2025ApJ...995..140H

Preliminary astrometric data from the fourth data release of the Gaia mission revealed a 33 M_⊙ dark companion to a metal-poor red giant star, deemed Gaia BH3. This system hosts both the most massive known stellar-origin black hole and the lowest-metallicity star yet discovered in orbit around a black hole. The formation pathway for this peculiar stellar-black hole binary system has yet to be determined, with possible production mechanisms that include isolated binary evolution and dynamical capture. The chemical composition of the stellar companion in Gaia BH3 (hereafter BH3⋆) can help constrain the potential formation mechanisms of this system. Here, we conduct the most comprehensive chemical analysis of BH3⋆ to date using high resolution spectra obtained by the Tull Coudé Spectrograph on the 2.7 m Harlan J. Smith Telescope at McDonald Observatory to constrain potential formation mechanisms. We derived 29 elemental abundances ranging from lithium to thorium and find that BH3⋆ is an α-enriched ([α/Fe] = 0.41), r-I neutron-capture star ([Eu/Fe] = 0.57). We conclude that BH3⋆ shows no chemical peculiarities (defined as deviations from the expected chemical pattern of an r-I halo red giant) in any elements, which is in alignment with both the dynamical capture and isolated binary evolution formation scenarios. With an upper limit detection on Th, we use the Th/Eu chronometer to place limits on the cosmochronometric age of this system. These observations lay the groundwork for heavy-element chemical analysis for subsequent black hole and low-metallicity stellar binaries that will likely be found in Gaia DR4.