Everything about Accretion Disc totally explained
An
accretion disc (or
accretion disk) is a structure formed by diffuse material in orbital motion around a central body. The central body is typically either a young star, a
protostar, a
white dwarf, a
neutron star, or a
black hole. Instabilities within the disc redistribute angular momentum, causing material in the disc to spiral inward towards the central body. Gravitational energy released in that process is transformed into heat and emitted at the disk surface in the form of electromagnetic radiation. The frequency range of that radiation depends on the central object. Accretion discs of young stars and protostars radiate in the infrared, those around neutron stars and black holes in the X-ray part of the spectrum.
Accretion Disc Physics
In the 1940's models were first derived from basic physical principles. In order to agree with observations those models had to invoke a yet unknown mechanism for angular momentum redistribution. If matter is to fall inwards it must lose not only gravitational energy but also lose
angular momentum. Since the total angular momentum of the disc is conserved, the angular momentum loss of the mass falling into the center has to be compensated by an angular momentum gain of the mass far from the center. In other words, angular momentum should be
transported outwards for matter to accrete. According to the Rayleigh stability criterion,
» .
Most astrophysical discs don't meet this criterion and are therefore prone to this magnetorotational instability. The magnetic fields present in astrophysical objects (required for the instability to occur) are believed to be generated via
dynamo action.
Analytic models of sub-Eddington accretion discs (thin discs, adafs)
When the accretion rate is sub-Eddington and the opacity very high, the standard thin accretion disc is formed. It is geometrically thin in the vertical direction (has a disc-like shape), and is made of a relatively cold gas, with a negligible radiation pressure. The gas goes down on very tight spirals, resembling almost circular, almost free (Keplerian) orbits. Thin discs are relatively luminous and they've thermal electromagnetic spectra, for example not much different from that of a sum of black bodies. Radiative cooling is very efficient in thin discs. The classic 1974 work by Shakura and Sunyaev on thin accretion discs is one of the most often quoted papers in modern astrophysics. Thin discs have been independently worked out by Lynden-Bell, Pringle and Rees. Pringle contributed in the past thirty years many key results to accretion disc theory, and wrote the classic 1981 review that for many years was the main source of information about accretion discs, and is still very useful today.
When the accretion rate is sub-Eddington and the opacity very low, an adaf is formed. This type of accretion disc was prophesied in 1977 by Ichimaru in a paper that was ignored almost by everybody for twenty years. (Some elements of the adaf model were present in the influential 1982 ion-tori paper by Rees, Phinney, Begelman and Blandford, however.)
Adafs started to be intensely studied by many authors only after their rediscovery in the mid 1990 by Narayan and Yi, and independently by Abramowicz, Chen, Kato, Lasota (who coined the name adaf), and Regev. Most important contributions to astrophysical applications of adafs have been made by Narayan and his collaborators. Adafs are cooled by advection (heat captured in matter) rather than by radiation. They are very radiatively inefficient, geometrically extended, similar in shape to a sphere (or a "corona") rather than a disc, and very hot (close to the virial temperature). Because of their low efficiency, adafs are much less luminous than the Shakura-Sunyaev thin discs. Adafs emit a power-law, non-thermal radiation, often with a strong Compton component.
Analytic models of super-Eddington accretion discs (slim discs, Polish doughnuts)
The theory of highly super-Eddington black hole accretion,M>>M
Edd, was developed in the 1980s by Abramowicz, Jaroszynski, Paczynski, Sikora and others in terms of "Polish doughnuts" (the name was coined by Rees). Polish doughnuts are low viscosity, optically thick, radiation pressure supported accretion discs cooled by advection. They are radiatively very inefficient. Polish doughnuts resemble in shape a fat torus (a doughnut) with two narrow funnels along the rotation axis. The funnels collimate the radiation into beams with highly super-Eddington luminosities.
Slim discs (name coined by Kolakowska) have only moderately super-Eddington accretion rates,
M≥M
Edd, rather disc-like shapes, and almost thermal spectra. They are cooled by advection, and are radiatively ineffective. They were introduced by Abramowicz, Lasota, Czerny and Szuszkiewicz in 1988.
Manifestations
Accretion discs are a ubiquitous phenomenon in astrophysics;
active galactic nuclei,
protoplanetary discs, and
gamma ray bursts all involve accretion discs. These discs very often give rise to jets coming from the vicinity of the central object. Jets are an efficient way for the star-disc system to shed angular momentum without losing too much mass.
The most spectacular accretion discs found in nature are those of
active galactic nuclei and of
quasars, which are believed to be massive black holes at the center of galaxies. As matter spirals into a
black hole, the intense gravitational gradient gives rise to intense frictional heating; the accretion disc of a black hole is hot enough to emit
x-rays just outside of the
event horizon. The large luminosity of quasars is believed to be a result of gas being accreted by supermassive black holes. This process can convert about 10 percent of the mass of an object into energy as compared to around 0.5 percent for
nuclear fusion processes.
In close binary systems the more massive primary component evolves faster and has already become a white dwarf, a neutron star, or a black hole, when the less massive companion reaches the giant state and exceeds its
Roche lobe. A gas flow then develops from the companion star to the primary. Angular momentum conservation prevents a straight flow from one star to the other and an accretion disc forms instead.
Accretion discs surrounding
T Tauri stars or
Herbig stars are called
protoplanetary discs because they're thought to be the progenitors of planetary systems. The accreted gas in this case comes from the molecular cloud out of which the star has formed rather than a companion star.
Further Information
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