"Cataclysmic variables" is a general term used to describe several classes of binary stars that undergo major brightenings or outbursts. The sub-class called dwarf novae are of particular interest because their outbursts, while not strictly regular in period, recur on timescales from a couple of weeks to months or years. Dwarf novae (DNe) consist of a white dwarf star and a normal low mass star locked in tight orbit about each other with orbital periods of (typically) 3 - 6 hours. Because of the proximity of the two stars, the normal star fills its Roche lobe (a theoretical surface where the gravity of the two stars balance one another) and transfers matter onto its companion. Because this material has angular momentum, it does not simply drop onto the companion star but rather spirals in, forming an accretion disk around the white dwarf. The accreting material is heated as it spirals in toward the white dwarf, generating copious UV and in some cases X-ray emission in its final plunge onto the white dwarf. The outbursts are thought to be due to instabilities in the rate of mass transfer, although whether this occurs due to changes in the transfer rate from the normal star or whether the mass transfer rate within the disk is responsible is a matter of debate. Look here for an artist's conceptual image of a cataclysmic variable star.
HUT was used to observe several such systems during Astro-1, including one object that was observed during its outburst phase. Each of these observations provided new and different information on the processes occurring in these systems, and demonstrated the power of the HUT wavelength range to provide new diagnostics for these stellar systems. However, these "snapshots" could not provide the comprehensive coverage needed to understand fully the cause of the outbursts and the heating that occurs on the white dwarf. The longer mission planned for Astro-2 should make it possible to study several DNe in more detail, and at least one throughout and entire outburst cycle. The American Association of Variable Star Observers (AAVSO), and international clearing house for compiling amateur astronomer observations of variable stars, will provide monitoring of key objects before, during, and after the Astro-2 mission and will provide quick notice of any objects entering the outburst phase.
In some cataclysmics, the white dwarf star has a strong magnetic
field; this field either partially or totally disrupts the accretion disk
and causes the accretion to take place instead onto the magnetic
poles of the white dwarf. HUT astronomers want to learn about how
accretion occurring in this way differs from disk accretion in DNe.
By measuring the light variations with time, the heating processes
occurring on the surface of the white dwarf star can be studied.
Although some magnetic systems were planned for Astro-1, none
were observed successfully. Hence, this will be a new area of
investigation for Astro-2.
