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.
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]