IRREGULAR SMALL-AMPLITUDE PULSATIONS IN YELLOW SUPERGIANT STAR MODELS

We examine the radial pulsation of hydrodynamic modes for less-massive supergiant stars in an attempt to explain the irregular small-amplitude pulsations observed in yellow supergiant stars (89 Her, for instance), which are believed to occur in the post-asymptotic giant branch. These pulsations are located in the bluer region of the HR diagram, beyond the conventional instability strip of the fundamental mode. It is shown that the higher overtone modes which take on the properties of the strange modes are pulsationally unstable over rather wide regions, which is compatible with observations. Using non-linear hydrodynamic simulations, we demonstrate that the pulsation in the non-linear regime shows chaotic behaviour for models with higher luminosities, for a wide range of parameters. We briefly discuss the transition from regular to chaotic oscillations.