  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 He-like 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 Ori C. We discuss these new results in the context of wind, coronal, and
hybrid wind-magnetic models of hot-star X-ray emission.
