Dwarf nova

Short description: Cataclysmic variable star, consisting of a close binary star system

Dwarf nova HT Cas seen in outburst (mag ~13.4) on November 2, 2010

A dwarf nova (pl. novae), or U Geminorum variable, is one of several types of cataclysmic variable star, consisting of a close binary star system in which one of the components is a white dwarf that accretes matter from its companion. Dwarf novae are dimmer and repeat more frequently than "classical" novae.[1]

## Overview

The first one to be observed was U Geminorum in 1855; however, the mechanism was not known until 1974, when Brian Warner showed that the nova is due to the increase of the luminosity of the accretion disk.[2] They are similar to classical novae in that the white dwarf is involved in periodic outbursts, but the mechanisms are different. Classical novae result from the fusion and detonation of accreted hydrogen on the primary's surface. Current theory suggests that dwarf novae result from instability in the accretion disk, when gas in the disk reaches a critical temperature that causes a change in viscosity, resulting in a temporary increase in mass flow through the disc, which heats the whole disc and hence increases its luminosity. The mass transfer from the donor star is less than this increased flow through the disc, so the disc will eventually drop back below the critical temperature and revert to a cooler, duller mode.[3][4]

Dwarf novae are distinct from classical novae in other ways; their luminosity is lower, and they are typically recurrent on a scale from days to decades.[3] The luminosity of the outburst increases with the recurrence interval as well as the orbital period; recent research with the Hubble Space Telescope suggests that the latter relationship could make dwarf novae useful standard candles for measuring cosmic distances.[3][4]

There are three subtypes of U Geminorum star (UG):[5]

* SS Cygni stars (UGSS), which increase in brightness by 2-6 mag in V in 1–2 days, and return to their original brightnesses in several subsequent days.
* SU Ursae Majoris stars (UGSU), which have brighter and longer "supermaxima" outbursts, or "super-outbursts," in addition to normal outbursts. Varieties of SU Ursae Majoris star include ER Ursae Majoris stars and WZ Sagittae stars (UGWZ).[6]
* Z Camelopardalis stars (UGZ), which temporarily "halt" at a particular brightness below their peak; a behavior termed a "standstill".[7] They are interpreted as occupying the border between the classes of dwarf nova and the more stable nova-like variables.[8]

In addition to the large outbursts, some dwarf novae show periodic brightening known as “superhumps”. They are caused by deformations of the accretion disk when its rotation is in resonance with the orbital period of the binary.

* AAVSO light curve of U Geminorum (SS Cygni type)

* Light curve of eclipsing dwarf nova HT Cassiopeiae during outburst, showing eclipses and superhumps (SU Ursae Majoris type)

* Light curve of Z Camelopardalis (Z Camelopardalis type)

## References

1. ↑ "GCVS Variability Types and Distribution Statistics of Designated Variable Stars According to their Types of Variability". February 12, 2009. http://www.sai.msu.su/groups/cluster/gcvs/gcvs/iii/vartype.txt.
2. ↑ Warner, Brian (July 1974). "Observations of Rapid Blue Variables – XIV: Z C HAMAELEONTIS". Monthly Notices of the Royal Astronomical Society 168 (1): 235–247. doi:10.1093/mnras/168.1.235.
3. ↑ 3.0 3.1 3.2 Simonsen, Mike, ed. "Introduction to CVs". Cataclysmic Variable Network. http://home.mindspring.com/~mikesimonsen/cvnet/id1.html.
4. ↑ 4.0 4.1 "Calibrating Dwarf Novae". Sky & Telescope: 20. September 2003.
5. ↑ Darling, David (2007-02-01). "U Geminorum star". http://www.daviddarling.info/encyclopedia/U/U_Geminorum_star.html.
6. ↑ Darling, David (2007-02-01). "SU Ursae Majoris star". Daviddarling.info. http://www.daviddarling.info/encyclopedia/S/SU_Ursae_Majoris_star.html.
7. ↑ Stehle, R.; King, A.; Rudge, C. (May 2001). "The standstill luminosity in Z Cam systems". Monthly Notices of the Royal Astronomical Society 323 (3): 584–586. doi:10.1046/j.1365-8711.2001.04223.x. Bibcode: 2001MNRAS.323..584S.
8. ↑ Osaki, Yoji (January 1996). "Dwarf-Nova Outbursts". Publications of the Astronomical Society of the Pacific 108: 39. doi:10.1086/133689. Bibcode: 1996PASP..108...39O.

