The origin of low spin of black holes in LIGO/Virgo mergers

LIGO/Virgo have reported six binary black hole (BH-BH) mergers. The effective spins of all of them are clustered around Xeff=0. However, the effective spin of one of them, GW170104, has an 82% probability of being negative, which would indicate a significant spin-orbit misalignment and seemingly supporting dynamical formation over isolated binary evolution. We show (i) as a proof-of-principle case, that GW170104 could have been formed through isolated binary evolution, and (ii) that the LIGO/Virgo measurements inform about, thus far unconstrained, angular momentum transport in massive stars. Massive stars can have inner transport of angular momentum and their strong winds may carry away substantial angular momentum, affecting the natal spin of the BHs created at the end of their lives. We present a physically motivated model for BH natal spins based on moderately efficient angular momentum transport. With this model we can explain the low effective spin of GW170104 in the context of isolated binary evolution. However, our predicted effective spin distribution of BH-BH mergers, showing a large fraction of high effective spins, is not consistent with LIGO/Virgo observations. If the effective spins of BHs observed by LIGO/Virgo continue to be small, it will indicate that angular momentum transport in massive stars is more efficient than adopted in our model and that BHs found in BH-BH mergers are born with low spins (a_spin=0).