2019MNRAS.482.4408G

We report the results of our study of the high-velocity (~160 km s^–1) runaway O star CPD-64°2731 and its associated horseshoe-shaped nebula discovered with the Wide-field Infrared Survey Explorer. Spectroscopic observations with the Southern African Large Telescope and spectral analysis indicate that CPD-64°2731 is a fast-rotating main-sequence O5.5 star with enhanced surface nitrogen abundance. We derive a projected rotational velocity of ~300 km s^–1 that is extremely high for this spectral type. Its kinematic age of ≃6 Myr, assuming it was born near the Galactic plane, exceeds its age derived from single star models by a factor of 2. These properties suggest that CPD-64°2731 is a rejuvenated and spun-up binary product. The geometry of the nebula and the almost central location of the star within it argue against a pure bow shock interpretation for the nebula. Instead, we suggest that the binary interaction happened recently, thereby creating the nebula, with a cavity blown by the current fast stellar wind. This inference is supported by our results of 2D numerical hydrodynamic modelling.