High-resolution X-ray spectroscopy reveals the special nature of Wolf-Rayet star winds
Oskinova, L.M.; Gayley, K.G.; Hamann, W.-R.; Huenemoerder, D.P.; Ignace, R.; Pollock, A.M.T.
We present the first high-resolution X-ray spectrum of a putatively
single Wolf-Rayet star. 400 ks observations of WR6 by the
XMM-Newton-telescope resulted in a superb quality high-resolution X-ray
spectrum. Spectral analysis reveals that the X-rays originate far out
in the stellar wind, more than 30 stellar radii from the photosphere,
and thus outside the wind acceleration zone where the line-driving
instability could create shocks. The X-ray emitting plasma reaches
temperatures up to 50MK, and is embedded within the un-shocked, "cool"
stellar wind as revealed by characteristic spectral signatures. We
detect a fluorescent Fe line at ~6.4 keV. The presence of fluorescence
is consistent with a two-component medium, where the cool wind is
permeated with the hot X-ray emitting plasma. The wind must have a very
porous structure to allow the observed amount of X-rays to escape. We
find that neither the line-driving instability nor any alternative
binary scenario can explain the data. We suggest a scenario where X-rays
are produced when the fast wind rams into slow "sticky clumps" that
resist acceleration. Our new data show that the X-rays in single
WR-star are generated by some special mechanism different from the one
operating in the O-star winds.
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