The R-Process Alliance: Chemo-Dynamically Tagged Groups of Halo $r$-Process-Enhanced Stars Reveal a Shared Chemical-Evolution History

Kavli Affiliate: Anna Frebel

| First 5 Authors: Dmitrii Gudin, Derek Shank, Timothy C. Beers, Zhen Yuan, Guilherme Limberg

| Summary:

We derive dynamical parameters for a large sample of 446 $r$-process-enhanced
(RPE) metal-poor stars in the halo and disk systems of the Milky Way, based on
data releases from the $R$-Process Alliance, supplemented by additional
literature samples. This sample represents more than a ten-fold increase in
size relative to that previously considered by Roederer et al., and, by design,
covers a larger range of $r$-process-element enrichment levels. We test a
number of clustering analysis methods on the derived orbital energies and other
dynamical parameters for this sample, ultimately deciding on application of the
HDBSCAN algorithm, which obtains 30 individual Chemo-Dynamically Tagged Groups
(CDTGs); 21 contain between 3 and 5 stars, and 9 contain between 6 and 12
stars. Even though the clustering was performed solely on the basis of their
dynamical properties, the stars in these CDTGs exhibit statistically
significant similarities in their metallicity ([Fe/H]), carbonicity ([C/Fe]),
and neutron-capture element ratios ([Sr/Fe], [Ba/Fe], and [Eu/Fe]). These
results demonstrate that the RPE stars in these CDTGs have likely experienced
common chemical-evolution histories, presumably in their parent satellite
galaxies or globular clusters, prior to being disrupted into the Milky Way’s
halo. We also confirm the previous claim that the orbits of the RPE stars
preferentially exhibit pericentric distances that are substantially lower than
the present distances of surviving ultra-faint dwarf and canonical dwarf
spheroidal galaxies, consistent with the disruption hypothesis. The derived
dynamical parameters for several of our CDTGs indicate their association with
previously known substructures, Dynamically Tagged Groups, and RPE Groups.

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