The characteristics of high velocity O and B stars which are ejected from supernovae in binary systems

Abstract

We perform binary population synthesis calculations to study the origin and the characteristics of runaway O and B stars which are ejected by the supernova explosion of the companion star in a binary system. The number of OB runaways can be explained from supernova ejections only if: high mass stars are preferentially formed in binaries, the initial mass ratio distribution is strongly peaked to unity and stars are rejuvenated to zero age upon accretion of mass from a companion star. Taking these requirements into consideration we conclude that at most 30% of the runaway O stars but possibly all runaway B stars obtain high velocities due to supernovae in evolving binaries. Stars which obtain high velocities via supernova ejections have the following characteristics: 1) at least 10% of the high velocity B stars and half the O stars have a mass greater than the turn off mass of the cluster in which they are born and would be observed as blue stragglers in the parent cluster, 2) their equatorial rotational velocities are proportional to their space velocity and 3) between 20% and 40% of the runaways have neutron star companions but less than 1% are visible as radio pulsars in part of the orbit.