Characterising the red giant companion of the black hole in the BH2 system

Context. The recently discovered Gaia BH2 binary system composed of a red giant star and a dormant black hole offers a great opportunity to investigate the formation of binary black holes. Aims. From this perspective, we performed an independent determination of fundamental parameters of the luminous giant star BH2*, a young thin disk object and high [ α /Fe]. A peculiarity of our investigation is the adoption of stellar models specifically tailored to Galactic red giant branch stars with chemical abundances and [ α /Fe] scaling calibrated over a large sample of objects. Methods. We derived the estimated system parameters using the SCEPtER pipeline, which relies on spectroscopic and global asteroseismic constraints from literature investigations and utilises a large grid of stellar models. We explored the robustness of the determination by adopting two different corrections for Δ ν values from the literature to account for the current uncertainty on this quantity. Results. The estimated masses ranged from M = 1.19 ± 0.05 M ⊙ to M = 1.26 ± 0.05 M ⊙ . The global age of the system was determined to be 4.8 ± 0.5 (sys) ± 0.7 (rand) Gyr. These estimates are consistent with recent findings but exhibit a significantly reduced uncertainty. The radius of BH2* was estimated to be between 8.23 ± 0.12 and 8.47 ± 0.13 R ⊙ . To explore potential merging or accretion events in the evolutionary history of BH2*, we conducted a supplementary radius estimation based on surface brightness–colour relations utilising V and K magnitudes along with Gaia DR3 parallax data. This estimate, based on two validated relations, indicated a significantly lower radius range of 7.50 ± 0.23 to 7.80 ± 0.23 R ⊙ . However, this discrepancy was not large enough to rule out a mere fluctuation. Finally, we explored the possibility of inferring non-single-star evolutionary scenarios for BH2* based on its chemical abundance pattern. Principal component analysis (PCA) using α -element abundances and sodium revealed that the position of BH2* in the PCA space was extreme, even when compared to other young α -enhanced stars identified as suspect merging products. Conclusions. Further asteroseismic observations and carbon and nitrogen determinations would enable a more detailed characterisation of BH2* and provide further insights into its evolutionary history.