A Spherical, Non-Lte Blanketed Model Stellar Atmosphere for Phi CAS (foi(a))

By constructing a model atmosphere, I have calculated the theoretical energy distribution of the emergent radiation that best matches the observed energy distribution of the star phi Cas, and so derived its effective temperature and surface gravity. For an accurate comparison between theory and observations, it is desirable to determine the interstellar extinction "in situ" and then correct the observations for it. Fortunately, phi Cas is very likely a member of the open cluster NGC 457. Five early B-type stars near the main -sequence turnoff in NGC 457 were observed at low dispersion with the IUE satellite. In order to use the "pair method" to derive the extinction curve for NGC 457, I have developed a new method of choosing a lightly reddened comparison star. It consists of matching the equivalent widths of spectral features that are particularly strong and sensitive to temperature and luminosity. The UV extinction in NGC 457 deviates from the average extinction for the Milky Way and is uniform across the cluster. For the visible region, monochromatic fluxes of phi Cas were dereddened with a mean extinction curve. Thus, the intrinsic energy distribution of phi Cas was determined from 1500 to 5800 A. A detailed new spherical model atmosphere was constructed. The equation of transfer was solved with the constraints of hydrostatic and pure radiative equilibrium. The statistical equilibrium of 10^ 6 transitions was treated. Several tests have demonstrated this model to be reliable. Comparison of the observed UV and visible energy distributions and the H_ delta, CaIIK, and MgII h and k line profiles with the theoretical results indicates that the best model for phi Cas has T_ {rm eff} = 7200 +/- 100K, log g = 0.4 +/- 0.1, and radius = 263 +/- 34 R _odot. These results imply a mass of 6.3 +/- 3.6 M_odot , which agrees with the mass of the main-sequence turnoff of NGC 457. This model is the first self-consistent model for phi Cas.