Measuring the Core Rotation of Massive Stars Using Interacting Binaries

The rotation rates of massive stars have been observed to be high, while their black hole remnants can spin very slowly. Revealing the interior rotation profile of massive stars is a key component for understanding the slow spin observed in binary black hole mergers. However, because of the obscuring and thick H-rich envelope, robust measurements of interior rotation rates are still lacking.

Envelope-stripping, via mass transfer or common envelope ejection in interacting binary stars, offers an opportunity to study the exposed cores of massive stars and measure their rotation rates. We measure the projected rotational velocity of the cores of massive stars by fitting modeled spectra to observed spectra of recently discovered envelope-stripped stars in the Magellanic Clouds. Analyzing a ~4M_sun stripped star, we find that its rotation rate is consistent with zero, suggesting that the cores of massive stars spin slowly. This result appears to contradict the expectations from standard models of angular momentum transport, which predict that stellar cores spin-up with time. Measured rotation rates of a set of stripped stars will thus likely provide valuable constraints on how angular momentum is transported inside massive stars.