Evolutionary Status of W Vir Pulsating Variables

Abstract

Stellar evolution calculations for population II stars with initial composition $Y_{0}=0.25$, $Z_{0}=10^{-3}$ and the initial stellar mass $M_{0}=0.82M_{\odot}$ were carried out from the main sequence to the white dwarf stage. Twelve AGB and post-AGB evolutionary sequences were computed with different values of the parameter in the Blöcker mass loss rate formula ($0.01\leq\eta_{\textrm{B}}\leq 0.12$). Selected models of evolutionary sequences with masses $M=0.536M_{\odot}$, $0.530M_{\odot}$, and $0.526M_{\odot}$ that experience the loop in the Hertzsprung–Russel diagram due to the final helium flash were used as initial conditions for solution of the equations of hydrodynamics describing radial stellar oscillations. The region of instability to radial fundamental mode pulsations is shown to extend from the asymptotic giant branch to effective temperatures as high as $T_{\textrm{eff}}\approx 6\times 10^{3}$ K. Pulsation periods of hydrodynamic models are in the range from 15 to 40 day and agree with periods of W Vir pulsating stars. The models of intermediate spectral type fundamental mode pulsators with periods $\Pi>40$ day locate in the upper part of the Hertzsprung–Russel diagram in the region of semiregular pulsating variables. We conclude that W Vir pulsating variables are the low–mass post-AGB stars that experience the final helium flash.