  In this paper we adopt a chemical evolution model, which is an improved
version of the Chiappini, Matteucci and Gratton (1997) model, assuming two main
accretion episodes for the formation of the Galaxy. The present model takes
into account in more detail than previously the halo density distribution and
explores the effects of a threshold density in the star formation process,
during both the halo and disk phases. In the comparison between model
predictions and available data, we have focused our attention on abundance
gradients as well as gas, stellar and star formation rate distributions along
the disk. We suggest that the mechanism for the formation of the halo leaves
detectable imprints on the chemical properties of the outer regions of the
disk, whereas the evolution of the halo and the inner disk are almost
completely disentangled. This is due to the fact that the halo and disk
densities are comparable at large Galactocentric distances and therefore the
gas lost from the halo can substantially contribute to building up the outer
disk. We also show that the existence of a threshold density for the star
formation rate, both in the halo and disk phase, is necessary to reproduce the
majority of observational data in the solar vicinity and in the whole disk.
Moreover, we predict that the abundance gradients along the Galactic disk must
have increased with time and that the average [alpha/Fe] ratio in stars (halo
plus disk) slightly decrease going from 4 to 10 Kpcs from the Galactic center.
We also show that the same ratios increase substantially towards the outermost
disk regions and the expected scatter in the stellar ages decreases, because
the outermost regions are dominated by halo stars.
