Time-averaged ultraviolet observations of pulsating DA white dwarfs from the IUE Observatory.

Abstract

IUE observations in the range 1200-1900 A have been obtained for four ZZ Ceti stars. The spectra reveal the 1400 and 1600 A absorption features, common to all DA stars in that temperature range, but also previously unreported structure near 1520 A. These observations, together with available archival data for three additional ZZ Ceti stars, are analyzed with model atmosphere fluxes incorporating recently identified quasi-molecular opacities. The instability strip is found to cover the approximate range Te = 11,400-13,000 K, its exact location depending on the model grid used. The results also confirm the existence of a period-effective temperature correlation for ZZ Ceti stars, as predicted by pulsation theory.