AM Herculis star

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Binary star system with a white dwarf (in the middle of the disk) and a stream of matter

As AM Herculis stars , (Engl. Polars , GCVS -Systematikkürzel: AM ) are cataclysmic variable binary stars ( CVs ) denotes where the geometry of the mass transfer through the strong magnetic field of the accreting white dwarf is greatly affected.

properties

The field strength in these systems is high (between 7 and 230 Mega Gauss ) - enough to synchronize the rotation of the white dwarf with the movement of the orbit. This bound rotation and the lack of an accretion disk is characteristic of this type of double star. In contrast, in the DQ Herculis stars ( intermediate polars ), which are also magnetic, the white dwarf can rotate freely and the magnetic field is usually not strong enough to prevent the formation of an accretion disk.

As in the non-magnetic CVs , the low-mass main sequence star loses gaseous matter via the inner Lagrange point , which, however, after a ballistic case, is coupled to the magnetic field lines of the white dwarf and diverted into one or more polar regions. The radial impact on the white dwarf creates a shock wave that heats up to several million Kelvin. The hot plasma in the accretion region extends for a few hundred kilometers. The luminosity of a few 10 32  ergs per second emitted there is mainly released as infrared and optical cyclotron radiation and X-ray braking radiation .

The light of the AM Herculis stars in the infrared and optical spectral range is strongly (up to 30%) linearly and circularly polarized due to the cyclotron radiation , which gave these objects the nickname "polar" in analogy to the pulsars . The intensity in all spectral ranges is usually strongly modulated over the period of rotation of the white dwarf for geometric reasons. In addition to the primary radiation from the accretion region, reprocessed radiation components from the accretion current and the illuminated side of the companion star can be detected.

The orbital or rotation periods of the approx. 90 known AM Herculis stars are between 78 minutes and 14 hours, with more than half of all periods being observed below 2 hours.

The mass transfer in AM Herculis stars can decrease sharply at irregular or cyclical intervals over weeks or years. The optical brightness also drops in parallel to the X-ray brightness. In these so-called "low-states" the photosphere of the otherwise overexposed binary star components can be observed. The accretion rate typically fluctuates between 10 −10 in the active status and 10 −13 solar masses per year in the quiet status. The cause of the changes in the mass transfer rate between the two stars is assumed to be either a star spot at the inner Lagrange point of the red dwarf or a change in the configuration of the magnetic field in the binary star 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 is reprocessed radiation from the vicinity of the magnetic poles.

Occurrence in star catalogs

The General Catalog of Variable Stars currently lists only about 20 stars with the abbreviation AM , which means that 0.04% of all stars in this catalog belong to the class of AM Herculis stars.

Origin of the magnetic field

In isolated white dwarfs, around 10% show a magnetic flux density of more than 1  megagauss . Originally it was believed to be fossil fields from the chemically strange Bp and Ap stars , but the density of these stars is orders of magnitude below the 10% observed in white dwarfs. Among the cataclysmic variables, however, about 25% of these binary stars belong to the polar or intermediate polar . The magnetic fields probably arise during the joint envelope phase, when the later white dwarf develops into a red giant and expands to such an extent that the companion star orbits within its atmosphere. The dynamo field is generated by the movement of the companion and later remains frozen in the core of the red giant, which develops into a white dwarf after its atmosphere has been released. Depending on the distance between the two stars, the white dwarf and its companion star after the joint envelope phase, either a polar, a prepolar or, if the two stars merge, a massive magnetic white dwarf is formed. In prepolars there is only a small mass exchange due to wind accretion from the companion star to the white dwarf. The accretion rates only reach values ​​of around 10 −13 solar masses per year and result in a cooled white dwarf with temperatures below 10,000 K and, if at all, only weak X-rays. The prototype of the prepolar is WX LMi. The prepoles are difficult to distinguish from the polar in their low-state .

Well-known AM Herculis stars

AM Herculis , AN Ursae Majoris , HU Aquarii , UZ Fornacis , V1309 Orionis , EP Draconis , EF Eridani , AR Ursae Majoris

See also

Web links

Individual evidence

  1. ↑ Lexicon of Astronomy . Spectrum Academic Publishing House, 1995, ISBN 3-86150-145-7 .
  2. ^ B. Warner: Cataclysmic variable stars. 1995, ISBN 0-521-41231-5 .
  3. ^ Cuno Hoffmeister , G. Richter, W. Wenzel: Veränderliche Sterne. JA Barth Verlag., Leipzig 1990, ISBN 3-335-00224-5 .
  4. ↑ List of known AM Herculis stars ( memento of the original from February 25, 2013 in the Internet Archive ) Info: The archive link has been inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. @1@ 2Template: Webachiv / IABot / www.arm.ac.uk
  5. ↑ M. Mouchet, JM Bonnet-Bidaud and D. de Martino: The X-ray emission of magnetic cataclysmic variables in the XMM-Newton era . In: Astrophysics. Solar and Stellar Astrophysics . 2011, arxiv : 1102.3594 .
  6. ↑ Zhibin Dai, Shengbang Qian, Linjia Li: The updated photometries and orbital period analysis for polar AM Herculis on the upper edge of period gap . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1307.5135v1 .
  7. ↑ Variability types General Catalog of Variable Stars, Sternberg Astronomical Institute, Moscow, Russia. Retrieved September 1, 2019 .
  8. ↑ E. Breedt, BT Gansicke, J. Girven, AJ Drake, CM Copper Wheat, SG Parsons, TR Marsh: The evolutionary state of short period magnetic white dwarf binaries . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1203.4711 .