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Accretion disc - Wikipedia, the free encyclopedia 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. en.wikipedia.org /wiki/Accretion_disc   (1064 words)

Accretion Disc Turbulence Accretion discs occur in a number of astrophysical settings, and over a huge range of scales (geometrical as well as in terms of energies involved). A crucial property of accretion discs is the rate of frictional (turbulent) dissipation in the disc. The "friction" allows angular momentum to be transported outwards in the disc, and allows mass to be transported inwards (eventually accreting onto the fl hole or onto the stellar surface, or else becoming ejected again in the jets). www.fys.ku.dk /nbiweb/side24706.htm   (330 words)

Abstracts Warps are seemingly common in accretion discs, and have been utilized to explain phenomena as diverse as the 35-day cycle in Her X-1 and the water masers in active galactic nuclei, such as NGC 4258. In contrast to the standard accretion disc, here viscous stresses are negligible: the disc accretes fast owing to its relatively low angular momentum and also owing to the radial pushing inwards by the infalling matter. The jet is collimated and confined by the azimuthal magnetic field that is produced by the dynamo in the disc and advected to the disc corona. www.tac.dk /meetings/discs2000/talks.html   (4450 words)

Introduction The typical temperatures of the first, so-called standard accretion disc model are hot enough (see Frank, King and Raine 1985 [6] for equations describing the structure of the standard disc) to ensure a very high degree of ionisation. This notwithstanding, nonviscous discs, with dynamics dominated by angular momentum removal via centrifugally driven winds, are conceivable, and models of such discs have been constructed (Königl 1989 [8], Lovelace, Romanova and Newman 1994 [11], Li 1995 [10]). The space is divided into two regions, the inside of the disc and the outside of the disc, by the disc's surface defined to coincide with its halfthickness. www.lpi.usra.edu /science/tom/mn/node1.html   (1225 words)

Diploma projects The accreted material forms an accretion disc around the white dwarf, however the accretion disc is unable to extend all the way down to the surface of the white dwarf, because the magnetic field of the white dwarf is strong enough to truncate the disc above the surface of the white dwarf. The accretion through the disc is driven by magnetic stresses, or equivalently currents, in the disc, which means that the current distribution can be calculated from the accretion flow. In the case of an AM Her the magnetic field is so strong that an accretion disc is never formed, and rather the accretion stream from the secondary is captured in the magnetosphere of the white dwarf before it can form an accretion disc. fy.chalmers.se /~torkel/Teaching/diploma.html   (1556 words)

SPH modelling of a counter rotating disc A timescale of years corresponds well with the timescale on which the accretion behaviour in GX 1+4 is observed to change, with a negative correlation between accretion rate and spin-up in some phases while the disc would be retrograde, and a positive one at other times when it is prograde (Chakrabarty et al. Thus, this system is ideally suited to study the possibility of forming retrograde discs, since the timescale for disc reversal would be much longer than that of the torque fluctuations on a timescale of one day or less that are common in all types of X-ray pulsars. We found that in the inner disc, once the sense of rotation of inflowing material was reversed (figure 3), the existing disc was rapidly driven inside the circularisation radius of the new counter-rotating matter. www.atnf.csiro.au /pasa/16_3/greenhill/paper/node4.html   (1002 words)

The mass of accretion disks A point is that, the accretion disc is made of a certain amount of gas and dust, and thus it inevitably generates a certain gravity field. In particular, the disc would have a mass close to that of the fl hole (approximately 9 million solar masses) and would be in a marginally stable state with respect to self-gravity. The contribution of the disc is in plain line, that of the fl hole is in dotted lines and the total curve is in red bold. www.obspm.fr /actual/nouvelle/dec02/ngc1068.en.shtml   (1074 words)

IAAT (Astronomy): Stellar Atmospheres Group - Accretion Discs Accretion discs are components of objects as diverse as protoplanetary systems, active galactic nuclei, cataclysmic variables or X-ray binaries. Accretion disc spectra are obtained at ground or space based observatories described in the next section, the calculation of synthetic spectra are shortly described here. The disc spectrum is calculated for different inclination angles, and the spectral lines are Doppler broadened according to the radial component of the Kepler rotation. astro.uni-tuebingen.de /groups/stellar/disc.shtml   (1075 words)

Introduction to CVs This is because the gas in one half of the disc is moving towards you, and its emission is thus blue-shifted, while the gas in the other half is moving away from you, and its emission is red-shifted. Because the accretion occurs almost perpendicularly to the surface of the white dwarf, copious X-ray and EUVB emission is liberated at the poles. The accretion process in the intermediate polars is through a disc with a disrupted inner radius (where the magnetic field is powerful enough to influence the flow of the gas), or an accretion stream as in the polars, or either, or both (just to prevent you from falling asleep). home.mindspring.com /~mikesimonsen/cvnet/id1.html   (1414 words)

