Intermediate polar

DQ Herculis star (English Intermediate polars ( IPs Abbr ) ) together with the AM Herculis stars the class of magnetic cataclysmic variables ( MCVs Abbr ), where by the strong magnetic field of the white dwarf, the Akkretionsgeometrie of mass transfer is greatly changed. The mass transfer to the white dwarf takes place, as generally described in cataclysmic variables, of a low-mass main sequence star that fills its Roche volume.

In contrast to the AM Herculis stars is the magnetic field strength of the white dwarf low (<10 Megagauss ), so that it can rotate freely and usually the formation of an accretion disk is not inhibited. Also compared to the AM Herculis stars significantly higher accretion rates also prevent spin -orbit coupling, or through angular momentum transfer at very short rotation periods of the white dwarf (eg AE Aqr 33 seconds ).

The accretion onto the surface of the white dwarf is along the magnetic field lines, where couples in the gaseous matter from the inner edge of the accretion disk. During the radial impact with the white dwarf a multi-million Kelvin hot plasma is produced in a compact, some hundred kilometers in diameter, accretion region. Where the emitted light power of up to 1033 ergs per second is released primarily as hard Röntgenbremstrahlung in the range 6-10 keV. If the rotation axis with respect to the magnetic poles so inclined there is a pulsating X-ray source. Infrared and optical cyclotron radiation, as well as their polarization, are difficult to detect in these objects, as mentioned in the spectral radiation of the accretion disk dominates with a continuum, which is interspersed with emission lines with Doppelpaeks. The variability in both the optical and in X-rays is associated with a variable mass transfer rate and the interaction in the magnetosphere of the white dwarf. Short-term flares are interpreted as a series of thermonuclear explosions on the surface of the compact star.

The orbital periods of about 90 known DQ Herculis stars are on average longer than that of the AM Herculis stars and are generally above the period gap of cataclysmic variables of 3 hours. The white dwarfs in the DQ Herculis stars have rotation periods between 33 seconds and 67 minutes. There is a crude correlation between the rotation and orbital period, wherein the rotation period is usually less than 1 /10 of the track period. However, the magnetic field densities of the polars and DQ Herculis stars are superimposed. It is believed that convert most of DQ Herculis stars with strong magnetic fields in polar after the orbital period of the orbit of the binary star system has reduced to values ​​of less than 3 hours. In contrast, it should not come at DQ Herculis stars with magnetic moments of less than 5 × 1033 Gcm3 to a synchronization of the rotation period of the white dwarf and the orbital period of the binary system.

Part of the DQ Herculis stars were times Supersoft X -ray sources that have transformed the accreted matter in a steady hydrogen burning on the surface of the white dwarf in helium. During this phase, the continuous accretion to the rotational period of the white dwarf has accelerated to the observed in DQ Herculis stars values ​​of some 10 seconds to a few minutes. The IPs are often not to accrete the entire flowing on the white dwarf matter in the situation. This is called the propeller mechanism in which the accretion stream is split into fireballs and can overcome the rapidly rotating, bound to the white dwarf magnetic field only to a small extent. The largest part, often more than 90 %, is accelerated by the propeller from the binary star system out. Overcomes one of the fireballs the magnetic field and is accreted onto the white dwarf so this leads to a flare. The fireballs of gas from the companion star have here a typical diameter of 10,000 km and a mass of 1014 tons at temperatures around 20,000 K.

Known DQ Herculis stars

  • DQ Her
  • GK Per