Euve spectroscopy of β canis majoris (B1 II-III) from 500 Å to 700 Å

Observations of the bright variable star β CMa (B1 II-III) made with 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 was detected by the EUVE spectrometers. This paper is primarily concerned with the portion of the spectrum that lies between the neutral helium ionization edge at 504 Å and an effective cutoff by interstellar absorption near 700 Å. As in our EUV analysis of the B2 II star ∈ CMa, we found that line-blanketed model atmospheres are not capable of predicting an energy distribution which matches observations in all wave-length regions. Consequently, we derived two set of basic 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 angular diameter, or require an exact agreement between models and the observed 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 either case, the star does not show the order of magnitude EUV excess that was seen in ∈ CMa. Neither model fits the data near 2200 Å, and additional photospheric opacity is proposed. The EUVE data also provide information concerning the low-density interstellar medium in the direction of β CMa. We derive a neutral hydrogen column density of ∼ 2 × 10¹⁸ cm^-2 and estimate a lower limit for the neutral helium column density of 1.4 × 10¹⁸ 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 absorption features, similar to that of ∈ CMa. Evidence for a stellar wind is seen in the O v 630 Å absorption feature. There is special interest in & CMa because it is among the brightest of the β Cephei class of variables. The pulsations in this class of star manifest 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 an analysis of the UV continuum by Beeckmans & Burger (1977), but our 68% confidence limits are significantly smaller than their 1 σ error bars. The general agreement implies that the pulsations do propagate between the layers where the optical and UV continua are formed and the layers where the EUV continuum forms, which is about six density scale heights higher in static models. The possibility that some pulsational energy deposition could occur within the outer photosphere is discussed. Our observations, taken over two time intervals separated by 70 days, resulted in the detection of the beat phenomenon owing to the three oscillation periods of β CMa.