# 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;[1] DBV, or V777 Her, stars, with helium-dominated atmospheres and the spectral type DB;[2] 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;[3][4] 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.[5][6] A subtype of DQV stars, with carbon-dominated atmospheres, has also been proposed,[7] and in May 2012, the first extremely low mass variable (ELMV) white dwarf was reported.[8]

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.[9]

 Types of pulsating white dwarf[4][7][8][10] DAV (GCVS: ZZA) DA spectral type, having only hydrogen absorption lines in its spectrum DBV (GCVS: ZZB) DB spectral type, having only helium absorption lines in its spectrum GW Vir (GCVS: ZZO) Atmosphere mostly C, He and O; may be divided into DOV and PNNV stars DQV DQ spectral type; hot, carbon-dominated atmosphere ELMV DA spectral type; ${\displaystyle \lesssim 0.2M_{\odot }}$

## DAV stars

Early calculations suggested that white dwarfs should vary with periods around 10 seconds, but searches in the 1960s failed to observe this.[11][12] 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.[13] 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.[14] In 1970, another white dwarf, Ross 548, was found to have the same type of variability as HL Tau 76;[15] in 1972, it was given the variable star designation ZZ Ceti.[16] 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.[17] 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.[18] 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.[19]

## DBV stars

In 1982, calculations by Don Winget and his coworkers suggested that helium-atmosphere DB white dwarfs with surface temperatures around 19,000 K should also pulsate.[20] Winget then searched for such stars and found that GD 358 was a variable DB, or DBV, white dwarf.[21] This was the first prediction of a class of variable stars before their observation.[22] In 1985, this star was given the designation V777 Her, which is also another name for this class of variable stars.[2][23] These stars have effective temperatures around 25,000K.[24]

## GW Vir stars

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.[5] This star (also the prototype for the class of PG 1159 stars) was observed to vary in 1979,[25] and was given the variable star designation GW Vir in 1985,[23] 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.[5][6] They are hot, with surface temperatures 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.)[26] It is believed that these stars will eventually cool and become DO white dwarfs.[5]

The periods of the vibrational modes of GW Vir stars range from about 300 to about 5,000 seconds.[26] How pulsations are excited in GW Vir stars was first studied in the 1980s[27] but remained puzzling for almost twenty years.[28] 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.[29]

## DQV stars

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.[30] 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 system with a carbon-oxygen accretion disk.[7]

