The spin rates of O stars in WR+O Magellanic Cloud binaries

Some massive, merging black holes (BHs) may be descendants of binary O stars. The evolution and mass transfer between these O stars determine the spins of their progeny BH. These will be measurable with future gravitational wave detectors, incentivizing the measurement of the spins of O stars in binaries. We previously measured the spins of O stars in Galactic Wolf-Rayet (WR)+O binaries. Here we measure the V_e sin I of four Large Magellanic Cloud (LMC) and two Small Magellanic Cloud (SMC) O stars in WR+O binaries to determine whether lower metallicity might affect the spin rates. We find that the O stars in Galactic and Magellanic WR+O binaries display average V_e sin I = 258 ± 18 and 270 ± 15 km s^-1, respectively. Two LMC O stars measured on successive nights show significant line width variability, possibly due to differing orbital phases exhibiting different parts of the O stars illuminated differently by their WR companions. Despite this variability, the V_e sin I are highly supersynchronous but distinctly subcritical for the O stars in all these binaries; thus we conclude that an efficient mechanism for shedding angular momentum from O stars in WR+O binaries must exist. This mechanism, probably related to Roche lobe overflow (RLOF)-created dynamo magnetic fields, prevents nearly 100 per cent breakup spin rates, as expected when RLOF operates, as it must, in these stars. A Spruit-Tayler dynamo and O star wind might be that mechanism.