EUVE SPECTROSCOPY OF EPSILON-CANIS-MAJORIS (B2-II) FROM 70-ANGSTROM TO 730-ANGSTROM

We present spectra of the brightest stellar source of extreme ultravio let (EUV) radiation longward of 400 Angstrom, the B2 II star, epsilon CMa. These data were taken with the three spectrometers aboard the NAS A Extreme Ultraviolet Explorer satellite (EUVE) during the first cycle of pointed observations. We report on our initial studies of the cont inuum and line spectrum of the stellar photosphere in the 320 to 730 A ngstrom region, and on the wind emission lines observed in the 170-375 Angstrom region. This is the first EUV spectrum of an early-type star , and thus makes epsilon 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 Angstrom) are found to be significantly greater than predicted by both LTE and non-LTE model atmospheres. Since epsilon CMa also exhibits a mid-infrared excess, th is points to the outer layers being warmer than the models indicate. T he 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 interst ellar medium in the immediate vicinity of the sun. All of the lines pr edicted to be strong from model atmospheres are present and several wi nd absorption features are also identified. We have detected emission lines from highly ionized iron, ranging from Fe+8 to Fe+15, which aris e in the X-ray producing region. The lines are consistent with the ROS AT PSPC observations if a multi-temperature emission model is used, an d the assumption is made that there is significant absorption beyond t hat of the neutral phase of the ISM. The EUV spectrum shows a strong H e II Lyman-alpha emission at 304 Angstrom. This line cannot be formed in the upper regions of the photosphere, but rather is due to the reco mbination of He++, which is itself produced by the X-ray and EUV (lamb da < 228 Angstrom) radiation present in the outer atmosphere. The spec trum also shows strong O III 374 Angstrom line emission produced by th e Bowen fluorescence mechanism, which has not previously been observed in the spectra of hot stars.