Dwarf Novae

Dwarf novae undergo multiple eruptions in which their brightness increases by about 2 – 5 magnitudes. Each eruption lasts between 2 and 20 days with the nova duration related to the interval between outbursts. This time interval is quasi-periodic and ranges from days to decades.

While classical and recurrent novae result from runaway thermonuclear reactions on the surface of a white dwarf, dwarf novae originate via a different mechanism. Two possible scenarios have been put forward to account for the outbursts, with the Disk Instability Model, currently popular amongst astronomers, able to reproduce many of the characteristics of dwarf nova eruptions.

In this model, the accretion disk surrounding the white dwarf is able to accumulate a certain amount of gas at a steady rate before becoming unstable. These instabilities cause material to be dumped more rapidly onto the white dwarf resulting in the release of large amounts of gravitational energy — the nova outburst. Once the accretion disk has lost enough mass, it once again becomes stable, the increased transfer of material to the white dwarf returns to normal levels, and the system returns to its quiescent state. The cycle is repeated once the disk again accumulates sufficient mass for instabilities within the accretion disk to arise.

The alternate model is the Mass Transfer Burst Model, where a sudden increase in the transfer of material from the companion star to the accretion disk causes the disk to collapse. The result is that matter is suddenly dumped onto the white dwarf releasing large amounts of gravitational energy — the nova outburst.

Due to the different origin for the outbursts, one of the features which distinguishes dwarf novae from classical and recurrent novae is that they do not eject a shell of material with the outburst. In addition, the increase in their brightness is much less than for other novae, hence the term dwarf in the name. They are the most abundant type of nova, with several hundred dwarf novae known.

See also: u geminorum stars.

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