The observational properties of the ZZ Ceti stars

Abstract

The ZZ Ceti stars are the pulsating white dwarfs lying within an instability strip on the white dwarf cooling sequence with edges at B-V colors of +0,16 and +0.29. A total of 13 confirmed ZZ Ceti stars have now been found, and all have DA spectral types, all have temperatures between 10500K and 13500K, and all have gravities between log g of 7.8 and 8.1. These properties, like all the properties of the ZZ Ceti stars, are normal for white dwarfs. Indeed, since both pulsating and non pulsating white dwarfs can be found within the instability strip, at present there is no way to distinguish observationally a pulsator from a non-pulsator other than by the immediate fact of its variability. All of the ZZ Ceti stars are multi-periodic with, in the case of HL Tau-76, more than 20 periods being simultaneously present in the light curve. The periods are always very long, ranging from 100 to 1200 seconds. In at least one variable, R548, the pulsation periods are exceptionally stable, with the upper limit on the rate of change of the periods being |p| < 10−12, but in other variables the periods appear to change on time scales as short as a few hours. All recent models for the variability of ZZ Ceti stars have invoked nonradial, g-mode pulsations. Qualitatively, the g-mode pulsations easily account for the length of the observed periods and for the occasional close spacing of the periods. While the g-mode interpretation is certainly at least partially correct, the light curve of the ZZ Ceti star GD 154 suggests that other complicating factors are at work. Although it too is multi-periodic, GD 154 has only one strong period, 1186 seconds. This period is so long that if it is a 'g-mode, it must be a high overtone, k ≈ 20-30. Thus, GD 154 seems to have been excited in a single high overtone. It has not been clearly demonstrated that a white dwarf can be excited in such a high overtone without exciting other overtones as well.