Program: GN-2007A-Q-9

White dwarf stars are the end point of evolution of around 97% of all stars born. As they cool, they pass through instability strips, where they are observed to be multi-periodic pulsators and each pulsation places an independent constraint on the stellar properties. Pulsations allow the determination of the stellar compositional layers, including the core, crucial to understand the progenitor's evolution, from AGB to planetary nebulae nuclei, born again phase, and their possible evolution to SNIa through accretion. To apply the properties derived from the pulsating stars to all white dwarf stars, and their progenitors, we must test if pulsation is a normal evolutionary state. As the instability strip is only 1200K wide, accurate temperatures and log g must be obtained and therefore the spectra must include the log g sensitive lines Hgamma to H9. We propose to obtain S/N> 60 optical spectra of the DA white dwarf stars for which the Sloan Digital Sky Survey spectra indicated temperatures inside the ZZ Ceti instability strip, but time series photometry show no evidence of variability. The Sloan spectra have insufficient S/N, particularly below 4000A where there are hydrogen lines whose strength can be used to measure surface gravity accurately. Theoretically and observationally, the location of the instability strip depends both on temperature and gravity. Our goal is to determine if the instability strip is pure or if some fraction of DA white dwarf stars do not pulsate as they evolve through the temperatures of the instability strip.