Pulsation properties of DA white dwarfs - Radial mode instabilities

The equations describing linear nonadiabatic, radial pulsations for models of compositionally stratified, evolving, DA white dwarfs are solved. A kappa-mechanism (radial mode) instability strip that is caused by the development of a hydrogen partial ionization zone during the evolutionary cooling of the models is found. Instabilities occur for radial modes; the periods range from 4 to 0.2 sec, and the e-folding growth times range from 2 x 10 to the 9th sec to 8 x 10 to the 2nd sec. The minimum growth times are found to occur in the unstable modes of shortest period. A comparison with previous calculations for nonradial pulsations suggests that the blue edge for the radial instability strip is approximately 1600 K hotter than the blue edge of the theoretical ZZ Ceti instability strip and that the maximum instability occurs at temperatures about 600 K hotter than the blue edge of the theoretical ZZ Ceti instability strip. The results are also seen as indicating that the development of this instability strip is insensitive to the mass of the surface hydrogen layer and to uncertainties in the hydrogen opacities.