Double-mode Cepheid period ratios from linear and nonlinear theory

In order to learn whether the periods of equilibrium stellar models, determined from linear nonadiabatic theory, are applicable to real stars, a nonlinear hydrodynamic claculation near its limit cycle has been analyzed for its periods. The model considered, with mass of 6.0 M/sub sun/, T/sub e/=5888 K, and 2820L/sub sun/, and the composition X=0.602, Z=0.044, is slowly switching its pulsation mode from the fundamental (6.03 d) to the first overtone (4.39 d). Therefore the ratios of periods can be obtained at two epochs with differing mode-amplitude ratios and nonlinear coupling between the two principal modes. The first overtone to fundamental period ratio is found to agree with the linear theory value regardless of the stage of mode-switching. It is concluded that the period ratios from linear theory are applicable to real radially finite-amplitude pulsating stars