High-Resolution Chandra Spectroscopy of (tau) Scorpii: A Narrow-Line X-Ray Spectrum from a Hot Star

Long known to be an unusual early-type star by virtue of its hard and strong X-ray emission, (tau) Scorpii poses a severe challenge to the standard picture of O-star wind-shock X-ray emission. The Chandra HETGS spectrum now provides significant direct evidence that this B0.2 star does not fit this standard wind-shock framework. The many emission lines detected with the Chandra gratings are significantly narrower than what would be expected from a star with the known wind properties of (tau) Sco, although they are broader than the corresponding lines seen in late-type coronal sources. While line ratios are consistent with the hot plasma on this star being within a few stellar radii of the photosphere, from at least one Helike complex there is evidence that the X-ray-emitting plasma is located more than a stellar radius above the photosphere. The Chandra spectrum of (tau) Sco is harder and more variable than those of other hot stars, with the exception of the young magnetized O star (theta)1 Ori C. We discuss these new results in the context of wind, coronal, and hybrid wind-magnetic models of hot-star X-ray emission.