Accretion disk

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

Unsolved problem 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

(more unsolved problems in physics)

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.

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,

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

where $\Omega$ 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.

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 $\alpha$ describing the effective increase of viscosity due to turbulent eddies within the disc. 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.

$\alpha$-Disc Model

Shakura and Sunyaev (1973) 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 $\nu =\alpha c_{\rm {s}}H$ where $c_{\rm {s}}$ is the sound speed, $H$ is the disc height, and $\alpha$ is a free parameter between zero (no accretion) and approximately one.

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 $\alpha$ parameter. Many of the observables depend only weakly on $\alpha$, so this theory is predictive even though it has a free parameter.

Using Kramers' law for the opacity it is found that

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

$T_{c}=1.4\times 10^{4}\alpha ^{-1/5}{\dot {M}}_{16}^{3/10}m_{1}^{1/4}R_{10}^{-3/4}f^{6/5}{\rm {K}}$

$\rho =3.1\times 10^{-8}\alpha ^{-7/10}{\dot {M}}_{16}^{11/20}m_{1}^{5/8}R_{10}^{-15/8}f^{11/5}{\rm {g\ cm}}^{-3}$

where $T_{c}$ and $\rho$ 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 $10^{10}{\rm {cm}}$, 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.

Magnetorotational Instability

Main article: Magnetorotational Instability

Balbus and Hawley (1991) proposed a mechanism which involves magnetic fields to generate the angular momentum transport. 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.

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

${\frac {\partial \Omega ^{2}}{\partial R}}>0$.

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.

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.

Unsolved problem 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?

(more 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.

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 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.

See also

• Accretion (science)
• Protoplanetary disc
• Solar Nebula
• Dynamo Theory

External links

• Professor John F. Hawley homepage
• Nonradiative Black Hole Accretion
• Accretion Discs on Scholarpedia
• Magnetic fields snare black holes' food - New Scientist

References

• Frank, Juhan (2002). Accretion power in astrophysics (Third Edition ed.). Cambridge University Press. ISBN 0-521-62957-8. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
• Krolik, Julian H. (1999). Active Galactic Nuclei. Princeton University Press. ISBN 0-691-01151-6.