Redefining the empirical ZZ Ceti instability strip

We use the new ZZ Ceti stars (hydrogen-atmosphere white dwarf variables; DAVs) discovered within the Sloan Digital Sky Survey ( Mukadam et al. 2004) to redefine the empirical ZZ Ceti instability strip. This is the first time since the discovery of white dwarf variables in 1968 that we have a homogeneous set of spectra acquired using the same instrument on the same telescope, and with consistent data reductions, for a statistically significant sample of ZZ Ceti stars. The homogeneity of the spectra reduces the scatter in the spectroscopic temperatures, and we find a narrow instability strip of width similar to 950 K, from 10,850 to 11,800 K. We question the purity of the DAV instability strip, as we find several nonvariables within. We present our best fit for the red edge and our constraint for the blue edge of the instability strip, determined using a statistical approach.