Redefining the Empirical ZZ Ceti Instability Strip

Abstract

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 $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.

Whole Earth telescope observations of the ZZ Ceti star HL Tau 76

“…It suggests that the location of the SDSS DA white dwarfs in the log g − T eff diagram suffers larger uncertainties. This may explain the overlap between pulsators and non-pulsators within the Mukadam et al (2004b) instability strip.…”

“…This is in disagreement with the instability strip derived from the SDSS newly discovered ZZ Ceti stars. Mukadam et al (2004b) suggest that it is not pure. Whether this results from a genuine cohabitation of pulsators and non-pulsators within the instability strip or from larger uncertainties in the atmospheric parameters derived from the SDSS needs to be examined.…”

“…The DA white dwarfs of the SDSS sample falling within the instability strip are fainter than those analyzed by Bergeron et al (2004). Their atmospheric parameters are derived from lower S/N spectra: for instance, the S/N of the spectrum for the brighest non variable star (WD 1338-0023, g = 17.1) in the Mukadam et al (2004b) sample is 35 (Kleinman et al 2004), while all the DA spectra analyzed by Bergeron et al (2004) have a S /N ≥ 80. It suggests that the location of the SDSS DA white dwarfs in the log g − T eff diagram suffers larger uncertainties.…”

Asteroseismological constraints on the structure of the ZZ Ceti star HL Tau 76

“…The ZZ Ceti instability strip takes a trapezoidal form in the H-R diagram because of the dependence of its blue and red boundaries on the stellar mass. However, the width and the limits of the instability strip differ in the two samples of Bergeron et al (2004) and of Mukadam et al (2004b). More importantly, while Bergeron et al (2004) find a pure instability strip, with no non-variable DA stars within the instability strip in their sample, Mukadam et al (2004b) reach a different conclusion from the analysis of the SDSS sample which contains non-variable stars inside the instability strip.…”

“…However, the width and the limits of the instability strip differ in the two samples of Bergeron et al (2004) and of Mukadam et al (2004b). More importantly, while Bergeron et al (2004) find a pure instability strip, with no non-variable DA stars within the instability strip in their sample, Mukadam et al (2004b) reach a different conclusion from the analysis of the SDSS sample which contains non-variable stars inside the instability strip. Whether this results from a genuine cohabitation of pulsators and non-pulsators within the instability strip or from larger uncertainties in the atmospheric parameters derived from the lower S/N SDSS spectra needs to be further investigated.…”