Polar (cataclysmic variable star)
As an AM Herculis stars, (English Polars, GCVS nomenclature abbreviation: AM ) are cataclysmic variable binary stars (CVs ) denotes where the geometry of the mass transfer is strongly affected by the strong magnetic field of the accreting white dwarf.
Properties
The field strength in these systems is high ( between 7-230 Megagauss ) - enough to synchronize the rotation of the white dwarf with the orbital motion. This synchronous rotation and the absence of an accretion disk is characteristic of this type of double star. Contrast, in the equally magnetic DQ Herculis stars (german intermediate polars ) rotate the white dwarf freely and the magnetic field is not strong enough to prevent the formation of an accretion disk in the rule.
As in the non-magnetic CVs of low-mass main sequence star loses gaseous matter through the inner Lagrangian point, but after ballistic fall couples to the magnetic field lines of the white dwarf and is deflected into one or more polar regions. During the radial impact with the white dwarf creates a shock wave that heats up to several million Kelvin. The hot plasma in the accretion region has an area of several hundred kilometers. The luminosity emitted there from some 1032 ergs per second is released mainly as infrared and optical cyclotron radiation and Röntgenbremstrahlung.
The light of the AM Herculis star in the infrared and optical spectral range is linear and circularly polarized cyclotron radiation due to the strong ( up to 30 %), which these objects the epithet ' Polar ' brought in analogy to the pulsars. The intensity in all spectral regions is modulated in the rule for geometric reasons strongly about the rotation period of the white dwarf. In addition to the primary radiation from the accretion reprozessierte radiation components from the accretion stream and the illuminated side of the companion star are detectable.
The circulation and rotation periods of the 90 known AM Herculis star are between 78 minutes and 14 hours, more than half of all the periods is observed below 2 hours.
The mass transfer in AM Herculis stars may decrease across strongly in irregular or cyclic intervals over weeks or years. Parallel to the X-ray brightness decreases and the optical brightness. In these so-called 'low -states ', the photospheres of otherwise outshone double star components are observed. The accretion rate typically varies between 10-10 in the active state and 10-13 solar masses per year in the quiet state. As a cause of the changes in the rate of mass transfer between the two stars is either a star spot at the inner Lagrangian point of the red dwarf is suspected or a change in the configuration of the magnetic field in the binary systems.
The X-ray radiation of the polar consists of a hard and a soft component. The hard component with energies above 2 keV is emitted directly from the plasma in the shock front. The soft component with energies between 30 and 50 electron volts reprozessierte radiation from the SURROUNDINGS the magnetic poles.
Origin of the magnetic field
With isolated white dwarfs show about 10% a magnetic flux density of more than 1 Megagauss. Initially it was assumed that it is fossil fields of the chemically peculiar Bp and Ap stars, but the density of these stars is orders of magnitude below the white dwarfs observed at 10%. Among the cataclysmic variables, however, about 25 % of these double stars are among the polars or intermediate polars. Probably the magnetic fields generated during the common envelope phase when the white dwarf later developed into a red giant and expand to the extent that the companion star orbits within its atmosphere. The dynamo field is generated by the motion of the companion and later remains in the core of the red giant's, which develops after shedding its atmosphere into a white dwarf, frozen. Depending on the distance between the two stars, the white dwarf and its companion star after the common envelope phase, either a polar, a Prepolar or in a merger of the two stars is a massive magnetic white dwarf. In Prepolaren only a small mass exchange takes place through a Windakkretion from the companion star to the white dwarf. The accretion rates only reach values of about 10-13 solar masses per year and bring in the sequence a cooled white dwarf with temperatures below 10,000 K, and if at all only a weak X-ray radiation. The prototype of the Prepolaren is WX LMi. The Prepolaren are difficult to distinguish from polars in their low state.