X-ray Emission from Early-Type Stars: EUVE, ROSAT, and ASCA Observations

I will present results from a ROSAT PSPC survey of 27 B stars and detailed EUVE and ASCA observations of 2 B stars, epsilon CMa (B2 II) and tau Sco (B0 V). These data are used to test the applicability of the wind-shock paradigm in the context of radiation-driven wind theory to the high-energy emission from OB stars. Contrary to the results of both the ROSAT and EINSTEIN all-sky surveys, I find that almost all B stars are X-ray emitters, albeit at much lower levels than O stars. The X-ray emission from most early-B stars is consistent with the predictions of the wind-shock picture, however higher than predicted mass loss rates are required to reconcile the observed X-ray properties of stars later than about B1 with the models. My combined analysis of the EUVE and ROSAT observations of epsilon CMa indicates that wind attenuation of the EUV and soft X-ray radiation is consistent with a wind-shock morphology and inconsistent with the existence of coronal X-ray emission. Also the temperature distribution in the hot plasma is qualitatively consistent with numerical models of the radiation-force instability wind shocks. ASCA and ROSAT spectra of tau Sco, on the other hand, indicate that the standard wind-shock picture cannot explain the high-energy properties of all early-type stars. The large X-ray flux and hard spectrum of tau Sco may be understood in terms of this object's youth, either through the interaction of its stellar wind with the remnant natal cloud, or via coronal activity related to its fossil magnetic field.