Cataclysmic variables (CVs) and X-ray transients (XTs) are generelly believed to be semi-detatched binaries containing compact objects (white dwarfs in CVs, neutron stars or black holes in XTs) and (usually) low-mass main-sequence stars. The Roche-overflown gas from the secondary star forms an accretion disk around the compact object. The chief source of the visual light of CVs (and XTs in outburst) is from the accretion disks. Several sorts of disk instabilities dramatically affect the luminosity of the disk, and then become observable as a variation in the optical flux. This feature not only provides us one of the best opportunities in direct investigation of the physics of the accretion disks through observations of CVs and XTs, but also enables us to reveal the nature of specific objects of astrophysical importance by applying the known physics of accretion disks.
Cataclysmic variables (CVs) are usually subdivided into dwarf novae and novalike variables. Dwarf novae show semi-periodic outbursts with a typical amplitude ranging from 2 to 6 magnitudes, and with a recurrence period of 10 to 1000 days. In contrast, novalike variables do not show prominent outburst activities.
Dwarf novae are further subclassified according to their light behavior:
1) SS Cyg stars showing rather regularly recurring outbursts,
2) Z Cam stars showing "standstills" during which the stars show little variation at brightness between maxima and minima,
Z Cam light curve
and 3) SU UMa stars showing two distinct types of outbursts, short (normal) outbursts and superoutbursts (for a review, see Warner 1985).
SU UMa light curve
As stated in the first section, the visual light of CVs for the most part reflects the energy output from the accretion disk, hence the cause of variation should primarily be sought in the accretion process itself.
Among several mechanisms to explain this rich variety of light variaion of CVs, only two of them seems to have remained viable: the mass-transfer instability (MTI) and the disk instability (DI). The former paradigm primarily assumes that the changing mass-transfer rate from the secondary produces the luminosity variation of the accretion disk; the latter, in contrast, does not assume change of mass-transfer rate, but the intrinsic instability of the accretion disk produces temporal changes in mass-accretion rate in the disk which is observed as quiescent and outburst states. The discrimination of these two paradigms in variable accreting system has been, whether explicitly or implicitly posed, always one of the main goals of both observers and theoreticians.
After a long period of debate, a fairly good
consensus in ordinary CVs seems to be reached between most of observers
and theoreticians concerning the natural explanation of dwarf nova
phenomenon: the disk instability model. The basic idea of the disk
instability is first proposed by Osaki (1974), which explained the
dwarf nova phenomenon as the following:
the disk accumulates the accreted mass
during quiescence and accretes it to the white dwarf during outburst.
The nature of disk instability which triggers such interchange of
disk status was not known at that time. Subsequent theoretical study
by Hoshi (1979) and by Meyer and Meyer-Hofmeister (1981) finally
discovered the thermal instability of the accretion disk due to the
partial ionization of the hydrogen.
This thermal instability has been shown to not only successfully reproduce the various light curves of SS Cyg-type dwarf novae (e.g. Cannizzo 1993) but also give a natural explanation of two basic types of CVs: the dwarf novae and novalike variables.
ER UMa stars and WZ Sge-type stars provide modern examples of discussions between MTI and DI interpreations.
The difference between dwarf novae and novalike variables is explained in the scheme of thermal instability theory, in that the higher mass-transfer rate in novalike variables produce thermally stable accretion disks.
The Z Cam stars have intermediate mass-transfer rates, and are believed to share properties of these two subclasses, that is, phases showing dwarf nova-type activity when the disk is thermally unstable, and standstills when the disk in thermally stable like nova-like variables.
Ritter's Catalogue: The Ritter-Catalog of Cataclysmic Binaries
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