Chemo-dynamical Analysis of a CNO-Enhanced Ultra Metal-poor Star ([Fe/H] < -4): Insights into Early Enrichment by Faint Population III Supernova

We report an independent identification of the ultra metal-poor (UMP) star ([Fe/H] =-4.06) Gaia DR3 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 about 6, kpc from the Sun. Abundance analysis under local thermodynamic equilibrium reveals significant enhancements, relative to the solar level, in carbon ([C/Fe] = +1.54, after evolutionary correction for depletion in the RGB), nitrogen ([N/Fe] = +2.64), and oxygen ([O/Fe] = +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 about 21-to-28,M and a low explosion energy of (0.3 at most E SN/(10 51, erg) at most 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.