Accretion disc

An accretion disk is in astrophysics a rotating disk around a central object, the material transported in the direction of the center ( accreted ). It may consist of atomic gas or dust ( standard disk ) or from different degrees of ionized gas ( plasma).

Mechanism of accretion in the accretion disk

Since a gas disk does not rotate as a rigid body, but differentially to the central object ( the inner regions rotate because of Kepler's laws faster), there are friction and shearing forces. Through these and other turbulent processes in the disk particles are transported towards the central object, so that the central object is gaining ground ( accreted ). These ( angular momentum conservation ) these particles have to pay their angular momentum outward. This is done by a particle 's angular momentum is transferred to another that is " pushed away " as a result of the central object.

The molecular viscosity is too small to be responsible for the angular momentum transfer ( in the required order of magnitude). Therefore, it is believed that the disk becomes turbulent, and the turbulence creates a viscosity. In weakly ionized disks assume the magnetic fields that carry the ions inevitably brings with it a major role. They cause instability (magneto rotational instability (MRI ) ), which lead to turbulence in the disc and thus to a dynamic viscosity. This finally allows accretion, ie feeding the disc material to the central body. The theory for the description of plasmas in magnetic fields is the magnetohydrodynamics ( MHD).

The temperature of a ring of an accretion is a function of density, viscosity, and rotation speed. They therefore rises in the direction of its center and can reach up to several million Kelvin in the transition layer. The radiation profile of an accretion disk is in a first approximation composed of the radiation of many rings of different temperature with different distance from the accreting object, ranging from infrared to hard X-rays.

The diameter of accretion disks ranging from a few hundred astronomical units to hundreds of parsecs in active galactic nuclei. The energy stored in the accretion disks of matter, the mass of the accreting object exceed by one to two orders of magnitude. These discs can be described as self - gravitating discs, because the information stored in the disc material, with its gravitational force, and stabilizes these together.

Occurrence of accretion disks

Typical accretion disks around young stars are located during and for some time after star formation. These include the T Tauri stars, Herbig and the FU Orionis stars. In older stars appear on accretion disks in binary star systems in which a mass flow from a donor takes place to a compact object. These include the star ratings of symbiotic stars, the cataclysmic binary stars and the X-ray binaries. The accreting compact objects are neutron stars, black holes and white dwarfs and less frequently to main-sequence stars in Algolsternen, Beta Lyrae stars and double periodic variables. Several orders of magnitude larger radii and mass transfer rates have been found in the accretion disk around the central black holes in galaxies. These manifest themselves depending on the viewing angle and accretion rate as quasars, active galactic nuclei, or Seyfert galaxies. In neutron stars and black holes gravitational potential energy is converted into accretion disks, so that by the viscosity in the differentially rotating disk, this light bright. This mechanism can vary depending on compactness, the quotient of the mass and radius of the object, up to 20 times as effective as the generation of radiation from nuclear processes, such as fusion.

Formation of accretion disks

A gas cloud can contract only under the influence of gravity when there is friction between encountered parts of different speed in any form. Otherwise, they would also by collisions in the Middle retain the same kinetic energy and do not take the permanence further down in the potential well of course. This dissipation is greater, the larger the relative speed. The dissipation is the smaller, the smaller the relative velocities of the particles. When all contractile cloud has a significant angular momentum then done parallel to the axis in the center encounters a higher speed than normal to the axis. Wherein these movements are damped more than the orbital angular momentum of which coincides with the total angular momentum. Once the components are moving somewhat in a plane, the relative velocity decreases significantly and it remains flat. There are models for the formation of accretion disks. These radiative processes play an essential role for the damping.

Disc Instability Model

Accretion disks oscillate in a number of classes of stars between a state of high and low mass transfer rates. This applies to both close binary systems such as the dwarf novae, AM Canum - Venaticorum star and X-ray binaries, low mass and single stars like the FU Orionis stars, which are classified into phases with low accretion rates as T Tauri stars. The viscosity of the material in the disc varies by a factor of 10 between the two states, and this is independent of the chemical composition since the accretion is in the AM - CVn stars almost exclusively of helium and usually dominated by the hydrogen in the other cases. In the case of a high viscosity, the disc heats up due to the higher internal friction and results in a large increase in the electromagnetic radiation. Using the Disc Instability Model ( German disc instability model ) suggests that the eruptions in the star classes describe quite well, but there is so far no physical cause for the sudden change in viscosity known.