Through thick and thin: kinematic and chemical components in the solar neighbourhood

We search for chemically distinct stellar components in the solar neighbourhood using a compilation of published data. Extending earlier work, we show that when the abundances of Fe, alpha elements and the r-process element Eu are considered together, stars separate neatly into two groups that delineate the traditional thin and thick disc components of the Milky Way. The group akin to the thin disc is traced by stars with [Fe/H] > -0.7 and [alpha/Fe] < 0.2. The thick disc-like group overlaps the thin disc in [Fe/H] but has higher abundances of alpha elements and Eu. Stars in the range -1.5 < [Fe/H] < -0.7 with low [alpha/Fe] ratios, however, seem to belong to a separate, dynamically cold, non-rotating component that we associate with tidal debris. The kinematically hot stellar halo dominates the sample for [Fe/H] < -1.5. These results suggest that it may be possible to define the main dynamical components of the solar neighbourhood using only their chemistry, an approach with a number of interesting consequences. With such definition, the kinematics of thin disc stars is found to be independent of metallicity: their average rotation speed remains roughly constant in the range -0.7 < [Fe/H] < +0.4, a result that argues against radial migration having played a substantial role in the evolution of the thin disc. The velocity dispersion of the thin disc is also independent of [Fe/H], implying that the familiar increase in velocity dispersion with decreasing metallicity is the result of the increasing prevalence of the thick disc at lower metallicities, rather than of the sustained operation of a dynamical heating mechanism. The substantial overlap in [Fe/H] and, likely, stellar age, of the various components might affect other reported trends in the properties of stars in the solar neighbourhood. A purely chemical characterization of these components would enable us to scrutinize these trends critically in order to understand which result from accretion events and which result from secular changes in the properties of the Galaxy.