Evidence for wind attenuation and a multitemperature plasma in the combined EUVE and ROSAT observations of epsilon canis majoris (B2 II)

We use both EUVE and ROSAT data sets to test three general pictures-coronal, wind shock, and external-for the production of the observed high-energy emission from the B giant, epsilon CMa (B2 II). Because of the very low interstellar opacity along its line of sight, epsilon CMa is the only early-type star that has strong emission lines detected with the EUVE spectrometers. The line spectrum provides the first solid observational evidence that the emission is thermal. Theoretical EUV spectra based upon two-temperature model fits to the ROSAT data predict too much flux, especially in the iron line complex near 175 Angstrom. We use progressively more complex models until we are able to achieve a fit to the combined data sets. We find that both a temperature distribution in the emitting plasma and some attenuation of the EUV and soft X-ray emission by the ionized stellar wind must be included in the models. The model fitting indicates that only 13% to 21% of the emission-line complex near 175 Angstrom escapes the wind. This amount is consistent with the wind shock model, in which the emitting material is distributed throughout the stellar wind. It is much more absorption than is predicted by the external source model, where all of the emitting material is at radii beyond the cold stellar wind. And it is significantly less absorption than is expected in the coronal model, given what is known about the star's mass-loss rate. The derived temperature distribution and wind filling factor of hot gas are also qualitatively consistent with our numerical simulations of wind shocks. We conclude that although the observed flux from epsilon CMa in the interval 54 eV <E <100 eV is approximately the same as that above 100 eV, because of wind attenuation the total generated radiation in the EUV band between 54 eV and 100 eV is 5 times greater than that in the X-ray region.