PULSATIONAL PROPERTIES OF POST-AGB STARS

Linear stability analysis and hydrodynamic calculations are performed for post-AGB star models using the new OPAL opacities of Iglesias et al. The linear pulsational calculations show that for low mass stars with L/M > 10(4)L./M. the low order radial modes are unstable across the whole HR-diagram. Excited are radial fundamental mode and first overtone, and in the intermediate temperature range (log(T(eff)) almost-equal-to 4.2) higher overtone modes become excited. With the new opacities the instability region is not confined to the extension of the Cepheid instability strip but extends much further towards higher effective temperatures. The behavior of nonlinear pulsations in selected models is presented. These calculations show that at higher temperatures the nonlinear models exhibit low amplitude almost-equal-to 0.001 - 0.04 semiregular pulsations. The amplitudes are higher close to the regions where the driving in the H/He ionization or Z-bump zones is most efficient. In the intermediate temperature range the amplitudes are much smaller.