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The TLUSTY model code of Hubeny \& Lanz are used to compute the B star photospheric spectrum in the ultraviolet and optical regions. These models treated H and He continua in NLTE, but the lines were assumed to be in detailed balance. A few models were computed with 5 levels of H and 14 levels of He I, plus continua, in NLTE. There were no significant differences between the models with the lines treated in detailed balance and in explicit NLTE. Solar abundances were assumed throughout this study. Effective temperatures are derived by fitting line profiles observed by the Hubble Space Telescope GHRS to the computed synthetic spectra. We find $T_{\rm eff}= 15300$ K, and a rotational velocity of about 150 km s$^{-1}$. The strengths of the Si II resonance lines in the ultraviolet are shown to be anomalously weak in the observed spectra, consistent with an overionization of Si II by 1.1 dex. Additional model atmospheres were computed treating the bound levels of Si II and Si III and the corresponding continua explicitly in NLTE. We will report on the results of these ongoing model studies of $\zeta$ Aur B and present the latest estimates of the stellar parameters $(T_{\rm eff}, \log g, {\rm and~} v_{rot})$.
This work is supported by HST Grant GO-3626.01-91A to the University of Colorado.
