Published online by Cambridge University Press: 25 May 2016
The interaction of the stellar wind of a supersonically moving massive star with its surrounding interstellar medium can result in the formation of an observable bow-shock. Recent studies at optical and infrared wavelengths indicate the presence of wind bow-shocks around several OB runaways, including the high-mass X-ray binary system Vela X-1. A large fraction of runaway stars do not seem to form wind bow-shocks. Obviously, when the local sound speed is high (∼ 100 km s−1), as is the case e.g., inside a hot superbubble, the (subsonic) space velocity would not be sufficient to form a bow-shock. Two-dimensional time-dependent hydrodynamical simulations indicate that the bow-shock is generally unstable; for certain combinations of ISM and wind parameters a bow-shock is not formed at all. The runaway nature of Wolf-Rayet stars in relation to the formation of wind bow-shocks is also discussed.