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| [[File:NGC 4261 Black hole.jpg|thumbnail|Image taken by Hubble space telescope of what may be gas accreting onto a black hole in elliptical galaxy NGC 4261]]
| | Hello. Let me introduce the writer. Her title is Emilia Shroyer but it's not the most feminine name out there. Doing ceramics is what adore performing. Puerto Rico is exactly where he and his spouse live. Hiring is her day job now but she's usually wanted her own company.<br><br>my page; [http://tomjones1190.Livejournal.com/ http://tomjones1190.Livejournal.com] |
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| {{unsolved|physics|Accretion disc jets: Why do the discs surrounding certain objects, such as the nuclei of active [[Galaxy|galaxies]], emit [[Relativistic jet|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 [[active galactic nucleus|active galactic nuclei]]), but their origin is still not well understood.}}
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| An '''accretion disc''' is a structure (often a [[circumstellar disk]]) formed by diffuse material in [[orbit]]al motion around a massive central body. The central body is typically a [[star]]. [[Gravity]] causes material in the disc to spiral inward towards the central body. Gravitational and frictional forces compress and raise the temperature of the material causing the emission of [[electromagnetic radiation]]. The frequency range of that radiation depends on the central object's mass. Accretion discs of young stars and [[protostar]]s radiate in the [[infrared]]; those around [[neutron star]]s and [[black hole]]s in the [[X-ray]] part of the [[Electromagnetic spectrum|spectrum]]. The study of oscillation modes in accretion discs is referred to as '''diskoseismology'''.<ref>{{cite journal | doi = 10.1086/170465 | last = Nowak | first = Michael A. | coauthors = Wagoner, Robert V. | title = Diskoseismology: Probing accretion disks. I - Trapped adiabatic oscillations | journal = Astrophysical Journal | year = 1991 | volume = 378 | pages = 656–664 | bibcode = 1991ApJ...378..656N}}</ref><ref>{{cite journal | doi = 10.1016/j.newar.2008.03.012 | last = Wagoner | first = Robert V. | title = Relativistic and Newtonian diskoseismology | journal = New Astronomy Reviews | year = 2008 | volume = 51 | issue = 10-12 | pages = 828–834 | bibcode = 2008NewAR..51..828W}}</ref>
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| == Manifestations ==
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| Accretion discs are a ubiquitous phenomenon in astrophysics; [[active galactic nuclei]], [[protoplanetary disc]]s, and [[gamma ray burst]]s 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]].
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| 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 follows the [[tendex line]] into a black hole, the intense [[gravitational gradient]] gives rise to intense [[friction]]al heating; the accretion disc of a black hole is hot enough to emit [[X-ray]]s 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.
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| In close [[binary system]]s 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.
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| Accretion discs surrounding [[T Tauri star]]s or [[Herbig Ae/Be stars|Herbig star]]s are called [[protoplanetary disc]]s because they are thought to be the progenitors of [[planetary system]]s. The accreted gas in this case comes from the [[molecular cloud]] out of which the star has formed rather than a companion star.
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| {{multiple image
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| | header = animations of black hole accretion
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| | align = left
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| | header_align = left/right/center
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| | direction = horizontal
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| | image1 = NASA-led Study Explains How Black Holes Shine in Hard X-rays.ogg
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| | width1 = 300
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| | alt1 =
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| | caption1 = This animation of supercomputer data takes you to the inner zone of the accretion disk of a stellar-mass black hole.
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| | image2 = Outflow from active galaxy NGC 3783 (artist’s impression).ogg
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| | width2 = 300
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| | alt2 =
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| | caption2 = This video shows an artist’s impression of the dusty wind emanating from the black hole at the centre of galaxy NGC 3783.
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| }}
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| <br clear="all"/>
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| == Accretion disc physics ==
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| [[Image:Accretion disk.jpg|thumb|250px|Artist's conception of a black hole drawing matter from a nearby star, forming an accretion disc.]]
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| In the 1940s, models were first derived from basic physical principles.<ref name="W1948">{{Citation | last=Weizsäcker | first=C. F. | year=1948 | title=Die Rotation Kosmischer Gasmassen | periodical=Z. Naturforsch. | volume=3a | issue= | pages=524–539 | url=
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| |bibcode = 1948ZNatA...3..524W }}</ref> 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 [[Magnetorotational instability|Rayleigh stability criterion]],
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| :<math>\frac{\partial(R^2\Omega)}{\partial R}>0,</math>
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| where <math>\Omega</math> represents the [[angular velocity]] of a fluid element and <math>R</math> its distance to the rotation center,
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| an accretion disc is expected to be a [[laminar flow]]. This prevents the existence of a [[Fluid dynamics|hydrodynamic]] mechanism for angular momentum transport.
