Towards a pure ZZ Ceti instability strip^*

¹ Instituto de Física, Universidade Federal do Rio Grande do Sul, 91501-900 Porto-Alegre, RS, Brazil e-mail: barbara@if.ufrgs.br
² Department of Astronomy and McDonald Observatory, University of Texas, Austin, TX 78712, USA
³ Departamento de Física e Química, Universidade de Caxias do Sul, Caxias do Sul, RS, Brazil
⁴ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A'ohoku Place, Hilo, HI 96720, USA
⁵ Gemini Observatory, Northern Operations Center, 670 North A'ohoku Place, Hilo, HI 96720, USA
⁶ Steward Observatory, University of Arizona, 933 N. Cherry Ave. Tucson, AZ 85721, USA
⁷ Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany

Received: 22 June 2006
Accepted: 15 September 2006

Abstract

Context.We have observed again two stars inside the ZZ Ceti instability strip that were previously classified as not-observed-to-vary (NOV) by Mukadam et al. (2004) and found them to be low-amplitude variables. Some evidence points to a pure ZZ Ceti instability strip; other evidence contests it.

Aims.The two stars previously classified as NOV have Sloan Digital Sky Survey (SDSS) spectroscopic effective temperatures that place them inside the ZZ Ceti instability strip, and they were “contaminating” the strip as constant stars, which could indicate that the instability strip was no longer a simple evolutionary stage. A pure instability strip indicates that pulsation is a normal phase which all DAs must go through.

Methods.We used effective temperatures derived from SDSS optical spectra by comparing them with model atmospheres to look for pulsators through time-resolved photometry and stars previously classified as NOV.

Results.Our new results indicate, but do not prove, a pure instability strip, because there are still other NOV stars that need to be observed again. Additionally, we have discovered five other ZZ Ceti stars based on their effective temperatures.