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

Abstract

The observed multiwavelength spectrum of the B1II--III star beta CMa is successfully reproduced, including the extreme ultraviolet (EUV) continuum observed by Extreme Ultraviolet Explorer (EUVE), with a non-local thermodynamic equilibrium fully line-blanketed spherical hydrostatic model atmosphere. The available spectrophotometry of beta CMa from 500 A to 25 mu m is best fitted with model parameters T_eff = 24000 K, log g = 3.5 and an angular diameter of theta_LD = 0.565 mas. We find that a neutral interstellar hydrogen column of N(H^0) ~= 2 x 10^18 cm^-2 provides the 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 investigation of spherical and plane-parallel models shows that the Lyman continuum predictions are quite sensitive to model geometry and surface gravity between effective temperatures 18 000 and 33 000 K. This result provides an explanation for the reported excesses between the observed EUV fluxes from beta CMa and epsilon CMa and plane-parallel model atmosphere predictions.