ASCAX‐Ray Spectroscopy of the Unusual B0 V Star τ Scorpii

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Abstract
We have obtained a high-quality ASCA spectrum of the MK standard B0 V star τ Sco in order to test the standard wind-shock picture of OB star X-ray production. The fluxes in three line complexes from ions indicative of hot plasma—Mg+10, Si+12, and S+14—are measured, and we also present a global spectral fit using a fairly standard multitemperature, optically thin, collisional equilibrium model. We were able to achieve a statistically good fit, but only by using the MeKaL plasma emission code (Mewe, Kaastra, & Liedahl) and fixing the elemental abundances at the photospheric values as determined by optical spectroscopy. The parameters of the model are T1 = 7 MK, EM1 = 3.5 × 1054 cm-3, T2 = 12 MK, EM2 = 8.1 × 1053 cm-3, T3 > 27 MK, EM3 > 3.0 × 1053 cm-3. The quantity of material with temperature in excess of 107 K on τ Sco is comparable to that with temperature in excess of 106 K on most other early B stars. The data cannot be explained by the standard line-force instability wind-shock mechanism. However, more unusual shock mechanisms involving magnetically confined wind shocks or interactions between infalling matter and the ambient stellar wind cannot be ruled out. Alternately, a dynamo driven by differential rotation could be powering coronal plasma. If magnetic fields are involved in any way, then the star's extreme youth could play a role.