ZZ Ceti stars: a new class of pulsating white dwarfs

The existence of the ZZ Ceti stars as a new, distinct class of variable stars is proven by means of a photometric survey for rapid luminosity variations among white dwarfs. The null results of the survey are used to show that the ZZ Ceti stars are all DA white dwarfs that fall in a restricted region centered at (B-V) = +0.20 in the two-color diagram. A light curve and power spectrum of the light curve is presented for all ZZ Ceti stars. In general, the ZZ Ceti stars have complex, multi-periodic light curves. The periods fall in the range from about 200 s to 1000 s, and the luminosity variations range from about 0.01 mag to 0.30 mag in amplitude. These variables are single, physically similar stars pulsating in nonradial g-modes. Only three mechanisms - rotation, excitation of multiple nonradial modes, and coupling between modes - are sufficient to explain virtually all of the photometric complexities exhibited by the ZZ Ceti variables. The ZZ Ceti stars are found to be normal white dwarfs which occur in the temperature range from about 10500 K to 13500 K on the DA cooling sequence. The onset of variability among DA white dwarfs is shown to be an evolutionary effect; it is shown that at least 40% of all DA white dwarfs will cool to become ZZ Ceti variables. The ZZ Ceti stars are by far the most numerous intrinsic variables in the galaxy.Time-resolved Stroemgren photometry is used to examine the changes of the effective temperature, the radius, and the effective surface gravity with time during a pulsation in order to evaluate the relative contributions of these parameters to the total luminosity variation