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A study of the moderately wide Wolf-Rayet spectroscopic binary HD 190918
Radial-velocity observations of the Wolf-Rayet spectroscopic binary HD190918 obtained from 25 spectrograms covering the yellow-green range arepresented. In general three absorption lines were measured to determinethe line-of-sight motion of the O star and one unblended emission line,He II lambda 5411.52, for the Wolf-Rayet star. A sharp C III lambda 5696emission line, as seen in most Of type spectra, was detected on eachspectrogram and measured. This line follows the predictedradial-velocity curve of the O star fairly well when the radialvelocities are shifted by an appropriate amount. New orbital elementshave been found for the O star, for the Wolf-Rayet star, and for the CIII emission line. The estimated systemic velocity is -20.9 +/- 0.7 km/sfor the O star, +70.1 +/- 4.6 km/s for the Wolf-Rayet star, and -34.2+/- 1.5 km/s for the sharp C III emission line. The systemic velocity ofthe O star is reasonable considering the expected line-of-sightcomponent of motion due to the peculiar motion of Population I stars,Galactic rotation, and reflex solar motion. We adopt the O-star systemicvelocity as a fiducial radial velocity for the binary HD 190918. Thisshows that the He II lambda 5411 line of the WN4.5 star is displacedlongward by 91.1 km/s, while the sharp C III line appears to be formedin a body of gas moving toward the observer by an additional 13.3 km/s.We discuss the implications of each possible solution including theswath traversed by the O star in the outer part of the line emittingregion of the Wolf-Rayet star and the possible generation of X-rays. Weconclude that our observations of the sharp C III lambda 5696 emissionline confirm the hydrodynamic models of Stevens, Blondin, and Pollockwhich show that extensive, chaotic tongues of cooling plasma are formedperpendicular to the line joining the stars in the case of collidingwinds in massive binary systems. We describe observational tests whichmay be used to confirm what type of model is best for the line-emittingregion of a Wolf-Rayet star.
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