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| {{redirect|V777 Herculis star|the star|GD 358}}
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| A '''pulsating white dwarf''' is a [[white dwarf]] [[star]] whose [[luminosity]] [[variable star|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<ref name="physrev">[http://adsabs.harvard.edu/abs/1990RPPh...53..837K Physics of white dwarf stars], D. Koester and G. Chanmugam, ''Reports on Progress in Physics'' '''53''' (1990), pp. 837–915.</ref><sup>, pp. 891, 895</sup>; '''DBV''', or '''V777 Her''', stars, with [[helium]]-dominated atmospheres and the spectral type DB<ref name="wden">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.</ref><sup>, p. 3525</sup>; and '''GW Vir''' stars, with atmospheres dominated by helium, [[carbon]], and [[oxygen]], and the spectral type [[PG 1159 star|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;<ref name="quirion">[http://adsabs.harvard.edu/abs/2007ApJS..171..219Q 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.</ref><sup>, §1.1, 1.2;</sup><ref>§1, [http://adsabs.harvard.edu/abs/2004A%26A...426L..45N 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.</ref> 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]].<ref name="quirion" /><sup>, § 1.1;</sup><ref name="obrien" /> A subtype of '''DQV''' stars, with [[carbon]]-dominated atmospheres, has also been proposed.,<ref name=dqv>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, {{bibcode|2008ApJ...678L..51M}}, {{doi|10.1086/588286}}</ref> and in May 2012, the first extremely low mass variable ('''ELMV''') white dwarf was reported.<ref name=hermes2012>[http://adsabs.harvard.edu/abs/2012ApJ...750L..28H 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</ref>
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| 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 [[asteroseismology|asteroseismological]] evidence about the interiors of white dwarfs.<ref>[http://dx.doi.org/10.1088/0953-8984/10/49/014 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.</ref>
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| {| class="wikitable" align="right" style="margin: 0 0 0.7em 0.7em;"
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| | colspan="2" align="center" | '''Types of pulsating white dwarf'''<ref name="quirion" /><ref>{{cite web | last = Association Française des Observateurs d'Etoiles Variables| first = | authorlink = | coauthors = | title =ZZ Ceti variables | work = | publisher = Centre de
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| Données astronomiques de Strasbourg| date = | url = http://cdsweb.u-strasbg.fr/afoev/var/ezz.htx| doi = | accessdate = 2007-06-06}}</ref><sup>, §1.1, 1.2;</sup><ref name=dqv /><ref name=hermes2012 />
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| | '''DAV''' ([[General Catalog of Variable Stars|GCVS]]: ''ZZA'') || DA [[White_dwarf#Atmosphere_and_spectra|spectral type]], having only [[hydrogen]] [[absorption line]]s in its spectrum
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| | '''DBV''' (GCVS: ''ZZB'') || DB spectral type, having only [[helium]] absorption lines in its spectrum
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| | '''GW Vir''' (GCVS: ''ZZO'') || Atmosphere mostly C, He and O; <br /> may be divided into '''DOV''' and '''PNNV''' stars
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| | '''DQV''' || DQ spectral type; hot, [[carbon]]-dominated atmosphere
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| |'''ELMV''' || DA spectral type; <math>\lesssim 0.2 M_{\odot}</math>
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| |}
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| ==DAV stars==
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| Early calculations suggested that white dwarfs should vary with periods around 10 seconds, but searches in the 1960s failed to observe this.<ref name="physrev" /><sup>, § 7.1.1;</sup><ref>[http://adsabs.harvard.edu/abs/1967ApJ...148L.161L 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.</ref> 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.<ref>[http://adsabs.harvard.edu/abs/1968ApJ...153..151L A New Short-Period Blue Variable], Arlo U. Landolt, ''Astrophysical Journal'' '''153''', #1 (July 1968), pp. 151–164.</ref> 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.<ref name="physrev" /><sup>, § 7.</sup> In 1970, another white dwarf, [[Ross 548]], was found to have the same type of variability as HL Tau 76;<ref>[http://adsabs.harvard.edu/abs/1971ApJ...163L..89L 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.