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G. E. de Messieres (Swarthmore College), C. Cardamone (Wellesley College), D. H. Cohen (Swarthmore College), J. J. MacFarlane (Prism Computational Sciences), S. P. Owocki, A. ud-Doula (Bartol Research Institute, Univ. of Delaware)
It is generally believed that O and early-B stars produce X-rays via shocks embedded in their highly supersonic stellar winds. However, several lines of evidence point to other mechanisms operating in at least some unusual OB stars (those that are perhaps young, magnetized, cooler, or have circumstellar disks). The B0 V star tau Sco has long been known to be unusual in several ways, and to have harder and stronger X-ray emission than most hot stars. With the unprecedented spectral resolution provided by the Chandra gratings, we can begin to quantitatively assess the different X-ray production mechanisms that have been proposed for this unusual star. We report on line ratio analyses (of density, temperature, and local UV radiation field), as well as line widths and centroids, in order to discriminate among the various models. The notable results are that the copius very hot (kT > 1 keV) plasma on this star is situated one or two stellar radii above the photosphere, and that it is quite stationary with respect to the star, in contrast to the appreciable wind velocity seen in UV lines. We discuss these new results in the context of several types of models, including those based on line-force instability, cloud-infall, magnetically confined wind shocks, and coronal magnetic reconnection.
This work was partially funded by NASA grant number NAG5-10088 and by the Keck Northeast Astronomy Consortium.