The R-Process Alliance: On J0010 - 1735, an Accreted, Chemically Unusual, Metal-poor Star with R-process Enhancement

We present a detailed chemo-dynamical analysis of the chemically peculiar very metal-poor ([Fe/H] =-2.22 plus or minus 0.11) Milky Way (MW) halo star, 2MASS J00101758 - 1735387 (hereafter J0010-1735), discovered by the R-Process Alliance. We derive abundances for 45 elements, including 28 neutron-capture elements, and confirm that is highly r-process-enhanced, with [Eu/Fe]= +1.30. The light, α, and Fe-peak element abundances ratios relative to Fe are all sub-solar, aside from C, Si, and most extremely, Ca. We investigate three possible formation scenarios based on the star's chemical abundances and orbital kinematics: whether it formed in the in-situ MW halo, in a MW dwarf galaxy satellite, or in a satellite of the Magellanic Cloud system. Using the position of in the [Mg/Mn]--[Al/Fe] plane, the abundance pattern of both the light and heavy elements, and the kinematics, we conclude that J0010-1735 most plausibly originated in a now-disrupted ultra-faint dwarf galaxy that was accreted into the MW halo. We examine potential chemical enrichment scenarios of J0010-1735, including one similar to the r-process-enhanced UFD Reticulum II, or enrichment via a sub-Chandrasekhar Type Ia SNe, potentially supplemented by a Ca-rich gap transient and by a separate neutron-rich site that facilitated r-process enrichment. A nuclear cosmochronometric age of 12.5 plus or minus4.2 Gyr is derived from the [Th/Eu] ratio, consistent with an ancient origin in one of the earliest star-forming systems in the universe.