A non-local thermodynamic equilibrium spherical line-blanketed stellar atmosphere model of the early B giant beta CMa

The observed multiwavelength spectrum of the B1II-III star beta CMa is succ essfully reproduced, including the extreme ultraviolet (EUV) continuum obse rved by Extreme Ultraviolet Explorer (EUVE), with a non-local thermodynamic equilibrium fully line blanketed spherical hydrostatic model atmosphere. T he available spectrophotometry of beta CMa from 500 Angstrom to 25 mu m is best fitted with model parameters T-eff = 24 000 K, log g = 3.5 and an angu lar diameter of theta(LD) = 0.565 mas. We find that a neutral interstellar hydrogen column of N(H-0) similar or equal to 2 x 10(18) cm(-2) provides th e best agreement between the model EUV flux and that observed by EUVE. We use model atmosphere fits together with Hipparcos distances to calculate radii, luminosities and ionizing fluxes for beta CMa and alpha Vir. An inv estigation of spherical and plane-parallel models shows that the Lyman cont inuum predictions are quite sensitive to model geometry and surface gravity between effective temperatures 18 000 and 33 000 K. This result provides a n explanation for the reported excesses between the observed EUV fluxes fro m beta CMa and epsilon CMa and plane-parallel model atmosphere predictions.