Shadow of a large disc casts new light on the formation of high mass stars The disc has a diameter of about 20,000 AU [3] - which is 500 times the distance of the farthest planet in our solar system - and is by far the largest circumstellar disc ever detected. The astronomers have observed the region in the rotational transitions of the 12CO, 13CO and C18O molecules, and in the adjacent continuum at 3 mm. Given the fact that the accretion process is still active, and that models predict that about 30-50% of the circumstellar material can be accumulated onto the central object, it is likely that in the present case a massive protostar is currently being born. www.eurekalert.org /pub_releases/2004-05/eso-soa051104.php   (1816 words)

The structure equations of MHD accretion discs The fundamental physical idea is that an accretion disc corona is formed in a similar fashion to the solar corona: by buoyant flux of magnetic field and subsequent magnetic heating. The magnetic field is generated in the accretion disc by dynamo action, at a rate of the same order as the gravitational power. In analogy with the Shakura-Sunyaev disc theory, we formulate vertically averaged thin disc equations by integrating the MHD equations between z = h and z = -h, where in our case h is the height of the disc-corona boundary. www.atnf.csiro.au /pasa/16_3/dekool/paper/node1.html   (1677 words)

X-ray Branch: binaries research Accretion results from the transfer of angular momentum outwards by an unknown viscous action between shearing material. However a broad range of physical conditions occur within the atmosphere of any accretion disc where the cool outer disc is a very different environment to the hot inner region. Astrophysical discs cannot be resolved in general because of their size and distance, and therefore our problem is that classical observations provide only a global picture of accretion disc behaviour. universe.gsfc.nasa.gov /xrays/research/disk_atmos.html   (332 words)

Research program We will acquire spectroscopic data of accretion discs with high time- and velocity-resolution over several energy bands, employing HST to detect the inner disc regions in the UV, and ground-based optical and IR instruments to detect the outer disc; e.g. Indirect accretion disc imaging: The accretion disc targets are too small to resolve directly, therefore we intend to use maximum entropy tomography techniques to spatially resolve disc spectra. Disc atmosphere models: The discipline of disc atmosphere building has now reached a sophistication with which the physical state of discs can be realistically modelled. lheawww.gsfc.nasa.gov /~still/research/node10.html   (463 words)

A Model of AC211   (Site not responding. Last check: 2007-11-04) The accretion disc has considerable vertical structure, probably in the form of a rim aropund the outer edge. The X-rays we detect are therefore from an 'accretion disc corona' which surrounds the compact object, but which is sufficiently large to be visible from Earth despite the disc structure. The relative sizes of the disc structure, accretion disc and accretion disc corona are uncertain, but the figure illustrates how the blueshifted (left-hand) side of the corona is obscured more than the red. www.ing.iac.es:8080 /PR/SH/SH8687/model.html   (154 words)

Research   (Site not responding. Last check: 2007-11-04) Accretion discs can be found in a wide variety of enviroments, from the small discs around compact stars and protostars, to the massive discs around fl holes in the center of galaxies. These outburst are though to occur when an instability is triggered within the accretion disc, and the disc makes a global transition from a relatively cool, low mass accretion rate state to a much hotter high mass accretion rate state. A recent result from Doppler imaging of CVs is the detection of a two armed spiral pattern in the accretion disc of the binary IP Pegasi. www.astro.soton.ac.uk /~ds/research.html   (841 words)

AAVSO: IP Pegasi, December 2001 Variable Star Of The Month Currently, the accepted "disc instability" theory for dwarf nova outbursts (Osaki 1974) states that outbursts occur when, in normal (quiescent state) accretion, the surface density and temperature of the disc reach an unstable state; the temperature and radiation level then rise quickly and material stored in the disc is dumped onto the primary star. In accretion discs, shocks have long been expected for this reason, and they have appeared in computer simulations which model discs using the equations of gas hydrodynamics. The disc in a tomogram is still circular, and centered on the white dwarf's velocity, but is 'inside-out', the low-velocity (outer) regions being nearer the origin. www.aavso.org /vstar/vsotm/1201.stm   (3054 words)

Slide 1 BL Lacs are without significant accretion disc emission, leaving the host galaxy to dominate the optical emission. There are two important elements to the structure of a radio quasar: the accretion disc around the fl hole, and the relativistic jet responsible for the radio emission. Rather, the emission from the accretion disc and broad-line region is intrinsically weak, and so cannot compete with the jet emission. www.phys.unsw.edu.au /~mwhiting/papers/IAUGA_jet_vs_disc_files/slide0001.htm   (1065 words)