## External links

* "New Method of Estimated Dwarf Novae Distances". 30 May 2003. https://spaceflightnow.com/news/n0305/30distance/.
* "SU Ursae Majoris". American Association of Variable Star Observers. https://www.aavso.org/vsots_suuma.
* "Amateur Astronomers and Dwarf Novae". European Southern Observatory. http://www.eso.org/sci/publications/messenger/archive/no.80-jun95/messenger-no80-43-45.pdf.
* "Activity at a glance (list of recently detected dwarf nova outbursts)". Cataclysmic Variable Network. https://sites.google.com/site/aavsocvsection/.

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White dwarf

Formation|

* Chandrasekhar limit
* PG 1159 star
* Stellar evolution
* Hertzsprung–Russell diagram
* Mira variable

Fate|

* Black dwarf
* Type Ia supernova
* Candidates
* Neutron star
* Pulsar
* Magnetar
* Related links
* Stellar black hole
* Related links
* Compact star
* Quark star
* Exotic star
* Extreme helium star
* Subdwarf B star
* Helium planet

In binary
systems|

* Nova
* Remnant
* List
* Dwarf nova
* Symbiotic nova
* Cataclysmic variable star
* AM CVn star
* Polar
* Intermediate polar
* X-ray binary
* Super soft X-ray source
* Binary pulsar
* X-ray pulsar
* List
* Helium flash
* Carbon detonation

Properties|

* Pulsating
* Urca process
* Electron-degenerate matter
* Quasi-periodic oscillations

Related|

* Planetary nebula
* List
* RAMBOs
* White dwarf luminosity function
* Timeline of white dwarfs, neutron stars, and supernovae

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Variable stars

Pulsating|

| Cepheids and
cepheid-like|

* Classical
* Type II (BL Herculis, W Virginis, RV Tauri)
* RR Lyrae
* Delta Scuti
* Rapidly oscillating Ap
* SX Phoenicis

|

Blue-white with
early spectra|

* Alpha Cygni
* Beta Cephei
* Slowly pulsating B-type
* PV Telescopii
* Blue large-amplitude pulsator

Long-period|

* Mira
* Semiregular
* Slow irregular

Other|

* Gamma Doradus
* Solar-like oscillations
* White dwarf

Eruptive|

| Protostar and PMS|

* Herbig Ae/Be
* Orion
* FU Orionis
* T Tauri

|

Giants and
supergiants|

* Luminous blue variable
* R Coronae Borealis (DY Persei)
* Yellow hypergiant

Eruptive binary|

* Double periodic
* FS Canis Majoris
* RS Canum Venaticorum

Other|

* Flare
* Gamma Cassiopeiae
* Lambda Eridani
* Wolf–Rayet

Cataclysmic|

* AM Canum Venaticorum
* Dwarf nova
* Luminous red nova
* Nova
* Polar
* Intermediate polar
* Supernova
* Hypernova
* SW Sextantis
* Symbiotic
* Symbiotic nova
* Z Andromedae

Rotating|

| Non-spherical|

* Rotating ellipsoidal

|

Stellar spots|

* BY Draconis
* FK Comae Berenices

Magnetic fields|

* Alpha² Canum Venaticorum
* Pulsar
* SX Arietis

Eclipsing|

* Algol
* Beta Lyrae
* Planetary transit
* W Ursae Majoris

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