Colliding Winds in Massive Binary Systems

Abstract

Stellar winds are a well-observed phenomenon, and in massive stars this process may result in significant mass loss from the system with dramatic evolutionary ramifications. In close binary systems of massive stars, the individual stellar winds will collide and form a shock front between the stars. The existence of this shock can be established through orbital phase–related variations in UV wind features and optical emission lines. High-density regions in the wind (near the photospheres and the shock region) will produce Ha and He i emission which can be used to map out the mass-flow structure of the system. The existence of a shock front between the stars may influence the balance of mass loss versus mass transfer in massive binary evolution since matter lost to one star as a result of Roche lobe overflow may hit the shock and be deflected before it can accrete onto the other star. I report here the results of an all-sky, high-resolution spectroscopic survey of massive binaries and identify those systems which show evidence of excess emission. Data were obtained in both the southern and northern hemispheres, and spectra from two other groups of observers, Gies and Kaper and their collaborators, were also included. The results are compared with two independent surveys of massive stars and show that binaries have a higher incidence and intensity of emission when compared to single stars of similar spectral type. Seven systems in my survey (plus four identified by earlier surveys) show significant phase-related variations in emission. I identify these as likely colliding-wind candidates. For three systems, HD 149404, HD 152248, and HD 163181, I present a simple model of the mass distribution and flow based on Ha, He i l6678, and in some cases, the UV resonance lines. Figure 1 shows, as an example, Ha profiles of the O8.5 I 1 O6.5 III binary HD 149404 plotted against heliocentric radial velocity. The profiles are arranged in order of orbital phase. Each spectrum is placed so that the continuum equals the phase of observation. The lower frame shows a gray-scale representation of the profile variations. Ha is composed of two broad emission components which appear to show clear orbital motion, as indicated by the opposite patterns in the gray-scale S diagram. However, the patterns are not those expected from the orbital motion of the stars. Analysis of the radial velocity curves suggests that these features arise in the gas flows or focused winds from the stars interacting with a shock region.