Clues to the formation of the Milky Way's thick disk
  We analyse the chemical properties of a set of solar vicinity stars, and show
that the small dispersion in abundances of \alpha-elements at all ages provides
evidence that the SFH has been uniform throughout the thick disk. In the
context of long time scale infall models, we suggest that this result points
either to a limited dependence of the gas accretion on the Galactic radius in
the inner disk (R<10 kpc), or to a decoupling of the accretion history and star
formation history due to other processes governing the ISM in the early disk,
suggesting that infall cannot be a determining parameter of the chemical
evolution at these epochs. We argue however that these results and other recent
observational constraints -- namely the lack of radial metallicity gradient and
the non-evolving scale length of the thick disk -- are better explained if the
early disk is viewed as a pre-assembled gaseous system, with most of the gas
settled before significant star formation took place -- formally the equivalent
of a closed-box model. In any case, these results point to a weak, or
non-existent inside-out formation history in the thick disk, or in the first
3-5 Gyr of the formation of the Galaxy. We argue however that the growing
importance of an external disk whose chemical properties are distinct from
those of the inner disk would give the impression of an inside-out growth
process when seen through snapshots at different epochs. However, the
progressive, continuous process usually invoked may not have actually existed
in the Milky Way.
