2005ApJ...624..307M

An X-ray search for possible compact companions of runaway OB stars has been conducted using pointed ROSAT observations. Of a list of 71 runaway stars, ROSAT exposures were available for 24, of which 13 are detected. These numbers are nearly 3 times larger than for a previously studied Einstein sample, and spectral information is exploited as well. Luminosities, hardness ratios, and long-term variability are as for normal OB stars and do not suggest the presence of collapsed companions. A result like this is often interpreted as support for dynamical ejection from a dense group rather than a supernova event in a binary as a production process for runaway stars. There are, however, several circumstances that may adversely affect the observability of a compact companion, or after a supernova explosion systems may be disrupted because of the large natal kick velocity imparted to the neutron star as a result of asymmetries in the explosions. It is noted that there is actually evidence for both of these production routes and that they may be expected to occur sequentially in the evolution of OB associations.