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Pulsating white dwarf
A pulsating white dwarf is a white dwarf star whose luminosity varies due to non-radial gravity wave pulsations within itself. Known types of pulsating white dwarfs include DAV, or ZZ Ceti, stars, with hydrogen-dominated atmospheres and the spectral type DA〔(Physics of white dwarf stars ), D. Koester and G. Chanmugam, ''Reports on Progress in Physics'' 53 (1990), pp. 837–915.〕, pp. 891, 895; DBV, or V777 Her, stars, with helium-dominated atmospheres and the spectral type DB〔White dwarfs, Gilles Fontaine and François Wesemael, in ''Encyclopedia of Astronomy and Astrophysics'', ed. Paul Murdin, Bristol and Philadelphia: Institute of Physics Publishing and London, New York and Tokyo: Nature Publishing Group, 2001. ISBN 0-333-75088-8.〕, p. 3525; and GW Vir stars, with atmospheres dominated by helium, carbon, and oxygen, and the spectral type PG 1159. (Some authors also include non-PG 1159 stars in the class of GW Vir stars.) GW Vir stars may be subdivided into DOV and PNNV stars;〔(Mapping the Instability Domains of GW Vir Stars in the Effective Temperature-Surface Gravity Diagram ), Quirion, P.-O., Fontaine, G., Brassard, P., ''Astrophysical Journal Supplement Series'' 171 (2007), pp. 219–248.〕, §1.1, 1.2;〔§1, (Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209 ), T. Nagel and K. Werner, ''Astronomy and Astrophysics'' 426 (2004), pp. L45–L48.〕 they are not, strictly speaking, white dwarfs but ''pre-white dwarfs'' which have not yet reached the white dwarf region on the Hertzsprung-Russell diagram.〔, § 1.1;〔 A subtype of DQV stars, with carbon-dominated atmospheres, has also been proposed.,〔SDSS J142625.71+575218.3: A Prototype for a New Class of Variable White Dwarf, M. H. Montgomery et al., ''Astrophysical Journal'' 678, #1 (May 2008), pp. L51–L54, , 〕 and in May 2012, the first extremely low mass variable (ELMV) white dwarf was reported.〔(SDSS J184037.78+642312.3: The First Pulsating Extremely Low Mass White Dwarf ), J.J. Hermes et al., ''The Astrophysical Journal Letters'', 750 (May 2012), p. L28〕
These variables all exhibit small (1%–30%) variations in light output, arising from a superposition of vibrational modes with periods of hundreds to thousands of seconds. Observation of these variations gives asteroseismological evidence about the interiors of white dwarfs.〔(Asteroseismology of white dwarf stars ), D. E. Winget, ''Journal of Physics: Condensed Matter'' 10, #49 (December 14, 1998), pp. 11247–11261. DOI 10.1088/0953-8984/10/49/014.〕
==DAV stars==
Early calculations suggested that white dwarfs should vary with periods around 10 seconds, but searches in the 1960s failed to observe this.〔, § 7.1.1;〔(Ultrashort-Period Stellar Oscillations. I. Results from White Dwarfs, Old Novae, Central Stars of Planetary Nebulae, 3C 273, and Scorpius XR-1 ), George M. Lawrence, Jeremiah P. Ostriker, and James E. Hesser, ''Astrophysical Journal'' 148, #3 (June 1967), pp. L161–L163.〕 The first variable white dwarf found was HL Tau 76; in 1965 and 1966, Arlo U. Landolt observed it to vary with a period of approximately 12.5 minutes.〔(A New Short-Period Blue Variable ), Arlo U. Landolt, ''Astrophysical Journal'' 153, #1 (July 1968), pp. 151–164.〕 The reason for this period being longer than predicted is that the variability of HL Tau 76, like that of the other pulsating variable white dwarfs known, arises from non-radial gravity wave pulsations.〔, § 7. In 1970, another white dwarf, Ross 548, was found to have the same type of variability as HL Tau 76;〔(High-Frequency Stellar Oscillations. VI. R548, a Periodically Variable White Dwarf ), Barry M. Lasker and James E. Hesser, ''Astrophysical Journal'' 163 (February 1971), pp. L89–L93.〕 in 1972, it was given the variable star designation ''ZZ Ceti''.〔(58th Name-List of Variable Stars ), B. V. Kukarkin, P. N. Kholopov, N. P. Kukarkina, N. B. Perova, ''Information Bulletin on Variable Stars'', #717, September 21, 1972.〕 The name ''ZZ Ceti'' also refers to this class of pulsating variable white dwarfs, which, as it consists of white dwarfs with hydrogen atmospheres, is also called ''DAV''.〔, pp. 891, 895. These stars have periods between 30 seconds and 25 minutes and are found in a rather narrow range of effective temperatures between about 12,500 and 11,100 K. The measurement of the rate of change of period with time for the gravity wave pulsations in ZZ Ceti stars is a direct measurement of the cooling timescale for a DA white dwarf, which in turn can give an independent measurement of the age of the galactic disk.
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