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

Robust determination of the fundamental parameters

¹ Dipartimento di Fisica “Enrico Fermi”, Università di Pisa, Largo Pontecorvo 3, I-56127 Pisa, Italy
² INFN, Sezione di Pisa, Largo Pontecorvo 3, I-56127 Pisa, Italy

^⋆ Corresponding author.

Received: 10 July 2025
Accepted: 21 August 2025

Abstract

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 Δν 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.