Kinematics of Highly R-Process-Enhanced Field Stars: Evidence for an
  Accretion Origin and Detection of Several Groups from Disrupted Satellites
  We present kinematics of 35 highly r-process-enhanced ([Eu/Fe] >= +0.7)
metal-poor (-3.8 < [Fe/H] < -1.4) field stars. We calculate six-dimensional
positions and velocities, evaluate energies and integrals of motion, and
compute orbits for each of these stars using parallaxes and proper motions from
the second Gaia data release and published radial velocities. All of these
stars have halo kinematics. Most stars (66%) remain in the inner regions of the
halo (< 13 kpc), and many (51%) have orbits that pass within 2.6 kpc of the
Galactic center. Several stars (20%) have orbits that extend beyond 20 kpc,
including one with an orbital apocenter larger than the Milky Way virial
radius. 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 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.
