A 70 { ~ { } M } _ { \odot } black hole was discovered in Milky Way disk in a long period ( P = 78.9 days ) and almost circular ( e = 0.03 ) detached binary system ( LB-1 ) with a high ( Z \sim 0.02 ) metallicity 8 { ~ { } M } _ { \odot } B star companion .
Current consensus on the formation of black holes from high metallicity stars limits the black hole mass to be below 20 { ~ { } M } _ { \odot } due to strong mass loss in stellar winds .
So far this was supported by the population of Galactic black hole X-ray binaries with Cyg X-1 hosting the most massive \sim 15 { ~ { } M } _ { \odot } black hole .
Using the Hurley et al .
2000 analytic evolutionary formulae , we show that the formation of a 70 { ~ { } M } _ { \odot } black hole in high metallicity environment is possible if stellar wind mass loss rates , that are typically adopted in evolutionary calculations , are reduced by factor of 5 .
As observations indicate , a fraction of massive stars ( \sim 10 \% ) have surface magnetic fields which , as suggested by Owocki et al .
2016 , may quench the wind mass-loss , independently of stellar mass and metallicity .
We also computed detailed stellar evolution models and we confirm such a scenario .
A non-rotating 85 { ~ { } M } _ { \odot } star model at Z = 0.014 with decreased winds ends up as a 71 { ~ { } M } _ { \odot } star prior core-collapse with a 32 { ~ { } M } _ { \odot } helium core and a 28 { ~ { } M } _ { \odot } CO core .
Such star avoids pair-instability pulsation supernova mass loss that severely limits black hole mass and may form a \sim 70 { ~ { } M } _ { \odot } black hole in the direct collapse .
Stars that can form 70 { ~ { } M } _ { \odot } black holes at high Z expand to significant size with radius of R \gtrsim 600 { ~ { } R } _ { \odot } ( thanks to large H-rich envelope ) , however , exceeding the size of LB-1 orbit ( semi-major axis a \lesssim 350 { ~ { } R } _ { \odot } ) .
Therefore , we can explain the formation of black holes upto 70 { ~ { } M } _ { \odot } at high metallicity , but unable to explain how a binary star system like LB-1 could have formed without invoking some exotic scenarios .