Date Published:February 1, 2007
Context: We have observed again two stars inside the ZZ Ceti instabilitystrip that were previously classified as not-observed-to-vary (NOV) byMukadam et al. (2004) and found them to be low-amplitude variables. Someevidence points to a pure ZZ Ceti instability strip; other evidencecontests it.
Aims: The two stars previously classified as NOVhave Sloan Digital Sky Survey (SDSS) spectroscopic effectivetemperatures that place them inside the ZZ Ceti instability strip, andthey were "contaminating" the strip as constant stars, which couldindicate that the instability strip was no longer a simple evolutionarystage. A pure instability strip indicates that pulsation is a normalphase which all DAs must go through.
Methods: We used effectivetemperatures derived from SDSS optical spectra by comparing them withmodel atmospheres to look for pulsators through time-resolved photometryand stars previously classified as NOV.
Results: Our new resultsindicate, but do not prove, a pure instability strip, because there arestill other NOV stars that need to be observed again. Additionally, wehave discovered five other ZZ Ceti stars based on their effectivetemperatures.Partially based on observations at Observatório do Pico dosDias/LNA; the Southern Astrophysical Research telescope, a collaborationbetween CNPq-Brazil, NOAO, UNC, and MSU; and McDonald Observatory of TheUniversity of Texas at Austin.