A dwarf nova (pl. ), 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 accretion disk matter from its companion. Dwarf novae are dimmer and repeat more often than "classical" novae.
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.
They are similar to classical
in that the white dwarf is involved in periodic outbursts, but the mechanisms are different.
nova 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.
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.
There are three subtypes of U Geminorum star (UG):
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SS Cygni stars (UGSS), which increase in brightness by 2–6 mag in V band in 1–2 days, and return to their original brightnesses in several subsequent days.
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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).
-
Z Camelopardalis stars (UGZ), which temporarily "halt" at a particular brightness below their peak; a behavior termed a "standstill".
In addition to the large outbursts, some dwarf novae show periodic brightening known as “”. They are caused by deformations of the accretion disk when its rotation is in resonance with the orbital period of the binary.
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