WZ Sagittae

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WZ Sagittae A/B
Observation data
Epoch J2000      Equinox J2000
Constellation Sagitta
Right ascension 20h 07m 36.50s[1]
Declination +17° 42′ 14.8″[1]
Apparent magnitude (V) 15.20[2]
7.0 - 15.5 B[3]
Characteristics
Spectral type DApe D
U−B color index +1.45[4]
B−V color index +1.49[4]
Variable type Dwarf nova[2]
Astrometry
Radial velocity (Rv) -51[2] km/s
Parallax (π) 22.97 ± 0.15[5] mas
Distance 142.0 ± 0.9 ly
(43.5 ± 0.3 pc)
Details
Mass 0.85 ± 0.04/0.08[6] M
Other designations
WZ Sge, Sge 1913, 2MASS J20073649+1742147, WD 2003+17.[2]

WZ Sagittae (WZ Sge) is a cataclysmic dwarf nova[2] star system in the Sagitta constellation. it consists of a white dwarf primary being orbited by a low mass companion. The white dwarf is about 0.85 solar masses while the companion is only 0.08 solar masses. This implies that the companion is a spectral class L2 star, although this has yet to be confirmed.[6] The distance to this system has been determined by parallax measurements using the fine guidance sense on the Hubble Space Telescope, yielding a distance of 43.5 parsecs.[5]

WZ Sagittae is an ultrashort period cataclysmic nova, with outbursts observed in 1913, 1946, 1978 and 2001. During the well-observed 2001 outburst, the nova reached a peak visual magnitude of 8.21. The 1913 event was the brightest of the observed outbursts, reaching a photographic magnitude of 7.0.[5]

This nova is classified as a SU Ursae Majoris class star, which is a subclass of dwarf nova that produces what are termed superoutbursts spaced several months apart, interspaced with normal outbursts every few weeks. The normal outbursts typically last 2−3 days, while a superoutburst lasts a few weeks. However, WZ Sagittae is unusual in that it is only observed to emit superbursts.[7]

The outbursts of a dwarf nova are caused when matter from a ring-like accretion disk becomes unstable. In this system, the companion star is sufficiently close to the white dwarf that the tidal bulge of the former overlaps the Roche limit, allowing matter to pass across and accumulate onto the disk. When the disk reaches a critical temperature, the gas collapses onto the white dwarf resulting in the release of gravitational potential energy.[8]

A superoutburst may be caused by a tidal interaction of the accretion disk with the donor star, resulting in a greater deposition of matter on the white dwarf. In the case of WZ Sagittae, however, magnetic effects may act to enhance the deposition, resulting in the lack of normal outbursts.[7] Because of the unique timing differences in the rate of superoutbursts of this nova, it been designated the prototype star for a WZ Sagittae subclass.

The orbital period of this system is 1.361 hours. Based upon observations of eclipses of the hot spot on the white dwarf (created by infalling material), the orbital plane of this system is inclined by 76° ± 6° to the line of sight from the Earth.[5]

References[edit]

  1. ^ a b Cutri, R. M. et al (June 2003). 2MASS All Sky Catalog of point sources. NASA. Bibcode:2003tmc..book.....C. 
  2. ^ a b c d e "V* WZ Sge -- Dwarf Nova". SIMBAD. Centre de Données astronomiques de Strasbourg. Retrieved 2009-09-03. 
  3. ^ "GCVS Query=WZ Sge". General Catalogue of Variable Stars @ Sternberg Astronomical Institute, Moscow, Russia. Retrieved 2010-12-01. 
  4. ^ a b Krzeminski, W.; Kraft, Robert P (1964). "Binary Stars among Cataclysmic Variables. V. Photoelectric and Spectroscopic Observations of the Ultra-Short Binary Nova WZ Sagittae". Astrophysical Journal 140: 921−935. Bibcode:1964ApJ...140..921K. doi:10.1086/147995. 
  5. ^ a b c d Harrison, Thomas E.; Johnson, Joni J.; McArthur, B. E.; Benedict, G. F.; Szkody, Paula; Howell, Steve B.; Gelino, Dawn M. (2004). "An Astrometric Calibration of the MV-Porb Relationship for Cataclysmic Variables based on Hubble Space Telescope Fine Guidance Sensor Parallaxes". The Astronomical Journal 127: 460–468. Bibcode:2004AJ....127..460H. doi:10.1086/380228. 
  6. ^ a b Steeghs, Danny; Howell, Steve B.; Knigge, Christian; Gänsicke, Boris T.; Sion, Edward M.; Welsh, William F (September 2007). "Dynamical Constraints on the Component Masses of the Cataclysmic Variable WZ Sagittae". The Astrophysical Journal 667 (1): 442−447. arXiv:0706.0987. Bibcode:2007ApJ...667..442S. doi:10.1086/520702. 
  7. ^ a b Matthews, O. M.; Speith, R.; Wynn, G. A.; West, R. G. (February 2007). "Magnetically moderated outbursts of WZ Sagittae". Monthly Notices of the Royal Astronomical Society 375 (1): 105−114. arXiv:astro-ph/0611200. Bibcode:2007MNRAS.375..105M. doi:10.1111/j.1365-2966.2006.11252.x. 
  8. ^ "WZ Sagittae". Variable Star of the Month series. American Association of Variable Star Observers. September 2001. Retrieved 2009-02-21. 

External links[edit]