2003ApJ...591.1184F

We first review and rebut the arguments that have been put forward recently against the use of optical spectroscopy in the determinations of the atmospheric parameters of ZZ Ceti and neighboring stars in the logg-T_effdiagram. We reiterate instead the conclusion of Bergeron et al. that optical spectroscopy alone, when properly handled and modeled, leads to very accurate and reliable estimates of the surface gravities and effective temperatures of ZZ Ceti pulsators. The optical spectroscopy approach has had a yield of 100% so far in predicting the variability of candidate ZZ Ceti stars, and we present evidence here of its successful application to two more objects. We hence report the detection of multiperiodic luminosity variations in the light curves of two DA white dwarfs, MCT 0145-2211 and HE 0532-5605, selected on the basis of an analysis of their optical spectra. This brings the number of known ZZ Ceti stars to a total of 34. Our study reveals that MCT 0145-2211 has T_eff=11,550 K and logg=8.14, while HE 0532-5605 has T_eff=11,560 K and logg=8.49, which places them both inside the empirical instability strip uncovered by Bergeron et al. We find that the amplitudes, periods, and degree of complexity of the light curves are consistent with the positions of the stars rather near the red edge. Using the same homogeneous approach, we have, so far, analyzed high-quality optical spectra for some 103 nonvariable DA white dwarfs and all of the 34 known ZZ Ceti stars. The picture of the empirical instability strip that emerges is that of a pure strip, in which no nonvariable stars are found. It has a trapezoidal shape in the logg-T_effplane, with the blue edge showing a stronger dependence on the surface gravity than the red edge does.