astronomy Astronomy () is a natural science that studies astronomical object, celestial objects and phenomena. It uses mathematics, physics, and chemistry in order to explain their origin and chronology of the Universe, evolution. Objects of interest ...

, an intermediate polar (also called a DQ Herculis Star) is a type of

cataclysmic variable In astronomy, cataclysmic variable stars (CVs) are stars which irregularly increase in brightness by a large factor, then drop back down to a quiescent state. They were initially called novae (), since ones with an outburst brightness visible to ...

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in ...

system with a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

and a cool

main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

secondary star. In most cataclysmic variables, matter from the companion star is gravitationally stripped by the compact star and forms an

accretion disk An accretion disk is a structure (often a circumstellar disk) formed by diffuse material in orbital motion around a massive central body. The central body is typically a star. Friction, uneven irradiance, magnetohydrodynamic effects, and other ...

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 is a vector 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 to its own velocity and t ...

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 AM Herculis is a binary variable star located in the constellation Hercules. This star, along with the star ''AN Ursae Majoris'', is the prototype for a category of cataclysmic variable stars called '' polars'', or ''AM Her'' type ...

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 Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

at minimum: the prototype

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

, and the unusual slow

nova A nova (plural novae or novas) is a transient astronomical event that causes the sudden appearance of a bright, apparently "new" star (hence the name "nova", which is Latin for "new") that slowly fades over weeks or months. Causes of the dramati ...

GK Persei GK Persei (also Nova Persei 1901) was a bright nova first observed on Earth in 1901. It was discovered by Thomas David Anderson, an Edinburgh clergyman, at 02:40 UT on 22 February 1901 when it was at magnitude 2.7. It reached a maximum ...

System structure

In intermediate polar systems, material stripped from a

red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave (TV channel), Dave since 2009, gaining a ...

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 X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...

emitters. The x-rays are generated by high velocity particles from the accretion stream forming a

shock Shock may refer to: Common uses Collective noun *Shock, a historic commercial term for a group of 60, see English numerals#Special names * Stook, or shock of grain, stacked sheaves Healthcare * Shock (circulatory), circulatory medical emerge ...

as they fall onto the surface of the white dwarf star. As particles decelerate and cool before hitting the white dwarf surface,

bremsstrahlung ''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...

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 Earth's interior out into space, where it interacts with the solar wind, a stream of charged particles emanating from the Sun. The magneti ...

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 collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. w ...

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 In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways: *An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system ...

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 el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

emission line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

at 468.6 nm and

circular polarization In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to th ...

, in addition to the light curve periodicities described below.

Light curve periodicities

The

light curve In astronomy, a light curve is a graph of light intensity of a celestial object or region as a function of time, typically with the magnitude of light received on the y axis and with time on the x axis. The light is usually in a particular frequ ...

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 In radio communications, a sideband is a band of frequencies higher than or lower than the carrier frequency, that are the result of the modulation process. The sidebands carry the information transmitted by the radio signal. The sidebands ...

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 A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed, ...

of the light curve and producing a

power spectrum The power spectrum S_(f) of a time series x(t) describes the distribution of power into frequency components composing that signal. According to Fourier analysis, any physical signal can be decomposed into a number of discrete frequencies, ...

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

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiati ...

, 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