A Spherical Non-LTE Line-Blanketed Stellar Atmosphere Model of the Early B Giant epsilon CMa

Abstract

We use a spherical non-LTE fully line blanketed model atmosphere to fit the full multi-wavelength spectrum, including the extreme ultraviolet (EUV) continuum observed by the {\it Extreme Ultraviolet Explorer}, of the B2 II star \epscma. The available spectrophotometry of \epscma\ from 350 Å to 25 \micron\ is best fit with model parameters \Teff = 21750\,K, \Logg = 3.5, and an angular diameter of 0.77 mas. The close agreement between the model and the measured EUV flux from \epscma\ is a result of the higher temperatures at the formation depths of the \ion{H}{1} and \ion{He}{1} Lyman continua compared to other models. The realistic model treatment of early B giants with spherical geometry and NLTE metal line blanketing results in the prediction of significantly larger EUV fluxes compared with plane-parallel models. We find that our metal line blanketed spherical models show significantly warmer temperature structures, 1-3 kK at the formation depth of the Lyman continua, and predict stronger EUV fluxes, up to a factor of 5 in the \ion{H}{1} Lyman continuum, compared with plane-parallel atmospheres that have identical model parameters. In contrast, we find spherical and plane-parallel models that do not include metal line blanketing are nearly identical. Our \Teff = 21000 K, \Logg = 3.2, spherical NLTE model predicts more than twice as many hydrogen ionizing photons and over 200 times more neutral helium ionizing photons than a standard hydrostatic plane-parallel LTE model with the same stellar parameters.