The chemical evolution of the Milky Way: the Three Infall Model

We present a new chemical evolution model for the Galaxy that assumes three main infall episodes of primordial gas for the formation of halo, thick and thin disc, respectively. We compare our results with selected data taking into account NLTE effects. The most important parameters of the model are (i) the time-scale for gas accretion, (ii) the efficiency of star formation and (iii) a threshold in the gas density for the star formation process, for each Galactic component. We find that, in order to best fit the features of the solar neighbourhood, the halo and thick disc must form on short time-scales (˜0.2 and ˜1.25 Gyr, respectively), while a longer time-scale is required for the thin-disc formation. The efficiency of star formation must be maximum (10 Gyr-1) during the thick-disc phase and minimum (1 Gyr-1) during the thin-disc formation. Also the threshold gas density for star formation is suggested to be different in the three Galactic components. Our main conclusion is that in the framework of our model an independent episode of accretion of extragalactic gas, which gives rise to a burst of star formation, is fundamental to explain the formation of the thick disc. We discuss our results in comparison to previous studies and in the framework of modern galaxy formation theories.