HYDRODYNAMIC PULSATION MODELS OF YELLOW SUPERGIANT STARS IN THE POSTASYMPTOTIC GIANT BRANCH

It is shown that irregular oscillation with small amplitudes is yielded in radial pulsation of hydrodynamic models for yellow supergiant stars in the post-asymptotic giant branch (AGB). The driving modes are either the third overtone or the second overtone mode which takes the role of the third overtone mode for higher luminosity models. In nonlinear hydrodynamic simulations, subharmonic components are strongly enhanced in the nonlinear pulsations in the chaotic regime, and so the nonlinear pulsation can yield much longer time-scale variations. We briefly discuss observed properties of 89 Her (or UU Her) type stars in comparison with the results of the present simulation.