The Formation of a 70 M⊙ Black Hole at High Metallicity
Journal
Date Issued
2020
Author(s)
Belczynski, K.
•
Hirschi, R.
•
Kaiser, E. A.
•
Liu, Jifeng
•
Casares, J.
•
Lu, Youjun
•
O'Shaughnessy, R.
•
Heger, A.
•
Justham, S.
•
Abstract
A 70 ${M}_{\odot }$ black hole (BH) was discovered in the Milky Way disk in a long-period detached binary system (LB-1) with a high-metallicity 8 ${M}_{\odot }$ B star companion. Current consensus on the formation of BHs from high-metallicity stars limits the BH mass to be below 20 ${M}_{\odot }$ due to strong mass loss in stellar winds. Using analytic evolutionary formulae, we show that the formation of a 70 ${M}_{\odot }$ 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 ${M}_{\odot }$ star model at Z = 0.014 with decreased winds ends up as a 71 ${M}_{\odot }$ star prior to core collapse with a 32 ${M}_{\odot }$ He core and a 28 ${M}_{\odot }$ CO core. Such a star avoids the pair-instability pulsation supernova mass loss that severely limits BH mass and may form a ∼70 ${M}_{\odot }$ BH in the direct collapse. Stars that can form 70 ${M}_{\odot }$ BHs at high Z expand to significant sizes, with radii of R ≳ 600 ${R}_{\odot }$ , however, exceeding the size of the LB-1 orbit. Therefore, we can explain the formation of BHs up to 70 ${M}_{\odot }$ 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.
Volume
890
Issue
2
Start page
113
Issn Identifier
0004-637X
Ads BibCode
2020ApJ...890..113B
Rights
open.access
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