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Encyclopedia > Accretion disc

Artist's conception of a binary star system with one black hole and one main sequence star

Unsolved problems in physics: Accretion disc jets: Why do the discs surrounding certain objects, such as the nuclei of active galaxies, emit radiation jets along their polar axes? These jets are invoked by astronomers to do everything from getting rid of angular momentum in a forming star to reionizing the universe (in AGNs), but their origin is still not well understood

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 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. Download high resolution version (3000x2400, 111 KB) [1] File links The following pages link to this file: Black hole Accretion disc Categories: NASA images ... Download high resolution version (3000x2400, 111 KB) [1] File links The following pages link to this file: Black hole Accretion disc Categories: NASA images ... Image File history File links No higher resolution available. ... This is a list of some of the unsolved problems in physics. ... For other uses, see Galaxy (disambiguation). ... Relativistic Jet. ... A Protonononostar is an object that forms by contraction out of the really big gas of a giant molecular cloud in the interstellar medium. ... This article or section does not adequately cite its references or sources. ... For the Hugo Award-winning story by Larry Niven, see Neutron Star (story). ... For other uses, see Black hole (disambiguation). ...

1 Accretion Disc Physics
- 1.1 α-Disc Model
- 1.2 Magnetorotational Instability
2 Manifestations
3 See also
4 External links
5 References

Accretion Disc Physics

In the 1940's models were first derived from basic physical principles.^[1] 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, This gyroscope remains upright while spinning due to its angular momentum. ...

$frac{partial(R^2Omega)}{partial R}>0,$

where $Ω$ represents the angular velocity of a fluid element and $R$ its distance to the rotation center, an accretion disc is expected to be a laminar flow. This prevents the existence of an hydrodynamic mechanism for angular momentum transport. Laminar flow (bottom) and turbulent flow (top) over a submarine hull. ... Fluid dynamics is the sub-discipline of fluid mechanics dealing with fluids (liquids and gases) in motion. ...

On one hand, it was clear that viscous stresses would eventually cause matter to heat up and radiate away part of the gravitational energy. On the other hand viscosity itself was not enough to explain the transport of angular momentum to the exterior parts of the disc. Turbulence enhanced viscosity was the mechanism thought to be responsible of such angular momentum redistribution, although the origin of the turbulence itself was not well understood. The conventional phenomenological approach introduces an adjustable parameter $α$ describing the effective increase of viscosity due to turbulent eddies within the disc.^[2]^[3] In 1991, with the rediscovery of the magnetorotational instability (MRI), S. A. Balbus and J. F. Hawley established that a weakly magnetized disc accreting around a heavy compact central object was highly unstable, providing a direct mechanism for angular momentum redistribution.^[4] In fluid dynamics, turbulence or turbulent flow is a flow regime characterized by chaotic, stochastic property changes. ... For other uses, see Viscosity (disambiguation). ...

$α$ -Disc Model

Shakura and Sunyaev (1973)^[2] proposed turbulence in the gas as the source of an increased viscosity. Assuming subsonic turbulence and the disc height as an upper limit for the size of the eddies, the disc viscosity can be estimated as $ν = α c s H$ where $c s$ is the sound speed, $H$ is the disc height, and $α$ is a free parameter between zero (no accretion) and approximately one. The speed of sound varies depending on the medium through which the sound waves pass. ...

By using the equation of hydrostatic equilibrium, combined with conservation of angular momentum and assuming that the disc is thin, the equations of disk structure may be solved in terms of the $α$ parameter. Many of the observables depend only weakly on $α$ , so this theory is predictive even though it has a free parameter. Hydrostatic equilibrium occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. ... This gyroscope remains upright while spinning due to its angular momentum. ...

