cataclysmic variable

(astronomy) A star showing a sudden increase in the magnitude of light, followed by a slow fading of light; examples are novae and supernovae. Also known as explosive variable. In particular, a short-period binary star, one of whose components is a white dwarf star, capable of irregularly timed but recurrent outbursts of brightness by 2 to 10,000.

A type of close binary star system containing a cool star transferring material to its hotter, high-density, degenerate white dwarf companion. The mass transfer results in a large range of observed variability, including cataclysmic events called outbursts, which can increase the brightness of the systems by 2–10 magnitudes (a logarithmic scale with each magnitude being a factor of 2.5 in brightness) from quiescence, equivalent to a factor of 6–10,000 times in intensity. The specific behavior of each system and the extent and cause of the variability are related to whether the transferred material accumulates in an accretion disk surrounding the white dwarf, or whether it flows in a ballistic stream directly from the cool star to the white dwarf surface. Which of these processes will occur depends on the magnetic field strength of the white dwarf and the separation of the two stars in the binary, properties which are determined during the formation of the system. See also Magnitude (astronomy).

If the white dwarf magnetic field is under 100 tesla (1 megagauss), an accretion disk will form and extend close to the white dwarf surface, while a hot spot will form where the mass stream from the cool star hits the disk. Systems in which the mass transfer occurs in this way constitute the most common type among the roughly 1000 known cataclysmic variables. They can be further classified as novae, dwarf novae, and novalike systems. These categories probably represent different phases of evolution of similar systems, and each is determined by the current mass-transfer rate.

Novae are the most spectacular cataclysmic variables, with 7–10-magnitude (factor of 630–10,000 times) outbursts caused by thermonuclear runaways triggered when a critical mass of hydrogen builds up in the atmosphere of the white dwarf. Usually this occurs every few thousand years, although a handful of novae with high-mass white dwarfs and mostly giant cool stars recur on time scales of tens of years. See also Nova.

Dwarf novae have much smaller outbursts of 2–5 magnitudes or a factor of 6–100 times (a small number may be as large as 9 magnitudes or a factor of 4000), and they recur much more frequently, on time scales of weeks or months. Their light curves and theoretical models indicate that their outbursts are probably due to accretion disk instabilities, resulting in increased accretion onto the white dwarf when the disk reaches a critical density.

Novalike systems have the highest mass-transfer rates, so that their disks dominate the light output of the system and do not undergo the instability of outbursts.

At very high magnetic fields, over 1000 T (10 MG), and small separations (short orbital periods), the mass-transfer stream follows the magnetic field lines of the white dwarf to the white dwarf surface. This type of magnetic cataclysmic variable is termed an AM Her star (after its prototype, AM Herculis) or as a polar (after its accretion mode). The approximately 60 known polars were primarily discovered by x-ray satellites due to their large x-ray emission, caused by an accretion shock (producing hard x-rays) as the material is channeled to the magnetic pole of the white dwarf and by the subsequent heating of the white dwarf surface (producing soft x-rays). See also X-ray astronomy; X-ray telescope.

For magnetic fields of 100–1000 T (1–10 MG) and larger separations, the material will form an outer accretion disk ring and then flow from its inner edge to the white dwarf following the magnetic field lines. The dozen known systems that exhibit this behavior are termed DQ Her stars (after DQ Herculis) or intermediate polars (IPs). See also Binary star; Stellar evolution; Variable star; White dwarf star.