Fast Spinning O-Type Star with a Compact Companion

30 Doradus is an extremely active star-forming region located within the Large Magellanic Cloud. It contains ~1000 young and massive OB stars. Observations of single O-type stars in 30 Doradus show a two-component structure in the distribution of their rotational velocities. There is a low velocity peak centred at ~80 km/s and a high velocity tail extending to ~600 km/s. Such high rotational velocities are difficult to reconcile for single O-type stars. VFTS 399 is a rapidly rotating O-type star belonging to the high velocity tail. It has a projected equatorial rotational velocity of about 334 km/s. VFTS 399 is estimated to be ~100,000 times more luminous than the Sun.

X-ray observations of VFTS 399 show that its X-ray luminosity is too great to be generated by just a single star. Instead, the high X-ray luminosity suggests the presence of a compact companion accreting material from VFTS 399. The X-ray flux exhibits periodic pulsations which are consistant with the compact companion being a neutron star. These pulsations have a remarkably long period and suggest that the neutron star has already spun down significantly. The neutron star probably has an exceptionally strong magnetic field. These powerful magnetic field lines act as a “brake”, spinning down the neutron star.

The presence of a neutron star around VFTS 399 allows its high rotational velocity to be explained via the binary hypothesis. In this evolutionary pathway for the formation of single, rapidly rotating O-type stars, the progenitor of the neutron star was once a massive star with at least ~25 times the Sun’s mass. Having a massive companion in a close-in orbit spun-up VFTS 399 and kept its spin rate high. The massive companion went supernova and left behind VFTS 399 as a single, rapidly rotating O-type star with a putative neutron star companion.

Reference:
J. S. Clark et al. (2015), “The VLT-FLAMES Tarantula survey XX. The nature of the X-ray bright emission line star VFTS 399”, arXiv:1503.00930 [astro-ph.SR]