Kinematic and chemical components in the solar neighbourhood

Abstract

Abundance data on solar neighbourhood stars suggest the presence of chemically-distinct stellar components in the solar neighbourhood. When the abundances of Fe, α elements, and the r-process element Eu are considered together, stars separate neatly into two groups that delineate the thin and thick disk components of the Milky Way. The group akin to the thin disk is traced by stars of relatively high Fe content and low [α/Fe] ratios. The thick disk-like group overlaps the thin disk in [Fe/H] but has higher abundances of α elements and Eu. Fe-poor stars with low [α/Fe] ratios, however, seem to belong to a separate, dynamically-cold, non-rotating component likely associated with debris from past accretion events. The kinematically-hot stellar halo dominates the sample at the metal-poor end. 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. For example, the average rotation speed and velocity dispersion of thin disk stars is roughly independent of metallicity, a result unexpected in most current theories of thin-disk formation. In this scenario, the familiar increase in the velocity dispersion of disk stars with decreasing metallicity is the result of the increasing prevalence of the thick disk 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 allows the use of their kinematics to assess their origin, an powerful approach denied to traditional ways of apportioning stars to the various Galactic components.