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| On one hand, it was clear that viscous stresses would eventually cause the matter towards the center to heat up and radiate away some of its 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 for such angular-momentum redistribution, although the origin of the turbulence itself was not well understood. The conventional [[Phenomenology (science)|phenomenological approach]] introduces an adjustable parameter <math>\alpha</math> describing the effective increase of viscosity due to turbulent [[Eddy (fluid dynamics)|eddies]] within the disc.<ref name="SS1973">{{Citation | last1=Shakura | first1=N. I. | last2=Sunyaev | first2=R. A. | year=1973 | title=Black Holes in Binary Systems. Observational Appearance | periodical=Astronomy and Astrophysics | volume=24 | issue= | pages=337–355 | bibcode=1973A&A....24..337S
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| }}</ref><ref name="LBP1974">{{Citation | last1=Lynden-Bell | first1=D. | last2=Pringle | first2=J. E.| year=1974 | title=The evolution of viscous discs and the origin of the nebular variables | periodical=Mon. Not. R. Astr. Soc. | volume=168 | issue= | pages=603–637 | bibcode=1974MNRAS.168..603L
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| }}</ref> 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 would be highly unstable, providing a direct mechanism for angular-momentum redistribution.<ref name="BH1991">{{Citation
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| | last1=Balbus | first1=Steven A. | last2=Hawley | first2=John F. | year=1991 | title=A powerful local shear instability in weakly magnetized disks. I – Linear analysis | periodical=Astrophysical Journal | volume=376 | issue= | pages=214–233 | bibcode=1991ApJ...376..214B
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| | doi=10.1086/170270
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| }}</ref>
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| === α-Disc Model ===
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| [[Nikolai Shakura|Shakura]] and [[Rashid Sunyaev|Sunyaev]] (1973)<ref name="SS1973"/> 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 <math> \nu=\alpha c_{\rm s}H</math>
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| where <math>c_{\rm s}</math> is the [[sound speed]], <math>H</math> is the disc height, and <math>\alpha</math> is a free parameter between zero (no accretion) and approximately one. Note that in turbulent motion <math> \nu\approx v_{\rm turb} l_{\rm turb} </math>, where <math> v_{\rm turb} </math> is the velocity of turbulent cells relative to the mean gas motion, and <math> l_{\rm turb} </math> is the size of the largest turbulent cells, which is estimated as <math>l_{\rm turb} \approx H = c_{\rm s}/\Omega</math> and <math> v_{\rm turb} \approx c_{\rm s} </math>, where <math>\Omega = (G M)^{1/2} r^{-3/2}</math> is the Keplerian orbital angular velocity, <math>r</math> is the radial distance from the central object of mass <math>M</math>.<ref>{{Citation | last=Landau and Lishitz | year=1959 | title=Fluid Mechanics | edition=31}}</ref>
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| 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 <math>\alpha</math> parameter. Many of the observables depend only weakly on <math>\alpha</math>, so this theory is predictive even though it has a free parameter.
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| Using Kramers' law for the opacity it is found that
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| :<math>H=1.7\times 10^8\alpha^{-1/10}\dot{M}^{3/20}_{16} m_1^{-3/8} R^{9/8}_{10}f^{3/5} {\rm cm}</math>
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| <br>
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| :<math>T_c=1.4\times 10^4 \alpha^{-1/5}\dot{M}^{3/10}_{16} m_1^{1/4} R^{-3/4}_{10}f^{6/5}{\rm K}</math>
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| <br>
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| :<math>\rho=3.1\times 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}</math>
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| where <math>T_c</math> and <math>\rho</math> are the mid-plane temperature and density respectively.
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| <math>\dot{M}_{16}</math> is the accretion rate, in units of <math>10^{16}{\rm g\ s}^{-1}</math>,
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| <math>m_1</math> is the mass of the central accreting object in units of a solar mass, <math> M_\bigodot</math>, <math>R_{10}</math> is the radius of a point in the disc, in units of <math>10^{10}{\rm cm}</math>, and
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| <math>f=\left[1-\left(\frac{R_\star}{R}\right)^{1/2} \right]^{1/4}</math>, where <math>R_\star</math> is the radius where angular momentum stops being transported inwards.
