The Formation of a 70 M o˙ Black Hole at High Metallicity

K. Belczynski, R. Hirschi, E. A. Kaiser, Jifeng Liu, J. Casares, Youjun Lu, R. O'Shaughnessy, A. Heger, S. Justham, R. Soria

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Abstract

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 M 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 M 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.

Original languageEnglish
Article number113
Number of pages5
JournalThe Astrophysical Journal
Volume890
Issue number2
DOIs
Publication statusPublished - 20 Feb 2020

Cite this

Belczynski, K., Hirschi, R., Kaiser, E. A., Liu, J., Casares, J., Lu, Y., O'Shaughnessy, R., Heger, A., Justham, S., & Soria, R. (2020). The Formation of a 70 M o˙ Black Hole at High Metallicity. The Astrophysical Journal, 890(2), [113]. https://doi.org/10.3847/1538-4357/ab6d77