Evolution and Pulsations of Population I Post-AGB Stars

Abstract

Evolutionary calculations of population I stars with initial masses M₀ = 1 M_⊙, 1.5 M_⊙ and 2 M_⊙ were carried out up to the stage of the proto-planetary nebula. Selected models of post-AGB evolutionary sequences with effective temperatures 3.6 × 10³ K ≲ T_eff ≲ 2 × 10⁴ K were used as initial conditions in calculations of self-excited 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 Π ≈ 300 day to several days. Period fluctuations are due to nonlinear effects and are most prominent at effective temperatures T_eff < 5000 K. The amplitude of bolometric light variations is ΔM_bol ≈ 1 at T_eff ≲ 6000 K and rapidly decreases with increasing T_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.