A Detailed Chemical Analysis of the Red Giant Orbiting the Black Hole $Gaia$ BH3: From Lithium to Thorium

TL;DR

Gaia BH3 presents a metal-poor giant in a binary with a dormant black hole, providing a clean chemical record of early nucleosynthesis in a low-metallicity environment. The study conducts a comprehensive high-resolution spectroscopic analysis of BH3, deriving 29 elemental abundances spanning lithium to thorium. It finds the host star is alpha-enhanced with [alpha/Fe] = 0.41 and r-I neutron-capture with [Eu/Fe] = 0.57, showing no chemical peculiarities relative to an r-I halo red giant. Using an upper limit on thorium, they apply the Th/Eu cosmochronometer to place a cosmochronometric age constraint on the system. The results support both isolated binary evolution and dynamical capture formation scenarios and establish a groundwork for heavy-element chemical analyses of future Gaia DR4 black hole binaries.

Abstract

Preliminary astrometric data from the fourth data release of the $Gaia$ mission revealed a 33 M$_{\odot}$ 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 \bhstar) can help constrain the potential formation mechanisms of this system. Here, we conduct the most comprehensive chemical analysis of \bhstar\ to date using high resolution spectra obtained by the Tull Coudé Spectrograph on the 2.7m 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 \bhstar\ is an $α$-enriched ([$α$/Fe] = 0.41), r-I neutron-capture star ([Eu/Fe] = 0.57). We conclude that \bhstar\ 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.