ZZ ZZ Ceti stars

- These are non-radially pulsating white dwarfs that change their brightness with periods from 30 s to 25 min and amplitudes from 0".'001 to 0".'2 in V. They usually show several close period values. Flares of 1 magnitude are sometimes observed; however, these may be explained by the presence of close UV Ceti companions. These variables are divided into the following subtypes: ZZA (subtype) - ZZ Cet-type variables of DA spectral type (ZZ Cet) having only hydrogen absorption lines in their spectra. ZZB (subtype) - ZZ Cet-type variables of DB spectral type having only helium absorption lines in their spectra. ZZO (subtype) - ZZ Cet type variables of the DO spectral type showing He 11 and C IV absorption lines in their spectra. GCVS

ZZ Ceti stars are classed as pulsating stars, however, it is accepted that they cannot be radial pulsating stars because their periods are too long. In fact, multicolor observations have confirmed that the pulsation modes are non-radial g-modes. Usually, several periods are simultaneously excited and their frequencies are often split into close pairs by the slow rotation of the star.

The periods are usually considered to be extremely stable but unstable periods do occur, with period changes occurring over a few hours, and are probably caused by interactions of various periods. This phenomenon is generally described as the beating of closely spaced frequencies.

Single white dwarfs have masses near 0.6 solar masses

Observation

Bright stars Mixed amplitude ^^ Mixed periods <S> CCD or PEP

The GCVS lists approximately 30 ZZ Ceti-type variables with a handful occurring within novalike or dwarf nova systems. Most of these stars belong to the ZZA type.

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