Other
Scientific paper
Jan 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011aas...21733819s&link_type=abstract
American Astronomical Society, AAS Meeting #217, #338.19; Bulletin of the American Astronomical Society, Vol. 43, 2011
Other
Scientific paper
We present results of recent X-ray observations of nitrogen-type Wolf-Rayet (WR) stars, focusing on late WN7 - WN9 subtypes. These observations were obtained as part of a broader X-ray survey of single WN stars using XMM-Newton and Chandra aimed at determining their X-ray properties and identifying plausible emission mechanisms. None of the WN survey targets is known to be a member of a binary system. We report unambiguous detections of WR 78 (WN7h) and WR 79a (WN9ha), and a possible (faint) detection of WR 16 (WN8h). These new WNL detections, along with previous detections of WN2 - WN6 stars, demonstrate that WN stars are X-ray sources across their full range of spectral subtypes. The X-ray luminosities of WN stars span a broad range (even among stars of similar spectral subtype) with typical values log Lx = 31.5 - 33.5 (ergs/s). There is some evidence that Lx is proportional to wind kinetic energy. WN star X-ray spectra show higher absorption than expected from optical extinction, likely due to their powerful metal-rich winds. Their spectra reveal strong emission lines from ionized metals and spectral fits typically require two components consisting of cool (kT < 1 keV) and hot (kT > 2 keV) plasma. Only cool plasma is anticipated if the X-rays arise in radiative wind shocks, so the presence of hot plasma implies other as yet unidentified X-ray production mechanisms are at work. Magnetic fields may play a role, but in contrast to magnetically-active solar-like stars, WN stars do not show large-amplitude X-ray flares in observations typically spanning a half day. But, more extensive monitoring at higher time-resolution is needed. Alternatively, unseen companions at close separation could play a role in WN star X-ray emission in some cases (esp. higher Lx objects), either via their intrinsic emission or as colliding wind secondaries.
Guedel Manuel
Schmutz Werner
Skinner Steve L.
Zhekov Svet
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