Chemical evolution of the Milky Way: constraints on the formation of the
  thick and thin discs
  We study the evolution of Milky Way thick and thin discs in the light of the
most recent observational data. In particular, we analyze abundance gradients
of O, N, Fe and Mg along the thin disc as well as the [Mg/Fe] vs. [Fe/H]
relations and the metallicity distribution functions at different
Galactocentric distances. We run several models starting from the two-infall
paradigm, assuming that the thick and thin discs formed by means of two
different infall episodes, and we explore several physical parameters, such as
radial gas flows, variable efficiency of star formation, different times for
the maximum infall onto the disc, different distributions of the total surface
mass density of the thick disc and enriched gas infall. Our best model suggests
that radial gas flows and variable efficiency of star formation should be
acting together with the inside-out mechanism for the thin disc formation. The
timescale for maximum infall onto the thin disc, which determines the gap
between the formation of the two discs, should be $t_{max}\simeq 3.25$ Gyr. The
thick disc should have an exponential, small scale length density profile and
gas infall on the inner thin disc should be enriched. We compute also the
evolution of Gaia-Enceladus system and study the effects of possible
interactions with the thick and thin discs. We conclude that the gas lost by
Enceladus or even part of it could have been responsible for the formation of
the thick disc but not the thin disc.
