WZ Sagittae

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Double star
WZ Sagittae
AladinLite
Observation
dates equinox :  J2000.0 , epoch : J2000.0
Constellation arrow
Right ascension 20 h 07 m 36.5 s
declination + 17 ° 42 ′ 14.7 ″
Apparent brightness 7 to 15.53 mag
Typing
rel. Brightness
(G-band) 		(15.21 ± 0.01) mag
rel. Brightness
(J-band) 		(14.86 ± 0.04) mag
B − V color index 0.1 
Spectral class DAep (UG)
Variable star type UGWZ + E + ZZ 
Astrometry
Radial velocity 62.3 km / s
parallax (22.16 ± 0.04)  mas
distance 147  ly
45.1  pc  
Proper movement 
Rec. Share: (71.64 ± 0.06)  mas / a
Dec. portion: (-24.35 ± 0.05)  mas / a
Physical Properties
Dimensions 0.08 / 0.85  M ☉
Rotation time 81.63 min
Other names
and catalog entries
2MASS catalog 2MASS J20073649 + 1742147 [1]
Other names 1RXS J200736.4 + 174217, 1SXPS J200736.7 + 174215, 2E 2005.3 + 1733, 2E 4322, 2RXP J200736.6 + 174218, 2RXS J200736.5 + 174219, 3XMM J200736.5 + 174213, AAVSO 2003 + 17, CMC15 J200736 .5 + 174214, EGGR 136, GALEX J200736.6 + 174214, GSC2.3 N1US062532, HV 3518, Lanning 50, Sge 1913, Nova Sge 1913, UCAC4 539-122231, URAT1 539-535296, USNO-B1.0 1077- 0629939, WDS 20076 + 1741B, WISE J200736.55 + 174215.1, XMMOM J200736.5 + 174215

Template: Infobox Star / Maintenance / MagGTemplate: Infobox Star / Maintenance / MagJ

WZ Sagittae also WZ Sge is a dwarf nova , and thus a cataclysmically changeable binary star system , in the constellation Arrow . It consists of a white dwarf who is orbited by a companion of low mass. The white dwarf has about 0.85 M ☉ , the companion only 0.08 M ☉ . It follows that the companion must be a brown dwarf of the spectral class L2, although this has not yet been confirmed. The distance to this system was determined by parallax and is given as 45.1 pc (approx. 147light  years ).

In this system, the companion star is so close to the white dwarf that the mass transfer through the inner Lagrange point takes place at a comparatively low rate of 1.59 × 10 −10  M ☉ / year, corresponding to 10 10 tons / second to the accretion disk . The disk radius around the white dwarf is approx. 100,000 km. At the point where the mass flow hits the pane, a bright impact front forms ("bright spot"). This spot has a radius of approx. 10,000 km, an effective temperature of approx. 16,000 K and thus contributes around 15% to the total optical luminosity of the system.

The system is classified in a subclass of dwarf novae, namely the group of SU-Ursae-Majoris stars . In addition to the normal outbreaks, these usually also show super maxima, which are about 1 m brighter and last about twice as long. However, WZ Sagittae is unusual in that only super-eruptions have been observed.

The supermaxima of this dwarf nova arise when the plasma in the ring-like accretion disc becomes unstable by exceeding a critical density and leads to a sudden increase in viscosity , as a result of which the matter accumulated in the accretion disc is increasingly transferred to the white dwarf.

WZ Sagittae forms the prototype of a further subdivision of the SU Ursae Majoris stars, which have extremely long supercycles of more than 10,000 days. In these dwarf novae, also known as TOADs (Tremendous Outburst Amplitude Dwarf Novae), magnetic effects can additionally intensify the mass transfer, which means that no normal outbreaks occur, but only super maxima with a super cycle on the order of decades. Only in this subgroup does a super maximum immediately follow a previous super maximum, while in the other SU Ursae Majoris stars there is always a number of normal eruptions between two super maxima. Their bursts reach greater amplitudes of 6 to 8 mag.

Furthermore, in contrast to the other SU Ursae Majoris stars, the WZ Sagittae stars show early superhumps and, after the eruptions, small maxima, which are referred to as rebrightenings. The early superhumps already appear in the rise to the maximum with a period that roughly corresponds to the later superhump period and are attributed to spiral arms in the accretion disk or an increased mass transfer. An increased mass transfer could also be the cause of the rebrightenings.

history

WZ Sagittae is a very fast dwarf nova that outbreaks were observed in 1913, 1946, 1978 and 2001. The first maximum occurred on November 22, 1913 and was discovered in 1919 by Joanna C. S. Mackie while examining photographic plates taken at the Harvard College Observatory . The event of 1913 was the brightest of all observed outbreaks and reached a photographic brightness of 7.0 m . It was then named "Nova Sge 1913".

