We present kinematics of 35 highly r -process-enhanced ( [ Eu/Fe ] \geq + 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 } \gtrsim - 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 .