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The $([\alpha/{\rm Fe}],[{\rm Fe/H}])$ distribution of Milky Way stars showsat least two distinct sequences, which have traditionally been associated withthe thin and thick disc components. The abundance distribution variessystematically with location $R$ and $|z|$ across the Galaxy. Using ananalytical chemodynamical model that includes the effects of radial migrationand kinematic heating, we show that it is possible to reproduce the observedabundance distribution at different locations. Unlike some earlier models, ourscheme does not require a distinct thick disc component emerging from aseparate evolutionary path. The proposed model has a continuous star formationhistory and a continuous age velocity dispersion relation. Moreover,[$\alpha$/Fe] is constant for stellar ages less than 8 Gyr, but increasessharply for older stars over a time scale of 1.5 Gyr. The gap between the twosequences 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 thehigh metallicity end, we find a sequence of stars having different ages with asimilar birth radius that originated in the inner disc. Our model successfullyexplains the uniformity of the locus of the high-[$\alpha$/Fe] sequence acrossdifferent locations. The low-[$\alpha$/Fe] sequence contains stars withdifferent birth radii that owes its existence to radial migration. For thelow-[$\alpha$/Fe] sequence, angular momentum is anti-correlated with [Fe/H],and the model can reproduce this trend at different Galactic locations. Ifradial migration is not included, the model fails to generate the doublesequence and instead shows only a single sequence. Our simple scheme has majoradvantages over earlier chemodynamical models, as we show.

Chemical Enrichment and Radial Migration in the Galactic Disk -- the origin of the [$\alpha/\rm Fe]$ Double Sequence