Finding r-II Sibling Stars in the Milky Way with the Greedy Optimistic Clustering Algorithm

Abstract R -process enhanced stars with [Eu/Fe] ≥ +0.7 (so-called r -II stars) are believed to have formed in an extremely neutron-rich environment which a rare astrophysical event (e.g., neutron-star merger) occurred. This scenario is supported by the existence of ultra-faint dwarf galaxy, Reticulum II, where most highly elements. In this scenario, some small fraction galaxies around Milky Way were enhanced. When each r-enhanced galaxy accreted Way, it deposited many Galactic halo similar orbital actions. To search for remnants -enhanced systems, we analyzed distribution actions N = 161 solar neighborhood using Gaia EDR3 data. Since observational uncertainty not negligible, applied newly developed greedy optimistic clustering method our sample stars. We found six clusters that orbits and chemistry, one new discovery. Given apparent phase-mixed member stars, interpret these good candidates completely disrupted merged ancient Way.