SPECTROSCOPIC STUDIES AND ATMOSPHERIC PARAMETERS OF PULSATING DA WHITE-DWARF (ZZ-CETI) STARS

We present optical spectrophotometry of a sample of 10 ZZ Ceti stars, obtained with the aim of providing independent estimates of the atmospheric parameters of these stars. This is accomplished by a study of high Balmer lines (Hγ-H8), which permits the separation of surface gravity from effective temperature effects. The observations are analyzed in terms of new grids of synthetic spectra, which incorporate the new occupation probability formalism of Hummer and Mihalas to calculate the atomic level populations. The ZZ Ceti stars studied in this work all have temperatures between 11,320 and 13,420 K. The average surface gravity for the sample is log g = 8.03, with σ(log g) = 0.25. Our analysis confirms the lower-than-average gravity of R548 (log g = 7.80), suspected from earlier photometry, as well as the somewhat larger-than-average gravity of G226-29 (log g = 8.20) which may, however, be insufficient to account for its short periods. The implications of our determinations for a few objects of particular astrophysical interest are discussed.