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 accretion discs physics is governed by a non-linear combination of many processes, including gravity, hydrodynamics, viscosity, radiation and magnetic fields. 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 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 disks in the Universe
2. Basic physics of accretion discs
3. Analytic models of accretion discs
      3.1. Thin discs
      3.2. Thick discs
4. Numerical simulations
5. Temporal behavior
6. The observational appearance
7. The fundamental unsolved problems
8. References

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A short summary of the basic properties of accretion discs

</table> </table>

Disc's type

Property


Proto-planetary
accretion discs
Accretion discs in black hole (BH) or neutron star (NS) binaries
Accretion discs around white dwarfs (WD) in cataclysmic binaries
Accretion discs in quasars
and other AGNs
Accretion discs in gamma ray burst (GRB) sources

Image

Figure 1: Image:proto-planetary-190x190.jpg Figure 2: Image:accretion-disc.jpg Figure 3: Image:Wz-sge-disc-spirals.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 sourrended 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.
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.
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.
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).
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 center. 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. For more details see the sub-section 2. Basic physics of accretion discs

Angular momentum transport

 ???  ???  ???  ???  ???

Cooling

 ???  ???  ???  ???  ???

Central accreting object

1 Msun star 3-10 Msun BH,
or ~1 Msun NS 		~1 Msun WD 106-109Msun BH 3-10 Msun BH

Disc's size
Rin-Rout

 ???-??? cm
10-2-200 AU 		106-1011 cm
 ??? 		109-1010 cm
 ??? 		1013-1018 cm x[M/107Msun]
0.3 pc ??? 		 ???
3-30 RS

Disc's temperature
Tmax-Tout

103-101 K 107-103 K 105-103 K 105-102 K 1010-109 K

Time behavior, oscillations

 ???  ???  ???  ???  ???

Theoretical models

Numerical simulations Thin discs, slim discs, adafs Thin discs Thick discs, slim discs Thick discs, numerical

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 (2008, Nordita, Stockholm, Sweden) 
Invited by: Dr. Eugene M. Izhikevich, Editor-in-Chief of Scholarpedia, the peer-reviewed open-access encyclopedia
Retrieved from "http://www.scholarpedia.org/article/Accretion_discs"
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