cataclysmic binary

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A semidetached binary that undergoes outbursts. The term is usually restricted to cataclysmic variables of the dwarf nova, nova, and nova-like types, excluding any binary that is a supernova, symbiotic star, or X-ray binary. In a cataclysmic binary a cool main-sequence star, usually in the spectral type range G8 to M8, loses mass via Roche lobe overflow onto its white dwarf companion. If the white dwarf is non-magnetic, the overflowing material forms an accretion disk around the white dwarf; the disk is cool at the outer edge, and becomes progressively hotter towards the inner edge, from which material falls preferentially onto the equatorial regions of the white dwarf. By contrast, if the white dwarf possesses a magnetic field the accretion flow is more or less dominated by the magnetic field and falls onto the white dwarf near its magnetic poles. When the magnetic field is sufficiently strong, no disk can form; accretion then occurs directly from the Roche lobe of the cool dwarf along magnetic field lines and onto the poles through an accretion column. Such stars are known as AM Herculis stars after the type star, or polars, because of the strong polarization in the light from them. For fields of intermediate strength, a disk may form but with a hole in the middle; accretion then occurs from the entire inner edge of the disk along field lines onto the magnetic poles, forming an accretion curtain. In this case, the star is usually called an intermediate polar.

Subjects: Astronomy and Astrophysics.

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