Thesis   (Site not responding. Last check: 2007-11-04) The distribution of line emission across the accretion disc is resolved using Doppler tomography and reveals a two armed spiral pattern in a variety of emission lines. The disc simulations also indicate that similar disc structure is expected in other systems carrying large discs such as dwarf novae in outburst and nova-like variables. For small, cool accretion discs, tidally driven spiral waves can be present but are too tightly wrapped to be resolved in the strong emission lines. www.astro.soton.ac.uk /~ds/thesis.html   (398 words)

Accretion discs The matter in an accretion disc is in general on a Keplerian orbit around the accreting object, that is it is held in place by the balance between gravity and the centrifugal force. In the process the accretion disc is heated by the released potential energy, which it radiates away. The turbulence in its turn is heating the accretion disc, but it is also re-generating the magnetic field, which was needed to derive the energy in the first place. fy.chalmers.se /~torkel/Pop_sci/disc.html   (719 words)

NSC Project 2003024 The most natural candidate is the differential rotation of the accretion disc, which can only be understood within a magnetohydrodynamic framework, since the Keplerian rotation in a geometrically thin accretion disc is hydrodynamically stable but magnetohydrodynamically unstable. There is now working going on in refining the simulations such that they allow for a more accurate modelling of the accretion disc and in particular its vertical structure. A related problem that we are working on is to understand the interaction between an accretion disc and a magnetized star, such as the neutron star in an X-ray pulsar or a T Tauri-star, a young solar-like star that has not yet lost its protoplanetary disc. www.nsc.liu.se /projects/previousprojects/project_2003024.html   (511 words)

Animations   (Site not responding. Last check: 2007-11-04) Evolution of a protoplanetary accretion disc in which the cooling time is sufficiently small for fragmentation to occur. The evolution of a protoplanetary disc with a mass equal to that of the central star. The evolution of a self-gravitating disc undergoing a co-planar encounter with an object with a mass equal to that of the disc and that is in a parabolic orbit. faculty.ucr.edu /~krice/animations.html   (176 words)

Black Holes: In the thin disc approximation pressure gradients in the radial direction are neglected, although the structure in the direction perpendicular to the disc is maintained by internal pressure gradients. The current interest in the theory of the structure, evolution and stability of thick accretion discs is due to the possibility that thick discs may be relevant to the understanding of the central power sources in radio galaxies and quasars. A thick disc has funnels along its rotation axis, and these may be related to the collimination of beams of particles and relativistic gases observed to be ejected at very high velocities from active galactic nuclei. astroa.physics.metu.edu.tr /~burcin/proje400/project.html   (5373 words)

Accretion during Binary Star Formation II. Gaseous Accretion and Disc Formation   (Site not responding. Last check: 2007-11-04) When a `seed' protobinary system forms within a collapsing molecular cloud core, its final state is determined by the accretion of, or interaction with, the remaining cloud material as it falls on to the system. The binary's mass ratio and orbit and the formation of circumstellar and circumbinary discs are all dependent on the dynamics of this accretion process. In this paper, we study the effects of accretion on the binary's mass ratio and separation. www.astro.ex.ac.uk /people/mbate/Preprints/gaseous/gaseous.html   (191 words)

ESA - Science - Science News Release This artist's impression shows the thick dust torus that astronomers believe surrounds supermassive fl holes and their accretion discs, like the one harboured in the nucleus of the spiral galaxy NGC 4388. Galaxies in which a torus blocks the light from the central accretion disc are called `Seyfert 2’ types and are usually faint to optical telescopes. Gas in the accretion disc close to the fl hole reaches high speeds and temperatures (over 100 million degrees, hotter than the Sun) as it races toward the void. www.esa.int /esaSC/Pr_16_2004_s_en.html   (1145 words)

Be stars   (Site not responding. Last check: 2007-11-04) Due to the gravitational pull of the white dwarf, the companion star is distorted and material from its outer layers escapes, forming an accretion disc around the white dwarf. When the accretion stream hits the accretion disc, a ``hot spot'' is produced. The material in the disc slowly spirals down towards the neutron star, gradually losing angular momentum through viscous interactions (a general term used in Astrophysics to describe all the not-very-well understood forces that a very hot plasma can experience in the presence of a strong magnetic field). xmm.u-strasbg.fr /staff/ignacio/novae.html   (452 words)

[No title] The response of the disc is expected to have the structure of these optimal perturbations at the time of their peak growth, which is unrelated to the structure of the least damped mode. However, the excitation of discs is not expected to be solely that in the form of an optimal perturbation. Consequently when the disc is excited by internally generated or externally imposed noise it will support high perturbation variance that will be organized by the governing non-normal operator into the shape of the evolving optimal perturbations of the flow. www.cc.uoa.gr /~akakour/papers/samos.doc   (1981 words)

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