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 dwarf 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 spectral type ''PG 1159'' and ''[WCE]''.^{1, p. 894} DAV, DBV and GW Vir 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.^[3]

'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'	PG 1159 spectral type, having carbon and oxygen doublet in its spectrum
''Types of pulsating white dwarf''[4]

Contents

DAV stars

DBV stars

GW Vir stars

References

See also

External links and further reading

DAV stars

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;[5]} 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.^[6] 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^[7]; in 1972, it was given the variable star designation ''ZZ Ceti''^[8]. 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.^[9] 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.^[10]

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.^{[11], p. L67.} Winget then searched for such stars and found that GD 358 was a variable DB, or ''DBV'', white dwarf.^[12] This was the first prediction of a class of variable stars before their observation.^{14, p. 89.} In 1985, this star was given the designation ''V777 Her'', which is also another name for this class of variable stars.^{[13]; 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 somthime refered as ''DOV'' stars are the ''GW Vir'' stars. Their prototype is PG 1159-035, which is also the prototype for the spectral class of PG 1159 stars. The PG 1159 stars are not, strictly speaking, white dwarfs as their position on the Hertzsprung-Russell diagram is in transition between the asymptotic giant branch and the white dwarf region; they are often called ''pre-white dwarfs''.^{[14], p. 72; 16} They are hot, with surface temperatures between 75,000 K and 200,000 K^[15], and are characterized by atmospheres with little hydrogen and absorption lines for helium, carbon and oxygen. They may have relatively low surface gravities (log ''g'' ≤ 5.5) and may still be fusing helium.^{[16], § 2.1.1, 2.1.2, Table 2.} It is thought that their atmospheric composition is odd because they are stars which, after they have left the AGB, have reignited helium fusion. As a result, a PG 1159 star's atmosphere is a mixture of material which was between the hydrogen- and helium-burning shells of its AGB star progenitor.^, §1. These stars are believed to eventually lose mass, cool, and become DO white dwarfs.^{15; [17], §4.} PG 1159-035 was observed to vary in 1979^[18], and was given the variable star designation ''GW Vir'' in 1985¹³; this is also an alternate name for the class of DOV stars.^{1, p. 894.}
The periods of the vibrational modes of GW Vir stars range from about 300 to about 5,000 seconds.^{[19], §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.^{19, §1.}

References

1. Physics of white dwarf stars, D. Koester and G. Chanmugam, ''Reports on Progress in Physics'' '53' (1990), pp. 837–915.
2. 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. 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.
4. ZZ Ceti variables
5. 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.
6. A New Short-Period Blue Variable, Arlo U. Landolt, ''Astrophysical Journal'' '153', #1 (July 1968), pp. 151–164.
7. 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.
8. 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.
9. On the Purity of the ZZ Ceti Instability Strip: Discovery of More Pulsating DA White Dwarfs on the Basis of Optical Spectroscopy
10. G117-B15A - How is it evolving?
11. 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.
12. 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.
13. 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.
14. 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.
15. Observational constraints on the evolutionary connection between PG 1159 stars and DO white dwarfs, S. D. Huegelmeyer, S. Dreizler, K. Werner, J. Krzesinski, A. Nitta, and S. J. Kleinman. arXiv:astro-ph/0610746.
16. The Elemental Abundances in Bare Planetary Nebula Central Stars and the Shell Burning in AGB Stars, Klaus Werner and Falk Herwig, ''Publications of the Astronomical Society of the Pacific'' '118', #840 (February 2006), pp. 183–204
17. Determination of Mass-Loss Rates of PG 1159 Stars from Far-Ultraviolet Spectroscopy, Lars Koesterke and Klaus Werner, ''Astrophysical Journal'' '500' (June 1998), pp. L55–L59.
18. 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.
19. 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.
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).

See also

★ Instability strip

★ G117-B15A

External links and further reading

★ Variable White Dwarf Data Tables, Paul A. Bradley, 22 March 2005 version. Accessed online June 7, 2007.

★ A Progress Report on the Empirical Determination of the ZZ Ceti Instability Strip, A. Gianninas, P. Bergeron, and G. Fontaine, arXiv:astro-ph/0612043.

★ 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.