The Formation of a 70 M_⊙ Black Hole at High Metallicity

A 70 M_⊙ black hole (BH) was discovered in the Milky Way disk in a long-period detached binary system (LB-1) with a high-metallicity 8 M_⊙  B star companion. Current consensus on the formation of BHs from high-metallicity stars limits the BH mass to be below 20 M_⊙  due to strong mass loss in stellar winds. Using analytic evolutionary formulae, we show that the formation of a 70 M_⊙  BH in a high-metallicity environment is possible if wind mass-loss rates are reduced by factor of five. As observations indicate, a fraction of massive stars have surface magnetic fields that may quench the wind mass-loss, independently of stellar mass and metallicity. We confirm such a scenario with detailed stellar evolution models. A nonrotating 85 M_⊙  star model at Z = 0.014 with decreased winds ends up as a 71 star prior to core collapse with a 32 M_⊙ He core and a 28 M_⊙ CO core. Such a star avoids the pair-instability pulsation supernova mass loss that severely limits BH mass and may form a ~70 M_⊙ BH in the direct collapse. Stars that can form 70 BHs at high Z expand to significant sizes, with radii of R ≳ 600 R_⊙, however, exceeding the size of the LB-1 orbit. Therefore, we can explain the formation of BHs up to 70 M_⊙ at high metallicity and this result is valid whether or not LB-1 hosts a massive BH. However, if LB-1 hosts a massive BH we are unable to explain how such a binary star system could have formed without invoking some exotic scenarios