ASCA X-RAY SPECTROSCOPY OF THE UNUSUAL B0-V STAR TAU-SCORPII

We have obtained a high-quality ASCA spectrum of the MK standard BO V star tau Sco in order to test the standard wind-shock picture of OB st ar X-ray production. The fluxes in three line complexes from ions indi cative of hot plasma-Mg+10, Si+12, and S+14 - are measured, and we als o present a global spectral fit using a fairly standard multitemperatu re, optically thin, collisional equilibrium model. We were able to ach ieve a statistically good fit, but only by using the MeKaL plasma emis sion code (Mewe, Kaastra, & Liedahl) and fixing the elemental abundanc es at the photospheric values as determined by optical spectroscopy. T he parameters of the model are T-1 = 7 MK, EM1 = 3.5 x 10(54) cm(-3), T-2 = 12 MK, EM2 = 8.1 x 10(53) cm(-3) T-3 > 27 MK, EM3 > 3.0 x 10(53) cm(-3). The quantity of material with temperature in excess of 10(7) K on tau Sco is comparable to that with temperature in excess of 10(6) K on most other early B stars. The data cannot be explained by the st andard line-force instability wind-shock mechanism. However, more unus ual shock mechanisms involving magnetically confined wind shocks or in teractions between infalling matter and the ambient stellar wind canno t be ruled out. Alternately, a dynamo driven by differential rotation could be powering coronal plasma. If magnetic fields are involved in a ny way, then the star's extreme youth could play a role.