$\alpha$/Fe] ratios at low metallicities ($-3.0<[Fe/H] <-1.0$) is not perfectly constant but it shows a slope, especially foroxygen. This slope is very well reproduced by our model with both sets ofyields. This is not surprising since realistic chemical evolution models,taking into account in detail stellar lifetimes, never predicted a completelyflat plateau. This is due either to the fact that massive stars of differentmass produce a slightly different O/Fe ratio or to the often forgotten factthat supernovae of type Ia, originating from white dwarfs, start appearingalready at a galactic age of 30 million years and reach their maximum at 1 Gyr.Comment: 32 pages, 9 figures, to be published in Ap