EUVE spectroscopy of ∈ Canis majoris (B2 II) from 70 to 730 Å

We present spectra of the brightest stellar source of extreme ultraviolet (EUV) radiation longward of 400 Å, the B2 II star, e CMa. These data were taken with the three spectrometers aboard the NASA Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the continuum and line spectrum of the stellar photosphere in the 320 to 730 Å region, and on the wind emission lines observed in the 170-375 Å region. This is the first EUV spectrum of an early-type star, and thus makes ∈ CMa the most comprehensively observed B star from the X-ray to infrared regimes. The radiation in both the H Lyman continuum and He I continuum (shortward of 504 Å) are found to be significantly greater than predicted by both LTE and non-LTE model atmospheres. Since ∈ CMa also exhibits a mid-infrared excess, this points to the outer layers being warmer than the models indicate. The anomalously large Lyman continuum flux, combined with the very low column density measured in the direction toward this star implies that it is the dominant source of hydrogen ionization of the local interstellar medium in the immediate vicinity of the sun. All of the lines predicted to be strong from model atmospheres are present and several wind absorption features are also identified. We have detected emission lines from highly ionized iron, ranging from Fe⁺⁸ to Fe⁺¹⁵, which arise in the X-ray producing region. The lines are consistent with the ROSAT PSPC observations if a multi-temperature emission model is used, and the assumption is made that there is significant absorption beyond that of the neutral phase of the ISM. The EUV spectrum shows a strong He II Lyman-α emission at 304 Å. This line cannot be formed in the upper regions of the photosphere, but rather is due to the recombination of He⁺⁺, which is itself produced by the X-ray and EUV (λ < 228 Å) radiation present in the outer atmosphere. The spectrum also shows strong O in 374 Å line emission produced by the Bowen fluorescence mechanism, which has not previously been observed in the spectra of hot stars.