The Instability Strip of ZZ Ceti White Dwarfs and Its Extension to the Extremely Low Mass Pulsators

The determination of the location of the theoretical ZZ Ceti instability strip in the log g - T-eff diagram has remained a challenge over the years, due to the lack of a suitable treatment for convection in these stars. We report here a detailed stability survey over the whole ZZ Ceti regime, including the very low masses where three pulsators have recently been found. With this in mind, we computed twenty-nine evolutionary sequences of DA models with various masses and chemical layering. These models are characterized by the so-called ML2/alpha = 1.0 convective efficiency and take into account the important feedback effect of convection on the atmospheric structure. We computed power spectra for these models with the Liege nonadiabatic pulsation code MAD, which is the only one to conveniently incorporate a full time dependent convection treatment and, thus, provides the best available description of the blue edge of the instability strip. On the other hand, given the failure of all nonadiabatic codes to properly account for the red edge of the strip, including MAD, we tested the idea that the red edge is due to energy leakage through the atmosphere. Using this approach, we found that our theoretical ZZ Ceti instability strip accounts remarkably well for the boundaries of the empirical strip.