Through Thick and Thin: Kinematic and Chemical Components in the Solar
  Neighbourhood
  We search for the existence of chemically-distinct stellar components in the
solar neighbourhood using published data. Extending earlier work, we show that
when the abundances of Fe, alpha elements, and the r-process element Eu are
considered, stars separate neatly into two groups that delineate the
traditional thin and thick disk of the Milky Way. The group akin to the thin
disk is traced by stars with [Fe/H]>-0.7 and alpha/Fe<0.2. The thick disk-like
group overlaps the thin disk 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. The kinematics of thin disk stars is then 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 disk. The velocity dispersion of
stars assigned to the thin disk is also independent of [Fe/H], implying that
the familiar increase in velocity dispersion with decreasing metallicity is the
result of the increased 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, probably, stellar age, of the various
components might affect other reported trends in the properties of stars in the
solar neighbourhood.
