The kinematical and binary properties of association and field O stars

Abstract

A catalog of 195 Galactic O-type stars brighter than V = 8.0 mag has been compiled to compare the velocity distribution and binary frequency among cluster and association, field, and runaway stars. Both the field stars and runaway stars have a larger dispersion in peculiar radial velocity, a more positive mean peculiar radial velocity, and a wider z-distribution than stars found in clusters and associations, which is consistent with the ejection of field and runaway stars from their birthplaces in associations. Visual binaries are common among stars in clusters and associations, but their incidence is a factor of 2 lower among field stars, and they are absent in the runaway stars. Similarly, there is a deficiency of spectroscopic binaries among field stars, and especially the runaway stars, relative to the numbers found in clusters and associations. Many of these properties can be understood in terms of ejection through close gravitational interactions with binary stars during an early high number density epoch in the evolution of clusters.