</ref> in 1972, it was given the [[variable star]] designation ''ZZ Ceti''.<ref>[http://adsabs.harvard.edu/abs/1972IBVS..717....1K 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.</ref> 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''.<ref name="physrev" /><sup>, pp. 891, 895.</sup> These stars have periods between 30 seconds and 25 minutes and are found in a rather narrow range of [[effective temperature]]s between about 12,500 and 11,100 [[Kelvin|K]].<ref>{{cite web | last = Bergeron | first = P.| authorlink = | coauthors = Fontaine, G| title =On the Purity of the ZZ Ceti Instability Strip: Discovery of More Pulsating DA White Dwarfs on the Basis of Optical Spectroscopy | work = | publisher = The Astrophysical Journal| date = | url = http://www.journals.uchicago.edu/doi/full/10.1086/379808 | doi = | accessdate = 2007-06-06}}</ref> 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 [[White_dwarf#Atmosphere_and_spectra|DA white dwarf]], which in turn can give an independent measurement of the age of the [[galactic disk]].<ref>{{cite web | last = Kepler| first = S.O.| authorlink = | coauthors = G. Vauclair, R. E. Nather, D. E. Winget, and E. L. Robinson | title = G117-B15A - How is it evolving? | work = White dwarfs; Proceedings of IAU Colloquium 114th, Hanover, NH, Aug. 15-19, 1988 (A90-32719 13-90) | publisher = Berlin and New York: Springer-Verlag| year = 1989| url = http://adsabs.harvard.edu/abs/1989whdw.coll..341K| doi = | accessdate = 2007-06-07}}</ref>
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| ==DBV stars==
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| In 1982, calculations by [[D. E. Winget]] and his coworkers suggested that helium-atmosphere DB white dwarfs with surface temperatures around 19,000 K should also pulsate.<ref>[http://adsabs.harvard.edu/abs/1982ApJ...252L..65W Hydrogen-driving and the blue edge of compositionally stratified ZZ Ceti star models], D. E. Winget, H. M. van Horn, M. Tassoul, G. Fontaine, C. J. Hansen, and B. W. Carroll, ''Astrophysical Journal'' '''252''' (January 15, 1982), pp. L65–L68.</ref><sup>, p. L67.</sup> Winget then searched for such stars and found that [[GD 358]] was a variable DB, or ''DBV'', white dwarf.<ref>[http://adsabs.harvard.edu/abs/1982ApJ...262L..11W Photometric observations of GD 358: DB white dwarfs do pulsate], D. E. Winget, E. L. Robinson, R. D. Nather, and G. Fontaine, ''Astrophysical Journal'' '''262''' (November 1, 1982), pp. L11–L15.</ref> This was the first prediction of a class of variable stars before their observation.<ref name="kawaler">White Dwarf Stars, Steven D. Kawaler, in ''Stellar remnants'', S. D. Kawaler, I. Novikov, and G. Srinivasan, edited by Georges Meynet and Daniel Schaerer, Berlin: Springer, 1997. Lecture notes for Saas-Fee advanced course number 25. ISBN 3-540-61520-2.</ref><sup>, p. 89.</sup> In 1985, this star was given the designation ''V777 Her'', which is also another name for this class of variable stars.<ref name="namelist67">[http://adsabs.harvard.edu/abs/1985IBVS.2681....1K The 67th Name-List of Variable Stars], P. N. Kholopov, N. N. Samus, E. V. Kazarovets, and N. B. Perova, ''Information Bulletin on Variable Stars'', #2681, March 8, 1985.</ref><sup>; </sup><ref name="wden" /><sup>, p. 3525</sup> These stars have effective temperatures around 25,000K.<ref name="physrev" /><sup>, p. 895.</sup>
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| <span id="DOV stars" />
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| ==GW Vir stars==
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| The third known class of pulsating variable white dwarfs is the ''GW Vir'' stars, sometimes subdivided into ''DOV'' and ''PNNV'' stars. Their prototype is [[PG 1159-035]].<ref name="quirion" /><sup>, §1.1.</sup> This star (also the prototype for the class of [[PG 1159 star]]s) was observed to vary in 1979,<ref>[http://adsabs.harvard.edu/abs/1979wdvd.coll..377M PG1159-035: A new, hot, non-DA pulsating degenerate], J. T. McGraw, S. G. Starrfield, J. Liebert, and R. F. Green, pp. 377–381 in ''White Dwarfs and Variable Degenerate Stars'', IAU Colloquium #53, ed. H. M. van Horn and V. Weidemann, Rochester: University of Rochester Press, 1979.</ref> and was given the variable star designation ''GW Vir'' in 1985,<ref name="namelist67" /> giving its name to the class. These stars are not, strictly speaking, white dwarfs; rather, they are stars which are in a position on the [[Hertzsprung-Russell diagram]] between the [[asymptotic giant branch]] and the white dwarf region. They may be called ''pre-white dwarfs''.