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

Abstract

Some massive, merging black holes (BH) may be descendants of binary O stars. The evolution and mass transfer between these O stars determines 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 sini of four LMC and two 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 sini = 258 ± 18 km/s and 270 ± 15 km/s, 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 sini are highly super-synchronous 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-created dynamo magnetic fields, prevents nearly 100% 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.

The IACOB project

“…Aside from projection effects, we can expect some differences between our empirical results and de Mink's simulations due to processes favoring a faster spin down than modeled, such as the formation of magnetic fields during mass accretion (as mentioned by de Mink et al 2013). Evidence supporting such processes (not necessarily a magnetic field) is found in the observation of super-synchronization but clearly subcritical rotation in OB+WR systems (Shara et al 2020). Moreover, we note that the de Mink et al ( 2013) simulation includes SB2 stars that have not been considered in our observational sample, and that the difference in metallicity -LMC for de Mink's study -could lead to a higher number of high-spin objects in their simulations.…”

An X-ray-quiet black hole born with a negligible kick in a massive binary within the Large Magellanic Cloud

“…However, one may expect mass accretors to reach critical rotation velocities [88] of ≈ 500 − 600 km s −1 . In contrast to the theoretical expectation, post-interaction O-type mass accretors (e.g., in Wolf-Rayet binaries) are commonly observed to rotate at sub-critical velocities, with typical velocities of 200-300 km s −1 [48,89]. The reasons for this are yet to be established, but could be related to boosted mass-loss of rapid rotators or magnetic fields produced during mass-transfer [89].…”

X-ray emission from BH+O star binaries expected to descend from the observed galactic WR+O binaries

“…The observed population of Be/X-ray binaries (Reig 2011) signifies that this spin-up may achieve near-critical rotation, with strong consequences for the mass outflow from the spun-up star, and the mass accretion onto the compact companion. The Galactic and LMC WR+O binaries do indeed also contain rapidly rotating O stars (Vanbeveren et al 2018;Shara et al 2020). However, while faster than average O stars, the analysed WR companions rotate on average with less than 50% of their critical rotational velocity, implying that the centrifugal force remains below 25% of the surface gravity at the stellar equator.…”