X‐Ray Counterparts of Runaway Stars

Abstract

An X-ray search for possible compactcompanions of runaway OB stars has been conducted using pointed ROSATobservations. Of a list of 71 runaway stars, ROSAT exposures were availablefor 24, of which 13 are detected. These numbers are nearly 3 timeslarger than for a previously studied Einstein sample, and spectralinformation is exploited as well. Luminosities, hardness ratios, andlong-term variability are as for normal OB stars and do not suggestthe presence of collapsed companions. A result like this is ofteninterpreted as support for dynamical ejection from a dense grouprather than a supernova event in a binary as a production processfor runaway stars. There are, however, several circumstances thatmay adversely affect the observability of a compact companion, orafter a supernova explosion systems may be disrupted because of thelarge natal kick velocity imparted to the neutron star as a result ofasymmetries in the explosions. It is noted that there is actuallyevidence for both of these production routes and that they may beexpected to occur sequentially in the evolution of OB associations.