Evolution and pulsations of population I post-AGB stars

Abstract

Evolutionary calculations of population I stars with initial masses $M_0=1M_\odot$, $1.5M_\odot$ and $2M_\odot$ were carried out up to the stage of the proto--planetary nebula. Selected models of post--AGB evolutionary sequences with effective temperatures $3.6\times 10^3\,\mathrm{K}\lesssim T_\mathrm{eff}\lesssim 2\times 10^4\,\mathrm{K}$ were used as initial conditions in calculations of self--escited stellar oscillations. For the first time the sequences of hydrodynamic models of radially pulsating post--AGB stars were computed using the self--consistent solution of the equations of radiation hydrodynamics and time--dependent convection. Within this range of effective temperatures the post--AGB stars are the fundamental mode pulsators with period decreasing as the star evolves from $\Pi\approx 300$ day to several days. Period fluctuations are due to nonlinear effects and are most prominent at effective temperatures $T_\mathrm{eff} < 5000$K. The amplitude of bolometric light variations is $\Delta M_\mathrm{bol}\approx 1$ at $T_\mathrm{eff} \lesssim 6000$K and rapidly decreases with increasing $T_\mathrm{eff}$. The theoretical dependence of the pulsation period as a function of effective temperature obtained in the study can be used as a criterion for the evolutionary status of pulsating variables suspected to be post--AGB stars.