In 1946, after 33 years of inactivity, it broke out again, and this time it was discovered by Kurt Himpel at the Heidelberg-Königstuhl State Observatory on the night of June 28-29 . During this eruption, the brightness increased from 12 m to 10 m in just 4 hours , and reached a maximum of 7.2 m . After the eruption in 1946, WZ Sagittae was considered a recurrent nova. Since recurrent novae are roughly periodic, it was deliberately monitored in 1978 by the American Association of Variable Star Observers in anticipation of another outbreak.

On November 28, 1978 it was measured by J. Bortle with a magnitude of ≤ 14.5 m . However, due to bad weather, he was unable to continue observing for three nights. On December 1, 1978, and 32.4 years after the second eruption, JT McGraw discovered a visual maximum of about 8.0 m at the McDonald Observatory at the University of Texas at Austin . McGraw's discovery telegram on December 1st triggered intensive efforts worldwide to study the star both spectroscopically and photometrically. Spectroscopic observations during the 1978 eruption demonstrated that WZ Sagittae exhibited properties that were more similar to a dwarf nova than a recurrent nova, and also showed light fluctuations known as superhumps for the first time. These superhumps are the defining features of dwarf novae of the SU Ursae Majoris star type. Therefore, WZ Sagittae was classified as a prototype for a new sub-category of this star type. The 2001 eruption, which reached a maximum visual luminosity of 8.21 m , appeared most similar to the 1978 eruption. It also showed a "dip" in brightness and then oscillated by about two orders of magnitude. WZ Sagittae, believed to be at the end of its evolution as a cataclysmically mutable binary star system, challenges both observers and theorists trying to understand the behavior and evolution of these stars.

Individual evidence

  1. ↑ a b c d e f g h WZ Sge. In: SIMBAD . Center de Données astronomiques de Strasbourg , accessed April 23, 2019 .
  2. ↑ a b c d e WZ Sge. In: VSX. AAVSO , accessed April 23, 2019 .
  3. ↑ a b Danny Steeghs et al .: Dynamical Constraints on the Component Masses of the Cataclysmic Variable WZ Sagittae . In: The Astrophysical Journal, Volume 667, Number 1 . 2007. arxiv : 0706.0987 . bibcode : 2007ApJ ... 667..442S . doi : 10.1086 / 520702 .
  4. ↑ a b Thomas E. Harrison et al .: An Astrometric Calibration of the M V -P orb Relationship for Cataclysmic Variables based on HST Fine Guidance Sensor Parallaxes . In: The Astronomical Journal, Volume 127, Number 1 . 2004. bibcode : 2004AJ .... 127..460H . doi : 10.1086 / 380228 .
  5. ↑ a b Robinson, EL; Nather, RE; Patterson, J .: A photometric study of the recurrent nova WZ Sagittae at minimum light . In: Astrophysical Journal, Part 1, vol. 219, Jan. 1, 1978, p. 168-182. . 1978. bibcode : 1978ApJ ... 219..168R . doi : 10.1086 / 155766 .
  6. ↑ a b O. M. Matthews, R. Speith, GA Wynn, RG West: Magnetically moderated outbursts of WZ Sagittae . In: Monthly Notices of the Royal Astronomical Society, Volume 375, Issue 1 . 2007. arxiv : astro-ph / 0611200 . bibcode : 2007MNRAS.375..105M . doi : 10.1111 / j.1365-2966.2006.11252.x .
  7. ↑ a b Patterson, J. et al .: A photometric study of the dwarf nova WZ Sagittae in outburst . In: Astrophysical Journal, Part 1, vol. 248, Sept. 15, 1981, p. 1067-1075. . 1981. bibcode : 1981ApJ ... 248.1067P . doi : 10.1086 / 159236 .
  8. ↑ Howell, Steve B .; Szkody, Paula; Cannizzo, John K .: Tremendous outburst amplitude dwarf novae . In: Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 439, no. 1, p. 337-345 . 1995. bibcode : 1995ApJ ... 439..337H . doi : 10.1086 / 175177 .
  9. ↑ Shinichi Nakagawa et al .: Multi-Color Photometry of the Outburst of the New WZ Sge-type Dwarf Nova, OT J012059.6 + 325545 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1304.1855v1 .
  10. ↑ Chikako Nakata et al .: WZ Sge-type dwarf novae with multiple rebrightenings: MASTER OT J211258.65 + 242145.4 and MASTER OT J203749.39 + 552210.3 . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.6712v1 .
  11. ↑ a b c Kate Davis: WZ Sagittae . In: Variable Star of the Month . April 13, 2010.