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], pp. 891, 895; DBV, or V777 Her, stars, with helium-dominated atmospheres and the spectral type DB[2], 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;[3], §1.1, 1.2;[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.[3], § 1.1;[5] A subtype of DQV stars, with carbon-dominated atmospheres, has also been proposed.[6]

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.[7]

Contents

DAV stars

Types of pulsating white dwarf[3][8], §1.1, 1.2;[6]
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

Early calculations suggested that white dwarfs should vary with periods around 10 seconds, but searches in the 1960s failed to observe this.[1], § 7.1.1;[9] 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.[10] 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.[1], § 7. In 1970, another white dwarf, Ross 548, was found to have the same type of variability as HL Tau 76;[11] in 1972, it was given the variable star designation ZZ Ceti.[12] 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.[1], 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.[13] 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.[14]

DBV stars

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.[15], p. L67. Winget then searched for such stars and found that GD 358 was a variable DB, or DBV, white dwarf.[16] This was the first prediction of a class of variable stars before their observation.[17], p. 89. In 1985, this star was given the designation V777 Her, which is also another name for this class of variable stars.[18][2], p. 3525 These stars have effective temperatures around 25,000K.[1], p. 895.

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.[3], §1.1. This star (also the prototype for the class of PG 1159 stars) was observed to vary in 1979,[19] and was given the variable star designation GW Vir in 1985,[18] 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.[3], § 1.1;[5] 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.)[3], Table 1 It is believed that these stars will eventually cool and become DO white dwarfs.[3], § 1.1.

The periods of the vibrational modes of GW Vir stars range from about 300 to about 5,000 seconds.[3], Table 1 How pulsations are excited in GW Vir stars was first studied in the 1980s[20] but remained puzzling for almost twenty years.[21] 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.[22], §1.

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.[23] 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.[6]

See also

References

  1. ^ a b c d e Physics of white dwarf stars, D. Koester and G. Chanmugam, Reports on Progress in Physics 53 (1990), pp. 837–915.
  2. ^ a b 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 0333750888.
  3. ^ a b c d e f g h 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.
  4. ^ §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.
  5. ^ a b The Extent and Cause of the Pre-White Dwarf Instability Strip, M. S. O'Brien, Astrophysical Journal 532, #2 (April 2000), pp. 1078–1088.
  6. ^ a b c 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, Bibcode2008ApJ...678L..51M, doi:10.1086/588286.
  7. ^ 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.
  8. ^ Association Française des Observateurs d'Etoiles Variables. "ZZ Ceti variables". Centre de Données astronomiques de Strasbourg. http://cdsweb.u-strasbg.fr/afoev/var/ezz.htx. Retrieved 2007-06-06. 
  9. ^ 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.
  10. ^ A New Short-Period Blue Variable, Arlo U. Landolt, Astrophysical Journal 153, #1 (July 1968), pp. 151–164.
  11. ^ 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.
  12. ^ 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.
  13. ^ Bergeron, P.; Fontaine, G. "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. http://www.journals.uchicago.edu/doi/full/10.1086/379808. Retrieved 2007-06-06. 
  14. ^ Kepler, S.O.; G. Vauclair, R. E. Nather, D. E. Winget, and E. L. Robinson (1989). "G117-B15A - How is it evolving?". White dwarfs; Proceedings of IAU Colloquium 114th, Hanover, NH, Aug. 15-19, 1988 (A90-32719 13-90). Berlin and New York: Springer-Verlag. http://adsabs.harvard.edu/abs/1989whdw.coll..341K. Retrieved 2007-06-07. 
  15. ^ 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.
  16. ^ 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.
  17. ^ 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 3540615202.
  18. ^ a b 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.
  19. ^ 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.
  20. ^ A Pulsation Mechanism for GW Virginis Variables, Arthur N. Cox, Astrophysical Journal 585, #2 (March 2003), pp. 975–982.
  21. ^ 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).
  22. ^ 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.
  23. ^ White dwarf stars with carbon atmospheres, Patrick Dufour, James Liebert, G. Fontaine, and N. Behara, Nature 450, #7169 (November 2007), pp. 522–524, Bibcode2007Natur.450..522D, doi:10.1038/nature06318

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