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]$=-2.22\pm 0.11$) 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 \thestar\ is highly $r$-process-enhanced, with [Eu/Fe]=$+1.30$. The light, $\alpha$, 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 $\mathit{\text{in-situ}}$ MW halo, in a MW dwarf galaxy satellite, or in a satellite of the Magellanic Cloud system. Using the position of \thestar\ 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 $12.5\pm 4.2$ Gyr is derived from the [Th/Eu] ratio, consistent with an ancient origin in one of the earliest star-forming systems in the universe.