EUVE SPECTROSCOPY OF BETA-CANIS-MAJORIS (B1-II-III) FROM 500-ANGSTROMTO 700-ANGSTROM

Observations of the bright variable star beta CMa (B1 II-III) made wit h the Extreme Ultraviolet Explorer (EUVE) are presented, We report on the continuous energy distribution, photospheric line identification, and the variability of the star, as well as the physical implications for the structure of the local interstellar medium. The star is one of the strongest EUV sources in the long-wavelength spectrometer of EUVE , and one of only two early-type stars whose photospheric continuum wa s detected by the EUVE spectrometers. This paper is primarily concerne d with the portion of the spectrum that lies between the neutral heliu m ionization edge at 504 Angstrom and an effective cutoff by interstel lar absorption near 700 Angstrom. As in our EUV analysis of the B2 II star epsilon CMa, we found that line-blanketed model atmospheres are n ot capable of predicting an energy distribution which matches observat ions in all wavelength regions. Consequently, we derived two set of ba sic parameters for the star (T-eff = 24,800 K, log g = 3.7; and T-eff = 23,250 K, log g = 3.5), depending whether we accept the measured ang ular diameter, or require an exact agreement between models and the ob served visual flux. For the higher T-eff model, the predicted EUV flux is in agreement with observations, while for the lower T-eff the star 's EUV continuum is about 5 times brighter than the predictions. In ei ther case, the star does not show the order of magnitude EUV excess th at was seen in epsilon CMa. Neither model fits the data near 2200 Angs trom, and additional photospheric opacity is proposed. The EUVE data a lso provide information concerning the low-density interstellar medium in the direction of beta CMa. We derive a neutral hydrogen column den sity of similar to 2 x 10(18) cm(-2) and estimate a fewer limit for th e neutral helium column density of 1.4 x 10(18) cm(-2). This indicates that along this sight line much of the hydrogen is ionized while the helium is neutral, The EUVE spectrum shows many strong photospheric ab sorption features, similar to that of epsilon CMa. Evidence for a stel lar wind is seen in the O v 630 Angstrom absorption feature. There is special interest in beta CMa because it is among the brightest of the beta Cephei class of variables, The pulsations in this class of star m anifest themselves primarily as periodic effective temperature changes . We find that the semiamplitude of the change is 108(-32)(+31) K for the primary period. This result is consistent with that derived from a n analysis of the UV continuum by Beeckmans & Burger (1977), but our 6 8% confidence limits are significantly smaller than their 1 sigma erro r bars, The general agreement implies that the pulsations do propagate between the layers where the optical and UV continua are formed and t he layers where the EUV continuum forms, which is about six density sc ale heights higher in static models. The possibility that some pulsati onal energy deposition could occur within the outer photosphere is dis cussed. Our observations, taken over two time intervals separated by 7 0 days, resulted in the detection of the beat phenomenon owing to the three oscillation periods of beta CMa.