Highly r-process-enhanced field stars kinematics

We apply three clustering methods to search for structure in phase space, and we identify eight groups. No abundances are considered in the clustering process, but the [Fe/H] dispersions of the groups are smaller than would be expected by random chance. The orbital properties, clustering in phase space and metallicity, and the lack of highly r-process-enhanced stars on disk-like orbits, indicate that such stars likely were accreted from disrupted satellites.

Comparison with the galaxy luminosity-metallicity relation suggests M_V_>~-9 for most of the progenitor satellites, characteristic of ultra-faint or low-luminosity classical dwarf spheroidal galaxies. Environments with low rates of star formation and Fe production, rather than the nature of the r-process site, may be key to obtaining the [Eu/Fe] ratios found in highly r-process-enhanced stars.