2017MNRAS.464.2066S

The black holes (BH) in merging BH-BH binaries are likely progeny of binary O stars. Their properties, including their spins, will be strongly influenced by the evolution of their progenitor O stars. The remarkable observation that many single O stars spin very rapidly can be explained if they accreted angular momentum from a mass-transferring, O-type or Wolf-Rayet (WR) companion before that star blew up as a supernova. To test this prediction, we have measured the spin rates of eight O stars in WR + O binaries, increasing the total sample size of such O stars' measured spins from 2 to 10. Polarimetric and other determinations of these systems' sin i allow us to determine an average equatorial rotation velocity from He I (He II) lines of v_e = 348 (173) km s^–1 for these O stars, with individual star's v_e from He I (He II) lines ranging from 482 (237) to 290 (91) km s^–1. We argue that the ∼100 per cent difference between He I and He II speeds is due to gravity darkening. Supersynchronous spins, now observed in all 10 O stars in WR + O binaries where it has been measured, are strong observational evidence that Roche lobe overflow mass transfer from a WR progenitor companion has played a critical role in the evolution of WR + OB binaries. While theory predicts that this mass transfer rapidly spins up the O-type mass gainer to a nearly breakup rotational velocity of v_e ∼ 530 km s^–1, the observed average v_e of the O-type stars in our sample is 65 per cent of that speed. This demonstrates that, even over the relatively short WR-phase time-scale, tidal and/or other effects causing rotational spin-down must be efficient. A challenge to tidal synchronization theory is that the two longest period binaries in our sample (with periods of 29.7 and 78.5 d) unexpectedly display supersynchronous rotation.