Formation of a $33,M_{odot}$ black hole in a low-metallicity binary

Formation of a $33,M_{odot}$ black hole in a low-metallicity binary
Kareem El-Badry
arXiv:2404.13047v1 Announce Type: new
Abstract: A $33,M_odot$ black hole (BH) was recently discovered in an 11.6-year binary only 590 pc from the Sun. The system, Gaia BH3, contains a $0.8,M_odot$ low-metallicity giant ($rm [M/H]=-2.2$) and is kinematically part of the Galactic halo, suggesting that the BH formed from a low-metallicity massive star. I show that orbits similar to that of Gaia BH3 are naturally produced through isolated binary evolution. The system’s period and eccentricity can result from a broad range of initial orbits with a modest natal kick ($v_{rm kick}lesssim 75,rm km,s^{-1}$) to the BH. I construct MESA models for metal-poor massive stars with initial masses ranging from $35-55,M_{odot}$, which reach maximum radii of $1150-1800,R_{odot}$ as red supergiants. Stars of this size would fit inside most plausible pre-supernova orbits for the system without overflowing their Roche lobes. In addition, models with moderately rapid initial rotation ($Omega/Omega_{rm crit} gtrsim 0.45$) undergo chemically homogeneous evolution and never expand to radii larger than $10,R_{odot}$. There are thus multiple channels through which a low-metallicity, extreme-mass ratio binary could produce a system like Gaia BH3. Dynamical formation scenarios are also viable, and there is little doubt that both isolated and dynamically-formed BH binaries with orbits similar to Gaia BH3 will be discovered in Gaia DR4. Only about 1 in 10,000 stars in the solar neighborhood have metallicities as low as Gaia BH3. This suggests that BH companions are dramatically over-represented at low-metallicity, though caveats related to small number statistics apply. The fact that the luminous star in Gaia BH3 has been a giant – greatly boosting its detectability – only for $sim$1% of the time since the system’s formation implies that additional massive BHs remain to be discovered with only moderately fainter companions.arXiv:2404.13047v1 Announce Type: new
Abstract: A $33,M_odot$ black hole (BH) was recently discovered in an 11.6-year binary only 590 pc from the Sun. The system, Gaia BH3, contains a $0.8,M_odot$ low-metallicity giant ($rm [M/H]=-2.2$) and is kinematically part of the Galactic halo, suggesting that the BH formed from a low-metallicity massive star. I show that orbits similar to that of Gaia BH3 are naturally produced through isolated binary evolution. The system’s period and eccentricity can result from a broad range of initial orbits with a modest natal kick ($v_{rm kick}lesssim 75,rm km,s^{-1}$) to the BH. I construct MESA models for metal-poor massive stars with initial masses ranging from $35-55,M_{odot}$, which reach maximum radii of $1150-1800,R_{odot}$ as red supergiants. Stars of this size would fit inside most plausible pre-supernova orbits for the system without overflowing their Roche lobes. In addition, models with moderately rapid initial rotation ($Omega/Omega_{rm crit} gtrsim 0.45$) undergo chemically homogeneous evolution and never expand to radii larger than $10,R_{odot}$. There are thus multiple channels through which a low-metallicity, extreme-mass ratio binary could produce a system like Gaia BH3. Dynamical formation scenarios are also viable, and there is little doubt that both isolated and dynamically-formed BH binaries with orbits similar to Gaia BH3 will be discovered in Gaia DR4. Only about 1 in 10,000 stars in the solar neighborhood have metallicities as low as Gaia BH3. This suggests that BH companions are dramatically over-represented at low-metallicity, though caveats related to small number statistics apply. The fact that the luminous star in Gaia BH3 has been a giant – greatly boosting its detectability – only for $sim$1% of the time since the system’s formation implies that additional massive BHs remain to be discovered with only moderately fainter companions.