WZ Sagittae
Encyclopedia
WZ Sagittae is a cataclysmic dwarf nova
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 mass
es 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. 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.
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.
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.
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
.
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. 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 to the line of sight from the Earth
.
Dwarf nova
A U Geminorum-type variable star, or dwarf nova is a type of cataclysmic variable starhttp://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt consisting of a close binary star system in which one of the components is a white dwarf, which accretes matter from its companion...
star system in the Sagitta constellation. it consists of a white dwarf
White dwarf
A white dwarf, also called a degenerate dwarf, is a small star composed mostly of electron-degenerate matter. They are very dense; a white dwarf's mass is comparable to that of the Sun and its volume is comparable to that of the Earth. Its faint luminosity comes from the emission of stored...
primary being orbited by a low mass companion. The white dwarf is about 0.85 solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es 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. The distance to this system has been determined by parallax
Parallax
Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight, and is measured by the angle or semi-angle of inclination between those two lines. The term is derived from the Greek παράλλαξις , meaning "alteration"...
measurements using the fine guidance sense on the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
, yielding a distance of 43.5 parsecs.
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
Photographic magnitude
Before the advent of photometers which accurately measure the brightness of astronomical objects, the apparent magnitude of an object was obtained by taking a picture of it with a camera. These images, made on photoemulsive film or plates, were more sensitive to the blue end of the visual spectrum...
of 7.0.
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.
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
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...
, 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
Potential energy
In physics, potential energy is the energy stored in a body or in a system due to its position in a force field or due to its configuration. The SI unit of measure for energy and work is the Joule...
.
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. 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 to the line of sight from the Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
.
External links
- AAVSO Variable Star of the Month. WZ Sagittae: September 2001 (HTML / PDF)
- AAVSO: Quick Look View of AAVSO Observations (get recent magnitude estimates for WZ Sge)