A Possible Formation Scenario of the Gaia BH1: Inner Binary Merger in Triple Systems

Abstract

Based on astrometric measurements and spectral analysis from Gaia DR3, two quiescent black hole (BH) binaries, Gaia BH1 and BH2, have been identified. Their origins remain controversial, particularly for Gaia BH1. By considering a rapidly rotating ( ω / ω _crit = 0.8) and strongly magnetized ( B _0 = 5000 G) merger product, we find that, at typical Galactic metallicity, the merger product can undergo efficient chemically homogeneous evolution. This results in the merger product having a significantly smaller radius during its evolution compared to that of a normally evolving massive star. Under the condition that the initial triple stability is satisfied, we use the Multiple Stellar Evolution code and the MESA code to identify an initial hierarchical triple that can evolve into Gaia BH1. It initially consists of three stars with masses of 9.03 M _⊙ , 3.12 M _⊙ , and 1 M _⊙ , with inner and outer orbital periods of 2.21 days and 121.92 days, and inner and outer eccentricities of 0.41 and 0.45, respectively. This triple initially experiences triple evolution dynamics instability (TEDI) followed by Roche lobe overflow (RLOF). During RLOF, the inner orbit shrinks, and tidal effects gradually suppress the TEDI. Eventually, the inner binary undergoes a merger through contact (or collision). Finally, using models of rapidly rotating and strongly magnetic stars, along with standard core-collapse supernova (SN) or failed supernova (FSN) models, we find that a postmerger binary (PMB) consisting of an 12.11 M _⊙ merger product and a 1 M _⊙ companion star (originally an outer tertiary) can avoid RLOF. After an SN or FSN with a low ejected mass of ∼0.22 M _⊙ and a low kick velocity ( ${46}_{-33}^{+25}\,\mathrm{km}\,{{\rm{s}}}^{-1}$ or ${9}_{-8}^{+16}\,\mathrm{km}\,{{\rm{s}}}^{-1}$ ), the PMB can form Gaia BH1 in the Galactic disk.

Keywords

• Black holes