Contributed Talk - Splinter HotStars

Friday, 15 September 2023, 15:05   (H 2036)

The rotation of single and binary O-stars in the Milky Way

Susanne Blex, Martin Haas, Rolf Chini
Astronomical Institute, Ruhr University Bochum

We determined the projected rotational velocity (vsini) of a complete sample of 170 single- and 70 double-lined Galactic O-stars, based on 3500 high-resolution multi-epoch optical spectra. The overall vsini statistic peaks at slow rotators (40-100 km/s) with a tail towards increased (100-200 km/s) and fast rotators (200-400 km/s). Compared to single stars, the binaries reveal a strong excess of increased rotators but a strong deficit of fast rotators. Two thirds of the double-lined binaries are close pairs with orbital periods shorter than 10 days. In most cases, both components are increased rotators, consistent with spin-orbit synchronization. Some have strong vsini differences as expected in a donor-gainer scenario. The remaining third of binaries have long periods and their separation is wide enough to discard spin-up by tidal friction or mass exchange. Typically, one component is an increased rotator and one is a slow rotator. We suggest that the wide-orbiting increased rotators gained their spin from a previous merging event, possibly as proto-star before reaching the main sequence. This could indicate that O-stars are mostly formed from merging of smaller fragments.