Pulsation properties of DA white dwarfs - Radial mode instabilities

We have solved the equations describing linear, nonadiabatic, radial pulsations for models of compositionally stratified, evolving, DA white dwarfs. We find a kappa-mechanism, radial mode, instability strip that is caused by the development of a hydrogen partial ionization zone during the evolutionary cooling of our models. Instabilities occur for radial modes with periods, product, in the range 4> or approx. =product> or approx. =0.2 s, with e-folding growth times, tau/sub e/, in the range 2 x 10/sup 9/> or approx. =tau/sub e/ > or approx. =8 x 10/sup 2/ s; the minimum growth times occur in the shortest period unstable modes. Comparison with our previous calculations for nonradial pulsations indicates that the blue edge for the radial instability strip is approx.1600 K hotter than the blue edge of the theoretical ZZ Ceti instability strip, and further, that the maximum instability occurs at temperatures approx.600 K hotter than the blue edge of the theoretical ZZ Ceti instability strip. Our results also indicate that the development of this instability strip is insensitive to the mass of the surface hydrogen layer and to uncertainties in the hydrogen opacities. We demonstrate that existing observations of the DA white dwars are insufficient to determine ifmore » such radial pulsations exist in the DA white dwarfs, and we evaluate the prospects for detecting these pulsations in the future.« less