astronomy Astronomy is a natural science that studies celestial objects and the phenomena that occur in the cosmos. It uses mathematics, physics, and chemistry in order to explain their origin and their overall evolution. Objects of interest includ ...

, an intermediate polar (also called a DQ Herculis Star) is a type of cataclysmic variable, binary star system with a

white dwarf A white dwarf is a Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

and a cool main-sequence secondary star. In most cataclysmic variables, matter from the companion star is gravitationally stripped by the compact star and forms an accretion disk around it. In intermediate polar systems, the same general scenario applies except that the inner disk is disrupted by the

magnetic field A magnetic field (sometimes called B-field) is a physical field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular ...

of the white dwarf. The name "intermediate polar" is derived from the strength of the white dwarf's magnetic field, which is between that of non-magnetic cataclysmic variable systems and strongly magnetic systems. Non-magnetic systems exhibit full accretion disks, while strongly magnetic systems (called polars or AM Herculis systems) exhibit only accretion streams which directly impact the white dwarf's magnetosphere. There were 26 confirmed intermediate polar systems as of 14 April 2006. This represents about 1% of the 1,830 total cataclysmic variable systems presented by Downes et al. (2006) in the Catalog of Cataclysmic Variables. Only two of them are brighter than 15th magnitude at minimum: the prototype

DQ Herculis DQ Herculis, or Nova Herculis 1934, was a slow, bright nova occurring in the northern constellation of Hercules (constellation), Hercules in December 1934. This cataclysmic variable star was discovered on 13 December 1934 by J. P. M. P ...

, and the unusual slow nova, GK Persei.

System structure

In intermediate polar systems, material stripped from a

red dwarf A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

secondary star flows into an accretion disk around the white dwarf, but the inner disk is truncated by the

of the white dwarf. In extreme instances, the disk can be fully disrupted, although this is uncommon. In the region where the disk is truncated, the gas in the disk begins to travel along the white dwarf's magnetic field lines, forming curved sheets of luminous material called ''accretion curtains''. Disk material passes through the curtains and then accretes onto the white dwarf near one of its magnetic poles.

Physical Properties

Intermediate polar systems are strong

x-ray An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

emitters. The x-rays are generated by high velocity particles from the accretion stream forming a shock as they fall onto the surface of the white dwarf star. As particles decelerate and cool before hitting the white dwarf surface, bremsstrahlung x-rays are produced and may subsequently be absorbed by gas surrounding the shock region. The

strength of white dwarfs in intermediate polar systems are typically 1 million to 10 million gauss (100–1000 teslas). This is about a million times stronger than the

Earth's magnetic field Earth's magnetic field, also known as the geomagnetic field, is the magnetic field that extends from structure of Earth, Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from ...

and towards the upper limit of magnetic field strengths that can be produced in a laboratory on Earth, but is much less than the magnetic field strength of

neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...

s. At the intersection of the accretion stream and the surface of the white dwarf, a hot spot is produced. Because the white dwarf has a dipole magnetic field, it will have one hot spot at each of its magnetic poles. As the white dwarf and its dipole magnetic field spin, the hot spots will spin also. Other defining characteristics of intermediate polars include a strong

Helium Helium (from ) is a chemical element; it has chemical symbol, symbol He and atomic number 2. It is a colorless, odorless, non-toxic, inert gas, inert, monatomic gas and the first in the noble gas group in the periodic table. Its boiling point is ...

II emission line at 468.6 nm and circular polarization, in addition to the light curve periodicities described below.

Light curve periodicities

The

light curve In astronomy, a light curve is a graph (discrete mathematics), graph of the Radiance, light intensity of a celestial object or region as a function of time, typically with the magnitude (astronomy), magnitude of light received on the ''y''-axis ...

of an intermediate polar may show several types of stable periodic changes in brightness. One periodicity is related to the orbital period of the binary star system. The orbital periods of confirmed intermediate polars range from 1.4 to 48 hours, with typical values between 3 and 6 hours. A second periodic signal originates from the rotation of the white dwarf spinning on its axis. The observational characteristic that most clearly defines an intermediate polar is the existence of a spin period signal that is shorter than the orbital period. The known periods range from 33 to 4022 seconds. The physical cause of optical spin-period oscillations is usually attributed to the changing viewing aspect of the accretion curtain as it converges near the white dwarf. A third light curve periodicity, the sideband period between the spin period and the orbital period, is also often present. All three periodic signals may be measured by taking a fourier transform of the light curve and producing a

power spectrum In signal processing, the power spectrum S_(f) of a continuous time signal x(t) describes the distribution of Power (physics), power into frequency components f composing that signal. According to Fourier analysis, any physical signal can be ...

. Intermediate polars produce spin and sideband periodicities in x-ray,

ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...

, and optical wavelengths. Although the source of the periods in all three wavelengths is ultimately the white dwarf spin, the exact mechanisms for producing the high-energy periodicities and the optical periodicities are thought to be different. In addition to the stable oscillations, unstable oscillations called "quasi-periodic oscillations" may appear and then die off after a few cycles. Quasi-periodic oscillations typically have periods between 30 and 300 seconds.

References

* * * Joseph Patterson,

External links

* * {{Variable star topics Stellar phenomena