Chemo-dynamical Analysis of a CNO-Enhanced Ultra Metal-poor Star ($[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]<-4$): Insights into Early Enrichment by Faint Population III Supernova

We report an independent identification of the ultra metal-poor (UMP) star ($[\mathrm{F}\mathrm{e}\mathrm{/}\mathrm{H}]=-4.06$) $Gaia$ DR3 \texttt{source_id} 4795913112968206720 (GDR3_479591) in the $Gaia$ mission’s Blue and Red Photometer ‘XP’ spectro-photometric catalog. We combine multi-band photometry, astrometry, and high-resolution spectroscopy to confirm GDR3_479591 as a red giant-branch (RGB) star located at $\sim 6\mathrm{k}\mathrm{p}\mathrm{c}$ from the Sun. Abundance analysis under local thermodynamic equilibrium reveals significant enhancements, relative to the solar level, in carbon ($[\mathrm{C}\mathrm{/}\mathrm{F}\mathrm{e}]=+1.54$, after evolutionary correction for depletion in the RGB), nitrogen ($[\mathrm{N}\mathrm{/}\mathrm{F}\mathrm{e}]=+2.64$), and oxygen ($[\mathrm{O}\mathrm{/}\mathrm{F}\mathrm{e}]=+2.62$). The CNO-enhanced GDR3_479591 is thus one of only 5 UMP stars with a detected oxygen abundance. The CNO excess is accompanied by enhancements in several other light elements, such as Na, Mg, Al, and Si. We demonstrate that the chemical pattern of GDR3_479591 can be reproduced by the yields of a single Population~III ‘faint’ supernova with progenitor mass of $\sim$21-to-28$M_{\odot }$ and a low explosion energy of ($0.3\le E_{\mathrm{S}\mathrm{N}}\mathrm{/}({10}^{51}\mathrm{e}\mathrm{r}\mathrm{g})\le 0.9$). Additionally, we use literature metal-poor stars to show that, contrary to recent propositions for high-redshift galaxies, a mild enhancement in [C/O] does not automatically translate to the high [C/Fe] typically observed in UMP stars in the Milky Way and its satellites. GDR3_479591 could not be dynamically associated with any of the most relevant accreted substructures in the Galactic halo, and we speculate that it was formed in an ultra-faint dwarf galaxy environment that later merged with the Milky Way.