Chemical Enrichment and Radial Migration in the Galactic Disk -- the
  origin of the [$\alpha/\rm Fe]$ Double Sequence
  The $([\alpha/{\rm Fe}],[{\rm Fe/H}])$ distribution of Milky Way stars shows
at least two distinct sequences, which have traditionally been associated with
the thin and thick disc components. The abundance distribution varies
systematically with location $R$ and $|z|$ across the Galaxy. Using an
analytical chemodynamical model that includes the effects of radial migration
and kinematic heating, we show that it is possible to reproduce the observed
abundance distribution at different locations. Unlike some earlier models, our
scheme does not require a distinct thick disc component emerging from a
separate evolutionary path. The proposed model has a continuous star formation
history and a continuous age velocity dispersion relation. Moreover,
[$\alpha$/Fe] is constant for stellar ages less than 8 Gyr, but increases
sharply for older stars over a time scale of 1.5 Gyr. The gap between the two
sequences is due to this sharp transition. We show that the high-[$\alpha$/Fe]
sequence at the low metallicity end is simply a pile-up of old stars. At the
high metallicity end, we find a sequence of stars having different ages with a
similar birth radius that originated in the inner disc. Our model successfully
explains the uniformity of the locus of the high-[$\alpha$/Fe] sequence across
different locations. The low-[$\alpha$/Fe] sequence contains stars with
different birth radii that owes its existence to radial migration. For the
low-[$\alpha$/Fe] sequence, angular momentum is anti-correlated with [Fe/H],
and the model can reproduce this trend at different Galactic locations. If
radial migration is not included, the model fails to generate the double
sequence and instead shows only a single sequence. Our simple scheme has major
advantages over earlier chemodynamical models, as we show.
