Cosmic phylogeny: reconstructing the chemical history of the solar
  neighbourhood with an evolutionary tree
  Using 17 chemical elements as a proxy for stellar DNA, we present a full
phylogenetic study of stars in the solar neighbourhood. This entails applying a
clustering technique that is widely used in molecular biology to construct an
evolutionary tree from which three branches emerge. These are interpreted as
stellar populations which separate in age and kinematics and can be thus
attributed to the thin disk, the thick disk, and an intermediate population of
probable distinct origin. We further find six lone stars of intermediate age
that could not be assigned to any population with enough statistical
significance. Combining the ages of the stars with their position on the tree,
we are able to quantify the mean rate of chemical enrichment of each of the
populations, and thus show in a purely empirical way that the star formation
rate in the thick disk is much higher than in the thin disk. We are also able
to estimate the relative contribution of dynamical processes such as radial
migration and disk heating to the distribution of chemical elements in the
solar neighbourhood. Our method offers an alternative approach to chemical
tagging methods with the advantage of visualising the behaviour of chemical
elements in evolutionary trees. This offers a new way to search for `common
ancestors' that can reveal the origin of solar neighbourhood stars.
