ULTRA-SHORT VARIABILITY: THE VIBRATING WHITE DWARF STARS.

Abstract

ABSTRACT The 29 white-dwarf stars that are known to exhibit variations sufficiently coherent to be classified as vibrating white dwarfs are grouped into three categories based on the stability of the oscillations and membership in close binary systems. The first category (occasional vibrators, binary) includes 15 objects that are members of close binary systems in which mass transfer is taking place between the components and that have exhibited low-amplitude short-period oscillations at some times but not at others. The second category (continual vibrators, binary) contains two objects, WZ Sge and DQ Her, that exhibit continual oscillations which have a stability millions of times greater than that observed for the occasional vibrators. The third category encompasses a group of 12 single DA white dwarfs that are variable in luminosity, exhibit complex but highly stable oscillations, and are referred to as ZZ Ceti stars. It is shown that the ZZ Ceti stars cluster in the H-R diagram in the region of maximum hydrogen opacity, lie on a linear extension of the Cepheid instability strip, and almost certainly exhibit g-mode oscillations.