In the thick of it: metal-poor disc stars in RAVE
  By selecting in the RAVE-DR4 survey the stars located between 1 and 2 kpc
above the Galactic plane, we question the consistency of the simplest
three-component model (thin disc, thick disc, halo) for the Milky Way. We
confirm that the metallicity and azimuthal velocity distribution functions of
the thick disc are not Gaussian. In particular, we find that the thick disc has
an extended metallicity tail going at least down to [M/H]=-2 dex, contributing
roughly at 3% of the entire thick disc population and having a shorter
scale-length compared to the canonical thick disc. The mean azimuthal velocity
of these metal-poor stars allows us to estimate the correlation between the
metallicity and the orbital velocity, which is an important constraint on the
formation mechanisms of the Galactic thick disc. Given our simple approach, we
find dVphi/d[M/H] ~ 50 km/s/dex, which is in a very good agreement with
previous literature values. We complete the study with a brief discussion on
the implications of the formation scenarios for the thick disc, and suggest
that given the above mentioned characteristics, a thick disc mainly formed by
radial migration mechanisms seems unlikely.
