ASCA observations of γ2Velorum (WC8+O9I): the variable X-ray spectrum of colliding winds.
Abstract
We present an analysis of two observations of the nearby Wolf-Rayet (WR)+O-star binary system γ2Velorum with the Advanced Satellite for Cosmology and Astrophysics (ASCA). These observations were taken at orbital phases φ=0.4 and 0.5, when the O star is close to being in front of the WR star. At phase φ=0.5, γ2Vel is approximately four times more luminous in X-rays than it is at φ=0.4. We fit the X-ray spectra using simple combinations of absorbed Raymond-Smith emission models. We find evidence of a cool (kT ∼0.3 keV) and a hot (kT ∼1.3 keV) component in each spectrum. The hotter component suffers from variable absorption which suggests that it originates in a localized region between the two stars. Between φ=0.4 and 0.5 the absorbing column for this hotter component drops by a factor of 3 which accounts for most of the corresponding increase in flux.
These observations are interpreted in terms of a colliding-wind model. The collision between the winds from the O star and the WR star generates a region of hot X-ray emitting gas. When the WR star is in front, the dense WR wind effectively absorbs all the X-ray flux from the wind collision. However, when the O star is in front the more diffuse wind of the O star allows substantially more flux from the wind collision to escape. Our ASCA data confirm the colliding-wind origin for the hard X-ray emission from y2 Vel, which we then fit to the ASCA spectra. This allows us to determine fundamental stellar wind parameters for both stellar components from the X-ray spectra. In particular, we determine a mass loss for the WC8 star of ∼3×10-5 Msun yr-1, a value 3 times lower than that determined by radio observations. The consequences of this are discussed. We consider that we are definitely seeing colliding-wind emission and also show the great potential for X-ray spectroscopy in determining fundamental wind parameters of WR (and other hot luminous) stars.- Publication:
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Monthly Notices of the Royal Astronomical Society
- Pub Date:
- December 1996
- DOI:
- 10.1093/mnras/283.2.589
- Bibcode:
- 1996MNRAS.283..589S
- Keywords:
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- Stellar Winds: Binaries;
- Stellar Winds: Wolf-Rayet Stars;
- Stellar Winds: X-Ray Variations;
- Stellar Winds: Collisions;
- Stellar Winds: Synthetic Spectra;
- binaries: general -- stars: early-type -- stars: individual: γ2 Velorum -- stars: mass-loss stars: Wolf-Rayet -- X-rays: stars