Using Kramers' law for the opacity it is found that

$H=1.7times 10^8alpha^{-1/10}dot{M}^{3/20}_{16} m_1^{-3/8} R^{9/8}_{10}f^{3/5} {rm cm}$

$T_c=1.4times 10^4 alpha^{-1/5}dot{M}^{3/10}_{16} m_1^{1/4} R^{-3/4}_{10}f^{6/5}{rm K}$

$rho=3.1times 10^{-8}alpha^{-7/10}dot{M}^{11/20}_{16} m_1^{5/8} R^{-15/8}_{10}f^{11/5}{rm g cm}^{-3}$

where $T c$ and $ρ$ are the mid-plane temperature and density respectively. $dot{M}_{16}$ is the accretion rate, in units of $10^{16}{rm g s}^{-1}$ , $m 1$ is the mass of the central accreting object in units of a solar mass, $M_bigodot$ , $R 10$ is the radius of a point in the disc, in units of $1010 cm$ , and $f=left[1-left(frac{R_star}{R}right) right]^{1/4}$ , where $R_star$ is the radius where angular momentum stops being transported inwards.

This theory breaks down when gas pressure is not significant. For example, if the accretion rate approaches the Eddington limit, radiation pressure becomes important and the disk will "puff up" into a torus or some other three dimensional solution like an Advection Dominated Accretion Flow (ADAF). Another extreme is the case of Saturn's rings, where the disk is so gas poor its angular momentum transport is dominated by solid body collisions and disk-moon gravitational interactions. In physics, the Eddington Limit is a natural limit to the luminosity that can be radiated by spherically symmetric accretion onto a compact object, like a black hole. ... In geometry, a torus (pl. ... This article is about the planet. ...

Magnetorotational Instability

Main article: Magnetorotational Instability

Hannes Alfven first showed that a magnetic field could provide a mechanism for the transfer of angular momentum, and this mechanism was incorporated into a theory of Fred Hoyle's on the origin of the Solar system (Shklovskii, 1966). Balbus and Hawley (1991) provide the following formulation: A simple system displaying this mechanism is a gas disc in the presence of a weak axial magnetic field. Two radially neighboring fluid elements will behave as two mass points connected by a massless spring, the spring tension playing the role of the magnetic tension. In a Keplerian disc the inner fluid element would be orbiting more rapidly than the outer, causing the spring to stretch. The inner fluid element is then forced by the spring to slow down, reduce correspondingly its angular momentum causing it to move to a lower orbit; the outer fluid element being pulled forward will speed up, increasing its angular momentum and move to a larger radius orbit. The spring tension will increase as the two fluid elements move further apart and the process runs away.^[5] The magnetorotational instability or MRI (also known as the Velikhov instability or Balbus-Hawley instability in the literature) is a fluid instability that arises when the angular velocity of a magnetized fluid decreases as the distance from the rotation center increases. ... Hannes Olof Gösta Alfvén (May 30, 1908; Norrköping, Sweden - April 2, 1995; Djursholm, Sweden) was a Swedish electrical power engineer. ... Sir Frederick Hoyle, FRS, (born on June 24, 1915 in Gilstead, Yorkshire, England â€“ August 20, 2001 in Bournemouth, England)[1] was a British astronomer, he was educated at Bingley Grammar School and notable for a number of his theories that run counter to current astronomical opinion, and a writer of... This article is about the Solar System. ... Iosif Samuilovich Shklovsky (Ио́сиф Самуи́лович Шкло́вский) (July 1, 1916 – March 3, 1985) was a Russian astronomer and astrophysicist. ...

It can be shown that in the presence of such a spring-like tension the Rayleigh stability criterion is replaced by

$frac{partialOmega^2}{partial R}>0$ .

Most astrophysical discs do not meet this criterion and are therefore prone to the magnetorotational instability. The magnetic fields present in astrophysical objects (required for the instability to occur) are believed to be generated via dynamo action.^[6] The Dynamo theory proposes a mechanism by which a celestial body such as the Earth generates a magnetic field. ...

Unfortunately, since the MRI is global in character it makes analytic models of accretion discs difficult to obtain. Instead, people now concentrate on numerical magnetohydrodynamic simulations to discover the workings of these astrophysical objects.^{[citation needed]}

Unsolved problems in physics: Accretion disc QPO's: Quasi-Periodic Oscillations happen in many accretion discs, with their periods appearing to scale as the inverse of the mass of the central object. Why do these oscillations exist? Why are there sometimes overtones, and why do these appear at different frequency ratios in different objects?

