Massive runaway and walkaway stars models

These are outnumbered by more than an order of magnitude by slower unbound companions, or "walkaway stars". This is a robust outcome of our simulations and is due to the reversal of the mass ratio prior to the explosion and widening of the orbit, as we show analytically and numerically. For stars more massive than 15M_{sun}_, we estimate that 10^+5^_-8_% are walkaways and only 0.5^+1.0^_-0.4_% are runaways, nearly all of which have accreted mass from their companion.

Our findings are consistent with earlier studies, however, the low runaway fraction we find is in tension with observed fractions of about 10%. Thus, astrometric data on presently single massive stars can potentially constrain the physics of massive binary evolution. Finally, we show that the high end of the mass distributions of runaway stars is very sensitive to the assumed black hole natal kicks and propose this as a potentially stringent test for the explosion mechanism.

We also discuss companions remaining bound which can evolve into X-ray and gravitational wave sources.