Accretion discs

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Author: Dr. Marek A. Abramowicz, Physics Department, Göteborg University, Sweden and N. Copernicus Astronomical Center, PAN, Warsaw, Poland
Author: Miss Odele Straub, N. Copernicus Astronomical Center PAN, Warsaw, Poland

Accretion discs are flattened astronomical objects made of rapidly rotating gas which slowly spirals onto a central gravitating body (accretor). The gravitational energy of infalling matter extracted in accretion discs powers stellar binaries, active galactic nuclei, proto-planetary discs and some gamma-ray bursts. The black hole accretion in quasars is the most powerful and the most efficient engine known in the whole Universe. The accretion discs physics is governed by a non-linear combination of many processes, including gravity, hydrodynamics, viscosity, radiation and magnetic fields. The high angular momentum of matter in an accretion disc is gradually transported outwards by stresses (turbulent, magnetic, etc.). This allows matter to gradually spiral inwards, towards the gravity centre. Matter's gravitational energy is degraded to heat. A fraction of the heat converts into radiation, which partially escapes and cools down the accretion disc.

The TABLE below gives a summary of some basic properties of different types of accretion discs. More information is provided in the sub-sections of this Scholarpedia article:

1. Observational evidence for accretion discs in the Universe
2. Basic physics of accretion discs
3. Analytic models of accretion discs
      3.1. Thin discs
      3.2. Thick discs
4. Temporal behaviour
5. Numerical simulations
6. Observational appearance
7. Fundamental unsolved problems
8. References

TABLE: A short summary of the basic properties of accretion discs

Type

Proto-planetary
Around white dwarfs
(WD) in cataclysmic binaries

 In black hole (BH)
or neutron star (NS) binaries		 In quasars
and other AGNs		 In gamma ray burst
(GRB) sources
Accretor
1 Msun star
~1 Msun WD
3-10 Msun BH; ~1 Msun NS
106 - 109Msun BH
3-10 Msun BH
Images
click the image		 Figure 1: Image:proto-planetary-190x190.jpg Figure 2: Image:Wz-sge-disc-spirals.jpg Figure 3: Image:accretion-disc.jpg Figure 4: Image:M87-jet.jpg Figure 5: Image:gamma-ray.jpg
Basic physics
The central part of a dense molecular cloud collapses to a proto-star surrendered by a proto-planetary accretion disc. Self gravity and sedimentation trigger the formation of planets. Bipolar outflows ("slow" jets) often emerge from proto-planetary discs.
U Gem is the prototype of a dwarf novae system, i.e. a close stellar binary, with "primary" being a WD with accretion disc. Disc's brightness in the visible light increases 100-fold every ~120 days and returns to the original level after a ~week, due to (mainly) a limit-cycle instability.
X-ray binaries (XRB) consist a mass loosing main-sequence "secondary" star and accreting BH or NS. Among XRBs, the soft X-ray transients (with BH or NS) show quasi-periodic outbursts. Most of the BH XRBs exhibit "fast" jets, and for this reason are called microquasars.
AGNs are supermassive BH at centers of galaxies. Accretion produces radiative power that often outshines the host galaxy. A large torus of gas and dust partially obscures the accretion disc. "Fast" (almost speed of light) jets emerge from many AGNs.
GRBs are the most energetic explosions in the universe. Models of GRBs invoke a black hole (M~3Msun) accreting matter at highly super-Eddington rates. Huge power of gamma-rays is possibly due to an extraction of the BH rotational energy (the Blandford- Znajek mechanism).
Angular momentum transport
Radial: in the inner disc region and at the surface, where the disc is sufficiently ionised (by X-rays, cosmic rays and collisions), via MRI induced turbulence; in weakly ionised regions via Hall instability
Vertical: via outflows and/or torque exerted by large scale magnetic fields.
Local: turbulent viscosity;
Global: direct dissipation by tidal spirals when the incoming supersonic flow shocks on the accretion disc
Local: viscosity induces a shear stress that transports angular momentum outwards; MRI drives a turbulent viscosity which also produces shear stresses;
Global: spiral shocks ?
In outer disc: far from the centre, global disturbances in the gravitational field of the host galaxy can lead to angular momentum removal of the matter;
In inner disc: friction between material on neighbouring orbits cause a slow outward transport of angular momentum
Viscous (turbulent) stresses: in the very optically thick mid-plane of the inner regions the large neutrino viscosity probably shuts off MRI, instead ohmic diffusion and Hall effect
Cooling
Black body radiation, convection, collisions
Black body radiation
Thin/slim disc: black body radiation, advection;
Adaf: advection, bremsstrahlung, Compton scattering
Thick disc (corona): Compton scattering, bremsstrahlung;
Thin disc: black body radiation
Inner region (< 140 RG): neutrino cooled;
Outer region: radiative inefficient, advection cooled

Size
Rin-Rout
 
 1011 - 1015 cm
10-2 - 200 AU 		106 - 1011 cm
10-7 - 10-2 AU 		109 - 1010 cm
10-4 - 10-3 AU		 106 - 1011 cm x [M/Msun]
10 - 106 RG 		105 - 10? cm x [M/Msun]
6 - 10? RG

Temperature
Tmax - Tout
 
 103 - 101 K 105 - 103 K 107 - 103 K 105 - 102 K 1010 - 109 K

Luminosity
 
 L << LEdd L << LEdd L << LEdd
L ~ LEdd		 L < LEdd
L > LEdd,  L >> LEdd 		L >> LEdd
Theoretical models
thick or thin discs (epoch depending), layered discs (with a magnetically inactive 'dead zone' in the mid-plane region)
thin discs (truncated and with funnel/column accretion if the WD is magnetised)
thin discs, slim discs, adafs
thick discs (corona), slim discs
thick discs, thin discs, hyper-accretion, ndaf
References Larson (2003)
Alexander (2008)		 McClintock, Remillard (2003)
Narayan, McClintock (2008)		 Lasota (2001)
Warner (2003)		 on-line compilation
Krolik (1998)		 Piran (2005)
Meszaros (2006)

The table is based on a lecture by Kristen Menou (November 2008, Nordita, Stockholm, Sweden) 


Invited by: Dr. Eugene M. Izhikevich, Editor-in-Chief of Scholarpedia, the peer-reviewed open-access encyclopedia
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