Image File history File links No higher resolution available. ... This is a list of some of the unsolved problems in physics. ...

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. An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ... The image above shows the optical afterglow of gamma ray burst GRB-990123 taken on January 23, 1999. ...

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. An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... This view, taken with infrared light, is a false-color image of a quasar-starburst tandem with the most luminous starburst ever seen in such a combination. ... For other uses, see Black hole (disambiguation). ... In the NATO phonetic alphabet, X-ray represents the letter X. An X-ray picture (radiograph) taken by Röntgen An X-ray is a form of electromagnetic radiation with a wavelength approximately in the range of 5 pm to 10 nanometers (corresponding to frequencies in the range 30 PHz... For the science fiction film, see Event Horizon (film). ... The deuterium-tritium (D-T) fusion reaction is considered the most promising for producing fusion power. ...

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. A three-dimensional representation of the Roche potential in a binary star with a mass ratio of 2, in the co-rotating frame. ...

Accretion discs surrounding T Tauri stars are called protoplanetary discs because they are 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. Drawing of a T-Tauri star with a circumstellar accretion disk T Tauri stars are a class of variable stars named after their prototype - T Tauri. ... A protoplanetary disc (also protoplanetary disk, proplyd) is an accretion disc surrounding a T Tauri star. ...

External links

References

^ Weizsacker, C. F. (1948), "Die Rotation Kosmischer Gasmassen", Z. Naturforsch. 3a: 524-539
^ ^a ^b Shakura, N. I. & R. A. Sunyaev (1973), "Black Holes in Binary Systems. Observational Appearance", Astronomy and Astrophysics 24: 337-355, <http://adsabs.harvard.edu/abs/1973A&A....24..337S>
^ Lynden-Bell, D. & J. E. Pringle (1974), "The evolution of viscous discs and the origin of the nebular variables", Mon. Not. R. Astr. Soc. 168: 603-637, <http://adsabs.harvard.edu/abs/1974MNRAS.168..603L>
^ Balbus, Steven A. & John F. Hawley (1991), "A powerful local shear instability in weakly magnetized disks. I - Linear analysis", Astrophysical Journal 376: 214-233, <http://adsabs.harvard.edu/abs/1991ApJ...376..214B>
^ Balbus, Steven A. (2003), "Enhanced Angular Momentum Transport in Accretion Disks", Annu. Rev. Astron. Astrophys. 41: 555-597, <http://arxiv.org/abs/astro-ph/0306208>
^ Rüdiger, Günther & Rainer Hollerbach (2004), The Magnetic Universe: Geophysical and Astrophysical Dynamo Theory, Wiley-VCH, ISBN 3-527-40409-0

Frank, Juhan; Andrew King; Derek Raine (2002). Accretion power in astrophysics, Third Edition, Cambridge University Press. ISBN 0-521-62957-8.
Krolik, Julian H. (1999). Active Galactic Nuclei. Princeton University Press. ISBN 0-691-01151-6.

Categories: All articles with unsourced statements | Articles with unsourced statements since June 2007 | Binary stars | Black holes | Unsolved problems

Results from FactBites:

Accretion disc - Wikipedia, the free encyclopedia (1064 words)
	Viscosity within the disc generates heat and saps orbital momentum, causing material in the disc to spiral inward until it impacts in an accretion shock on the central body if the body is a star, or slips toward the event horizon if the central body is a fl hole.
	Accretion discs are a ubiquitous phenomenon in astrophysics; active galactic nuclei, protoplanetary discs, and gamma ray bursts are only a few phenomena in which they are thought to occur.
	The most spectacular accretion discs found in nature are those of active galactic nuclei and quasars, which are believed to be massive fl holes at the center of galaxies.

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Accretion Disc Physics var ACE_AR = {Site: '756743', Size: '300250'};

α-Disc Model