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| The Shakura-Sunyaev α-Disc model is both thermally and viscously unstable.
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| An alternative model, known as the <math>\beta</math>-disk, which is stable in both sense assumes that the viscosity is proportional to the gas pressure <math>\nu \propto \alpha p_{\mathrm{gas}}</math>.
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| <ref>{{Citation | last=Lightman and Eardley | year=1974 | title=Black Holes in Binary Systems: Instability of Disk Accretion | periodical=The Astrophysical Journal, | first2=Douglas M. | volume=187 | issue= | last2=Eardley| pages=1 | bibcode=1974ApJ...187L...1L | doi=10.1086/181377 | first1=Alan P. }}</ref>
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| <ref>{{Citation | last=Piran | year=1978 | title=The role of viscosity and cooling mechanisms in the stability of accretion disks | periodical=The Astrophysical Journal, | volume=221 | issue= | pages=652 | bibcode=1978ApJ...221..652P | doi=10.1086/156069 | first1=T. }}</ref>
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| Note that in the standard Shakura-Sunyaev model, viscosity is proportional to the total pressure <math> p_{\mathrm{tot}} = p_{\mathrm{rad}} + p_{\mathrm{gas}} = \rho c_{\rm s}^2</math> since
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| <math>\nu = \alpha c_{\rm s} H = \alpha c_s^2/\Omega = \alpha p_{\mathrm{tot}}/(\rho \Omega)</math> .
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| The Shakura-Sunyaev model assumes that the disk is in local thermal equilibrium, and can radiate its heat efficiently. In this case, the disk radiates away the viscous heat, cools, and becomes geometrically thin. However, this assumption may break down. In the radiatively inefficient case, the disk may "puff up" into a [[torus]] or some other three dimensional solution like an Advection Dominated Accretion Flow ([[ADAF]]). The ADAF solutions usually require that the accretion rate is smaller than a few percent of the [[Eddington limit]]. Another extreme is the case of [[Rings of Saturn|Saturn's rings]], where the disk is so gas poor that its angular momentum transport is dominated by solid body collisions and disk-moon gravitational interactions. The model is in agreement with recent astrophysical measurements using [[gravitational lensing]].<ref>{{Citation | last=Poindexter | year=2008 | title=The Spatial Structure of An Accretion Disk | first3=Christopher S. | last3=Kochanek | periodical=The Astrophysical Journal, | first2=Nicholas | volume=673 | issue= 1| last2=Morgan| pages=34 | arxiv=0707.0003 | doi=10.1086/524190 | first1=Shawn |bibcode = 2008ApJ...673...34P | display-authors=1 }}</ref><ref>{{Citation | last=Eigenbrod | year=2008 | title=Microlensing variability in the gravitationally lensed quasar QSO 2237+0305 = the Einstein Cross. II. Energy profile of the accretion disk | periodical= Astronomy & Astrophysics, | volume=490 | issue= 3| pages=933 | arxiv=0810.0011 |doi = 10.1051/0004-6361:200810729 |bibcode = 2008A&A...490..933E | display-authors=1 | first1=A. | last2=Courbin | first2=F. | last3=Meylan | first3=G. | last4=Agol | first4=E. | last5=Anguita | first5=T. | last6=Schmidt | first6=R. W. | last7=Wambsganss | first7=J. }}</ref><ref>{{Citation | last=Mosquera | year=2009 | title=Detection of chromatic microlensing in Q 2237+0305 A | first3=E. | last3=Mediavilla | periodical=The Astrophysical Journal, | first2=J. A. | volume=691 | issue= 2| last2=Muñoz| pages=1292 | arxiv=0810.1626 | doi=10.1088/0004-637X/691/2/1292 | first1=A. M. |bibcode = 2009ApJ...691.1292M | display-authors=1 }}</ref><ref>{{Citation | last1=Floyd | year=2009 | title=The accretion disc in the quasar SDSS J0924+0219 | periodical= ArXiv:0905.2651v1 [astro-ph.HE] | arxiv=0905.2651|bibcode = 2009MNRAS.398..233F |doi = 10.1111/j.1365-2966.2009.15045.x | display-authors=1 | first1=David J. E. | last2=Bate | first2=N. F. | last3=Webster | first3=R. L. | volume=398 | pages=233 }}</ref>
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| === Magnetorotational instability ===
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| {{main|Magnetorotational instability}}
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| [[File:Protoplanetary disk HH-30.jpg|thumb|right|200px|HH-30, a [[Herbig–Haro object]] surrounded by an accretion disc]]
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| Balbus and Hawley (1991)<ref name="BH1991"/> 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.<ref name=""B2003>{{Citation
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| | last=Balbus
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| | first=Steven A.
