Colliding winds in O-type binaries

Abstract

In close binary systems of O-type stars, the individual stellar winds will collide between the stars to form shock fronts. The existence of the shock region can be established through observations of excess X-ray emission and orbital phase-related variations in UV wind features and optical emission lines. Here I report on work in progress on the orbital variations of the UV wind lines in a large sample of O-binaries that have been observed with the International Ultraviolet Explorer satellite. In most of these binaries, the blue absorption trough of the P-Cygni lines weakens when the strong wind component is behind its companion, in accordance with expectations for colliding winds. I describe how the radial velocity variations of the wind features differ from the orbital motions. Finally I discuss observations of Hα emission in these systems, with emphasis on HD 47129 (Plaskett's star). Such optical emission lines act as a probe of high density regions in the winds.