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 comp ared with those by conventional modes. It is shown that nonlinear puls ation with the strange modes has considerable amplitudes only at the h ydrogen ionization zone and in the atmosphere. This is sharp in contra st to that by conventional modes, and is identical with the characteri stics 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 luminos ity perturbation, other physical mechanisms must be involved for the i nstability along with the conventional kappa and gamma mechanisms, whi ch are familiar in Cepheid variables. We suggest that the density inve rsion in the H ionization region, including the adjacent HeI ionizatio n zone, may be the origin. Finally, we suggest that the different puls ation driving zones may cause the feature of the spatial amplitude var iations.