Hydrodynamic models of a Cepheid atmosphere. II - Continuous spectrum

Abstract

The hydrodyanmic models of a classical Cepheid computed by Karp have been used to study the continuous spectrum of Cepheids. The structure of each hydrodynamic model was used as a snapshot of the temperature and pressure structure of the atmospheric layers. After using line blocking factors to account for the effect of lines on the spectral energy distributions, broad-band UBVRI colors were calculated. The light and color curves of the model reproduce the amplitude and asymmetry observed in Cepheids in this period range. In addition, the (color, T/sub eff/) -relations derived from the models were found to agree with those derived independently. It is shown that loops in the (U-B, B- V) -diagram can be produced by a dependence of the continuous opacity on the electron pressure. Mean colors were computed using different averaging schemes, and it is shown that the intensity mean of the magnitudes best reproduces the colors of the equilibrium model. The location of the equilibrium model in the H- R diagram indicates that the zero point of the Sandage and Tammann period- luminosity relation is too high by 0.2 mag.