A NON-LTE ANALYSIS OF THE ZETA-AURIGAE B-TYPE SECONDARY .1. DETERMINATION OF THE FUNDAMENTAL STELLAR PARAMETERS

We present a non-LTE model atmosphere analysis of the B star secondary of zeta Aurigae (B5 V+K4 Ib) and determine its stellar parameters. A grid of model atmospheres and synthetic spectra were computed for stel lar parameters typical of mid-B stars, using the TLUSTY and SYNSPEC co des of Hubeny with the lines and continua of H and He calculated in no n-LTE. We observed zeta Aur with the Goddard High Resolution Spectrogr aph (GHRS) of the Hubble Space Telescope (HST) at several epochs near the 1993 eclipse. By carefully removing the circumstellar wind feature s at the two epochs furthest from eclipse, we recovered the intrinsic photospheric spectrum of the B star. The photospheric spectrum of zeta Aur B is compared to the grid of synthetic spectra, and the best fit is determined using a least-squares technique. We find T-eff = 15,400 +/- 300 K, log g = 3.9 +/- 0.1, and v sin i = 200 +/- 15 km s(-1). The corresponding spectral type, using the effective temperature scale of Underhill et al., is B5 V. The C I UV 5, 6, 7, and 9 resonance multip lets (1277-1281 Angstrom) and the Si II UV 4 (1260-1265 Angstrom) and UV 5 (1190-1197 Angstrom) resonance multiplets are observed to be much weaker than our models predict. We empirically determine departure co efficients of C I and Si II by varying the oscillator strengths of tra nsitions of each of these ions until a good match with the GHRS spectr a is obtained. For C I, we provide theoretical confirmation of these e mpirically determined departure coefficients by computing a more detai led model atmosphere including levels and transitions of C I, C II, an d C III treated in non-LTE. The synthetic spectra computed from this m odel are in good agreement with the GHRS observations, and the C I gro und-state departure coefficient is consistent with the empirically det ermined value. We examine several possible causes of the weakness of t he Si II lines and conclude that an underabundance due to non-LTE effe cts is the probable explanation. Previous model atmospheres including Si Ir computed in non-LTE show that the Si II resonance lines are form ed essentially in LTE. We suggest that autoionization of Si II (neglec ted in previous modeling) may shift the silicon ionization balance eno ugh to account for the weakness of the Si II lines.