Nonlinear Pulsation Driven by Strange Modes in Hydrodynamic Models

The characteristics of the nonlinear pulsation of hydrodynamic models, which have only strange modes as pulsational unstable modes, are compared with those by conventional modes. It is shown that nonlinear pulsation with the strange modes has considerable amplitudes only at the hydrogen ionization zone and in the atmosphere. This is sharp in contrast to that by conventional modes, and is identical with the characteristics of the unstable linear strange modes. We thus conclude that the unstable strange modes may be responsible for the nonlinear pulsation. We also discuss the physical origin of the instability of the strange modes. Since the modes are pulsationally unstable without any luminosity perturbation, other physical mechanisms must be involved for the instability along with the conventional kappa and gamma mechanisms, which are familiar in Cepheid variables. We suggest that the density inversion in the H ionization region, including the adjacent He I ionization zone, may be the origin. Finally, we suggest that the different pulsation driving zones may cause the feature of the spatial amplitude variations.