## Notes

1. ^ Koester & Chanmugam 1990, p. 891–895.
2. ^ a b Murdin, Paul, ed. (2001). Encyclopedia of Astronomy and Astrophysics. Bristol: Nature Publishing Group. p. 3525. ISBN 978-0-333-75088-9.
3. ^ Nagel, T.; Werner, K. (1 November 2004). "Detection of non-radial g-mode pulsations in the newly discovered PG 1159 star HE 1429-1209". Astronomy and Astrophysics. 426 (2): L45–L48. arXiv:astro-ph/0409243. Bibcode:2004A&A...426L..45N. doi:10.1051/0004-6361:200400079. ISSN 0004-6361. §1.
4. ^ a b Quirion, Fontaine & Brassard 2007, §1.1, 1.2.
5. ^ a b c d
6. ^ a b O'Brien, M. S. (1 April 2000). "The Extent and Cause of the Pre-White Dwarf Instability Strip". The Astrophysical Journal. 532 (2): 1078–1088. arXiv:astro-ph/9910495. Bibcode:2000ApJ...532.1078O. doi:10.1086/308613. ISSN 0004-637X.
7. ^ a b c Montgomery, M. H.; Williams, Kurtis A.; Winget, D. E.; Dufour, Patrick; DeGennaro, Steven; Liebert, James (2008). "SDSS J142625.71+575218.3: A Prototype for a New Class of Variable White Dwarf". The Astrophysical Journal Letters. 678 (1): L51. arXiv:0803.2646. Bibcode:2008ApJ...678L..51M. doi:10.1086/588286. ISSN 1538-4357.
8. ^ a b Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Brown, Warren R.; Kilic, Mukremin; Kenyon, Scott J. (1 May 2012). "SDSS J184037.78+642312.3: The First Pulsating Extremely Low Mass White Dwarf". The Astrophysical Journal Letters. 750 (2): L28. arXiv:1204.1338. Bibcode:2012ApJ...750L..28H. doi:10.1088/2041-8205/750/2/L28. ISSN 0004-637X.
9. ^ Winget, D. E. (1998). "Asteroseismology of white dwarf stars". Journal of Physics: Condensed Matter. 10 (49): 11247–11261. Bibcode:1998JPCM...1011247W. doi:10.1088/0953-8984/10/49/014. ISSN 0953-8984.
10. ^ Association Française des Observateurs d'Etoiles Variables. "ZZ Ceti variables". Centre de Données astronomiques de Strasbourg. Retrieved 2007-06-06.
11. ^ Koester & Chanmugam 1990, § 7.1.1.
12. ^ Lawrence, George M.; Ostriker, Jeremiah P.; Hesser, James E. (1 June 1967). "Ultrashort-Period Stellar Oscillations. I. Results from White Dwarfs, Old Novae, Central Stars of Planetary Nebulae, 3c 273, and Scorpius XR-1". The Astrophysical Journal Letters. 148: L161–L163. Bibcode:1967ApJ...148L.161L. doi:10.1086/180037. ISSN 0004-637X.
13. ^ Landolt, Arlo U. (1 July 1968). "A New Short-Period Blue Variable". The Astrophysical Journal. 153: 151–164. Bibcode:1968ApJ...153..151L. doi:10.1086/149645. ISSN 0004-637X.
14. ^ Koester & Chanmugam 1990, § 7.
15. ^ Lasker, Barry M.; Hesser, James E. (1 February 1971). "High-Frequency Stellar Oscillations.VI. R548, a Periodically Variable White Dwarf". The Astrophysical Journal Letters. 163: L89–L93. Bibcode:1971ApJ...163L..89L. doi:10.1086/180673. ISSN 0004-637X.
16. ^ Kukarkin, B. V.; Kholopov, P. N.; Kukarkina, N. P.; Perova, N. B. (1 September 1972). "58th Name-List of Variable Stars". Information Bulletin on Variable Stars. 717: 1. Bibcode:1972IBVS..717....1K. ISSN 0374-0676.
17. ^ Koester & Chanmugam 1990, pp. 891, 895.
18. ^ Bergeron, P.; Fontaine, G.; Billères, M.; Boudreault, S.; Green, E. M. (2004). "On the Purity of the ZZ Ceti Instability Strip: Discovery of More Pulsating DA White Dwarfs on the Basis of Optical Spectroscopy". The Astrophysical Journal. 600 (1): 404–8. arXiv:astro-ph/0309483. Bibcode:2004ApJ...600..404B. doi:10.1086/379808. ISSN 0004-637X.
19. ^ Kepler, S. O.; Vauclair, G.; Nather, R. E.; Winget, D. E.; Robinson, E. L. (1989). G117-B15A - How is it evolving?. IAU Colloq. 114: White Dwarfs. 328. pp. 341–345. doi:10.1007/3-540-51031-1_344.
20. ^ Winget, D. E.; van Horn, H. M.; Tassoul, M.; Fontaine, G.; Hansen, C. J.; Carroll, B. W. (1 January 1982). "Hydrogen-driving and the blue edge of compositionally stratified ZZ Ceti star models". The Astrophysical Journal Letters. 252: L67. Bibcode:1982ApJ...252L..65W. doi:10.1086/183721. ISSN 0004-637X.
21. ^ Winget, D. E.; Robinson, E. L.; Nather, R. D.; Fontaine, G. (1 November 1982). "Photometric observations of GD 358 - DB white dwarfs do pulsate". The Astrophysical Journal Letters. 262: L11–L15. Bibcode:1982ApJ...262L..11W. doi:10.1086/183902. ISSN 0004-637X.
22. ^ Kawaler, Steven D.; Novikov, I. D.; Srinivasan, G. (1997). Meynet, G.; Schaerer (eds.). Stellar remnants. Saas-Fee advanced course 25 lecture notes. Berlin: Springer. p. 89. ISBN 978-3-540-61520-0. Lecture notes for Saas-Fee advanced course number 25.
23. ^ a b Kholopov, P. N.; Samus, N. N.; Kazarovets, E. V.; Perova, N. B. (1 March 1985). "The 67th Name-List of Variable Stars". Information Bulletin on Variable Stars. 2681: 1. Bibcode:1985IBVS.2681....1K. ISSN 0374-0676.
24. ^ Koester & Chanmugam 1990, p. 895.
25. ^ McGraw, J. T.; Liebert, J.; Starrfield, S. G.; Green, R. (1979). PG1159-035: A new, hot, non-DA pulsating degenerate. IAU Colloq. 53: White Dwarfs and Variable Degenerate Stars. pp. 377–381. Bibcode:1979wdvd.coll..377M.
26. ^ a b Quirion, Fontaine & Brassard 2007, Table 1.
27. ^ Cox, Arthur N. (1 March 2003). "A Pulsation Mechanism for GW Virginis Variables". The Astrophysical Journal. 585 (2): 975–982. Bibcode:2003ApJ...585..975C. doi:10.1086/346228. ISSN 0004-637X.
28. ^ Cox, A. N. (1 May 2002). An Instability Mechanism for GW Vir Variables. Bulletin of the American Astronomical Society. 200. p. 85.07. Bibcode:2002AAS...200.8507C.
29. ^ Córsico, A. H.; Althaus, L. G.; Miller Bertolami, M. M. (1 October 2006). "New nonadiabatic pulsation computations on full PG 1159 evolutionary models: the theoretical GW Virginis instability strip revisited". Astronomy and Astrophysics. 458 (1): 259–267. arXiv:astro-ph/0607012. Bibcode:2006A&A...458..259C. doi:10.1051/0004-6361:20065423. ISSN 0004-6361. §1.
30. ^ Dufour, P.; Liebert, J.; Fontaine, G.; Behara, N. (November 2007). "White dwarf stars with carbon atmospheres". Nature. 450 (7169): 522–524. arXiv:0711.3227. Bibcode:2007Natur.450..522D. doi:10.1038/nature06318. ISSN 0028-0836. PMID 18033290.

## References

• Koester, D.; Chanmugam, G. (1990). "REVIEW: Physics of white dwarf stars" (PDF). Reports on Progress in Physics. 53 (7): 837–915. Bibcode:1990RPPh...53..837K. doi:10.1088/0034-4885/53/7/001. ISSN 0034-4885.
• Quirion, P.O.; Fontaine, G.; Brassard, P. (1 July 2007). "Mapping the Instability Domains of GW Vir Stars in the Effective Temperature-Surface Gravity Diagram". The Astrophysical Journal Supplement Series. 171 (1): 219–248. Bibcode:2007ApJS..171..219Q. doi:10.1086/513870. ISSN 0067-0049.