A Lifetime of Influence: Abstracts, Talks, and Posters
 

X-ray Emission from Wolf-Rayet Stars

S.L. Skinner
Univ. of Colorado, Boulder, CO

S.A. Zhekov
JILA, Univ. of Colorado, Boulder, CO

M. Guedel
ETH, Zurich, Switzerland

W. Schmutz
PMOD, Davos, Switzerland

K.R. Sokal
Univ. of Colorado, Boulder, CO

Previous X-ray studies of Wolf-Rayet (WR) stars have focused mainly on WR binary systems whose luminous high-temperature X-ray emission is thought to originate (at least partially) in colliding wind shocks. Much less is known about the X-ray emission of single WR stars, for which no evidence of binarity has yet been found. We present new results from an ongoing X-ray survey of single WR stars conducted with Chandra and XMM-Newton. Our objectives are to quantify the X-ray properties of single WRs and identify plausible emission mechanisms. Specifically, we would like to know if single WR stars emit predominantly soft X-rays (kT < 1 keV), as occurs in some O-type stars and is predicted for radiative wind shocks formed by the line-driven instability (LDI) mechanism. Observations to date have failed to detect any X-ray emission from single WC stars. The most stringent upper limit on X-ray luminosity is log L_x <= 29.82 ergs/s for the WC8 star WR 135, more than two orders of magnitude below that of X-ray bright WR binaries. In contrast, some WN stars have been detected. Recent Chandra observations of both WR 2 (WN2) and WR 134 (WN6) reveal luminous X-ray emission (log L_x > 32. erg/s) and a hot plasma component (kT >> 1 keV). The presence of high-temperature plasma is not in agreement with the cool plasma (kT < 1 keV) expected for radiative wind shocks. Either these X-ray bright WN stars harbor unseen companions or an alternative to the conventional radiative wind shock model will be needed.