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| | title=Enhanced Angular Momentum Transport in Accretion Disks
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| | journal=Annu. Rev. Astron. Astrophys.
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| | volume=41
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| | issue=1| year=2003
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| | pages=555–597
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| | arxiv=astro-ph/0306208
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| | doi=10.1146/annurev.astro.41.081401.155207
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| |bibcode = 2003ARA&A..41..555B }}</ref>
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| It can be shown that in the presence of such a spring-like tension the Rayleigh stability criterion is replaced by
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| :<math> \frac{d\Omega^2}{d \ln R}>0.</math>
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| Most astrophysical discs do not 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 theory|dynamo]] action.<ref name="RH2004">
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| {{ Citation
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| | last1=Rüdiger
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| | first1=Günther
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| | last2=Hollerbach
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| | first2=Rainer
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| | title=The Magnetic Universe: Geophysical and Astrophysical Dynamo Theory
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| | publisher=Wiley-VCH
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| | edition=
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| | year=2004
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| | isbn=3-527-40409-0}}</ref>
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| == Analytic models of sub-Eddington accretion discs (thin discs, ADAFs) ==
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| {{unsolved|physics|Accretion disc QPO's: [[Quasi-periodic oscillation|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?}}
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| 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 have thermal electromagnetic spectra, i.e. 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.
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| When the accretion rate is sub-Eddington and the opacity very low, an ADAF is formed. This type of accretion disc was predicted in 1977 by Ichimaru. Although Ichimaru's paper was largely ignored, some elements of the ADAF model were present in the influential 1982 ion-tori paper by Rees, Phinney, Begelman and Blandford.
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| 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.
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| == Analytic models of super-Eddington accretion discs (slim discs, Polish doughnuts) ==
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| The theory of highly super-Eddington black hole accretion, M>>M<sub>Edd</sub>, was developed in the 1980s by Abramowicz, Jaroszynski, [[Bohdan Paczyński|Paczyński]], 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.
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| Slim discs (name coined by Kolakowska) have only moderately super-Eddington accretion rates,
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| M≥M<sub>Edd</sub>, 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.
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| == See also ==
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| * [[Accretion (astrophysics)|Accretion]]
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| * [[Blandford–Znajek process]]
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| * [[Circumstellar disk]]
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| * [[Dynamo Theory]]
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| * [[Gravitational singularity]]
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| * [[Planetary ring]]
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| * [[Polar jet]]
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| * [[Solar Nebula]]
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| * [[Spin-flip]]
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| == References ==
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| {{Reflist}}
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| {{Refbegin}}
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| * {{Citation
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| | last=Frank
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| | first=Juhan
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| | coauthors=Andrew King; Derek Raine
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| | title=Accretion power in astrophysics
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| | publisher=Cambridge University Press
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| | edition=Third
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| | year=2002
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| | isbn=0-521-62957-8}}
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| * {{Citation
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| | last=Krolik
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| | first=Julian H.
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| | title=Active Galactic Nuclei
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| | publisher=Princeton University Press
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| | year=1999
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| | isbn=0-691-01151-6}}
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| {{Refend}}
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| == External links ==
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| {{commonscat|bullet=none|Accretion disks}}
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| * {{britannica|3072}}
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| * [http://www.astro.virginia.edu/~jh8h/ Professor John F. Hawley homepage]
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| * [http://www.astro.virginia.edu/VITA/papers/nraf2/section1.html Nonradiative Black Hole Accretion]
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| * [http://www.scholarpedia.org/article/Accretion_Discs Accretion Discs on Scholarpedia]
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| * [http://www.newscientistspace.com/article.ns?id=mg19025574.600&feedId=online-news_rss20 Magnetic fields snare black holes' food] – New Scientist
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| {{DEFAULTSORT:Accretion Disc}}
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| [[Category:Binary stars|-]]
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| [[Category:Black holes]]
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| [[Category:Unsolved problems in physics]]
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| [[Category:Vortices]]
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