Dynamically Tagged Groups of Very Metal-poor Halo Stars from the HK and
  Hamburg/ESO Surveys
  We analyze the dynamical properties of $\sim$1500 very metal-poor (VMP;
[Fe/H] $\lesssim -2.0$) halo stars, based primarily on medium-resolution
spectroscopic data from the HK and Hamburg/ESO surveys. These data, collected
over the past thirty years, are supplemented by a number of calibration stars
and other small samples, along with astrometric information from $Gaia$ DR2. We
apply a clustering algorithm to the 4-D energy-action space of the sample, and
identify a set of 38 Dynamically Tagged Groups (DTGs), containing between 5 and
30 member stars. Many of these DTGs can be associated with previously known
prominent substructures such as $Gaia$-Sausage/Enceladus (GSE), Sequoia, the
Helmi Stream (HStr), and Thamnos. Others are associated with previously
identified smaller dynamical groups of stars and streams. We identify 10 new
DTGs as well, many of which have strongly retrograde orbits. We also
investigate possible connections between our DTGs and $\sim$300 individual
$r$-process-enhanced (RPE) stars from a recent literature compilation. We find
that several of these objects have similar dynamical properties to GSE (5), the
HStr (4), Sequoia (1), and Rg5 (1), indicating that their progenitors might
have been important sources of RPE stars in the Galaxy. Additionally, a number
of our newly identified DTGs are shown to be associated with at least two RPE
stars each (DTG-2: 3, DTG-7: 2; DTG-27: 2). Taken as a whole, these results are
consistent with ultra-faint and/or dwarf spheroidal galaxies as birth
environments in which $r$-process nucleosynthesis took place, and then were
disrupted by the Milky Way.