<ref name="quirion" /><sup>, § 1.1;</sup><ref name="obrien">[http://adsabs.harvard.edu/abs/2000ApJ...532.1078O The Extent and Cause of the Pre-White Dwarf Instability Strip], M. S. O'Brien, ''Astrophysical Journal'' '''532''', #2 (April 2000), pp. 1078–1088.</ref> They are hot, with [[effective temperature|surface temperature]]s between 75,000 K and 200,000 K, and have atmospheres dominated by [[helium]], [[carbon]], and [[oxygen]]. They may have relatively low surface gravities (log ''g'' ≤ 6.5.)<ref name="quirion" /><sup>, Table 1</sup> It is believed that these stars will eventually cool and become DO white dwarfs.<ref name="quirion" /><sup>, § 1.1.</sup>
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| The periods of the [[vibrational modes]] of GW Vir stars range from about 300 to about 5,000 [[second]]s.<ref name="quirion" /><sup>, Table 1</sup> How pulsations are excited in GW Vir stars was first studied in the 1980s<ref>[http://adsabs.harvard.edu/abs/2003ApJ...585..975C A Pulsation Mechanism for GW Virginis Variables], Arthur N. Cox, ''Astrophysical Journal'' '''585''', #2 (March 2003), pp. 975–982.</ref> but remained puzzling for almost twenty years.<ref>[http://adsabs.harvard.edu/abs/2002AAS...200.8507C An Instability Mechanism for GW Vir Variables], A. N. Cox, p. 786, American Astronomical Society, 200th AAS Meeting, #85.07, in ''Bulletin of the American Astronomical Society'' '''34''' (May 2002).</ref> From the beginning, the excitation mechanism was thought to be caused by the so-called [[κ-mechanism]] associated with ionized [[carbon]] and [[oxygen]] in the envelope below the photosphere, but it was thought this mechanism would not function if helium was present in the envelope. However, it now appears that instability can exist even in the presence of helium.<ref name="newnon">[http://adsabs.harvard.edu/abs/2006A&A...458..259C New nonadiabatic pulsation computations on full PG 1159 evolutionary models: the theoretical GW Virginis instability strip revisited], A. H. Córsico, L. G. Althaus, and M. M. Miller Bertolami, ''Astronomy and Astrophysics'' '''458''', #1 (October 2006), pp. 259–267.</ref><sup>, §1.</sup>
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| ==DQV stars==
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| A new class of white dwarfs, with spectral type DQ and hot, carbon-dominated atmospheres, has recently been discovered by Patrick Dufour, James Liebert and their coworkers.<ref>White dwarf stars with carbon atmospheres, Patrick Dufour, James Liebert, G. Fontaine, and N. Behara, ''Nature'' '''450''', #7169 (November 2007), pp. 522–524, {{bibcode|2007Natur.450..522D}}, {{doi|10.1038/nature06318}}</ref> Theoretically, such white dwarfs should pulsate at temperatures where their atmospheres are partially ionized. Observations made at [[McDonald Observatory]] suggest that [[SDSS J142625.71+575218.3]] is such a white dwarf; if so, it would be the first member of a new, ''DQV'', class, of pulsating white dwarfs. However, it is also possible that it is a white dwarf [[binary star|binary system]] with a [[carbon]]-[[oxygen]] [[accretion disk]].<ref name=dqv />
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| ==See also==
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| *[[Instability strip]]
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| *[[G117-B15A]]
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| ==References==
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| {{reflist}}
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| ==External links and further reading==
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| *[http://astro.if.ufrgs.br/wdtables.html Variable White Dwarf Data Tables], Paul A. Bradley, 22 March 2005 version. Accessed online June 7, 2007.
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| *[http://adsabs.harvard.edu/abs/2006astro.ph.12043G A Progress Report on the Empirical Determination of the ZZ Ceti Instability Strip], A. Gianninas, P. Bergeron, and G. Fontaine, arXiv:astro-ph/0612043.
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| *[http://dx.doi.org/10.1088/0953-8984/10/49/014 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.
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| {{white dwarf}}
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| {{Variable star topics}}
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| [[Category:Pulsating white dwarfs| ]]
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