
	Where results make sense

Topic: Accretion disk

Related Topics

accretion disk

accretion disc

Accretion theory

compact disk

disk storage

disk mathematics

disk jockey

Compact Disk single

Redundant array of independent disks

hard disk drive

accretion disk

hard disk drive partitioning

In the News (Sun 23 Nov 08)

Alistair Darling

Aston Villa

Joe Paterno

Ole Miss

Confederations Cup

Kofi Annan

Lee Scott

Annika Sorenstam

Norm Coleman

Benji Madden

	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.
		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.
		The huge luminosity of quasars is believed to be a result of friction caused by gas and dust falling into the accretion discs of supermassive fl holes, which can convert about half of the mass of an object into energy as compared to a few percent for nuclear fusion processes.
	en.wikipedia.org /wiki/Accretion_disk (390 words)

	accretion disk
		In the case of young stars, accretion disks contain unconsolidated material, such as cosmic dust grains, which may subsequently accrete to form planets and other sizable objects (see planetary systems, formation).
		Accretion disks have been observed, or theorized to exist, in association with protostars, T Tauri stars, and other very young stars, Vega-type stars, and many interacting binary systems, including U Geminorum stars, novae, symbiotic stars, and W Serpentis stars, in which the secondary component is losing matter to the primary.
		In the latter case, accretion disks are thought to play a crucial role in the active galactic nucleus phenomenon.
	www.daviddarling.info /encyclopedia/A/accretiond.html (226 words)

	Accretion disc - Wikipedia, the free encyclopedia
		An accretion disc (or accretion disk) is a structure formed by material falling into a gravitational source.
		As matter spirals into a fl hole, the intense gravitational gradient gives rise to intense frictional heating; the accretion disc of a fl hole is hot enough to emit x-rays just outside of the event horizon.
		Protoplanetary discs are referred to as accretion disks when viewed as material falling into the central protostar.
	en.wikipedia.org /wiki/Accretion_disc (390 words)

	accretion disk atmospheres
		Irradiated disks whose structures are governed by the energy from hot central objects absorbed by the disk atmospheres, and 2.
		Disk models recreate these observations easily, but the problem of fitting three-dimensional disks to one-dimensional X-ray and optical light curves is poorly constrained.
		Astrophysical disks 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 disk behaviour.
	lheawww.gsfc.nasa.gov /~still/research/disk.html (1163 words)

	MHD Simulations of Stellar Magnetosphere - Accretion Disk Interaction (Site not responding. Last check: 2007-11-07)
		The magnetohydrodynamical evolution of the interaction region between the inner edge of an accretion disk and the magnetosphere of the central object is studied by means of time-dependent numerical simulations.
		The magnetosphere corotates with the central star and is threaded by one of three initial magnetic field topologies: 1) a dipole field which threads the disk continuously, 2) a dipole field excluded from the disk by surface currents, and 3) a dipole field continuously threading a disk superposed with a uniform axial magnetic field.
		Equatorial accretion results on a dynamical timescale unless the magnetic pressure of the magnetosphere exceeds the ram pressure of the accreting disk plasma; the latter we find to be a highly time dependent quantity.
	www.aas.org /publications/baas/v29n5/aas191/abs/S005007.html (327 words)

	Selected results - ADAF theory (Site not responding. Last check: 2007-11-07)
		Accretion disk theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disk.
		The present status of the solution for a low-luminous optically thin accretion disk model with advection is discussed and the limits for an advection dominated accretion flows (ADAF) imposed by the presence of magnetic field are analysed.
		For small accretion rates where the electron temperature is much less than the ion temperature, we show that the ohmic heating of the electrons gives a radiative efficiency that is reduced by a factor of g from that for a thin disk.
	astrosun.tn.cornell.edu /us-rus/adaf.htm (686 words)

	Relativistic Disk Images (Site not responding. Last check: 2007-11-07)
		Disk parameters are similar to the preceding two images, except with the inner radius at 1.24 R_g.
		Note that the disk is turbulent all the way down to the inner edge, and that the overall flow is approximately circular.
		The intensity of the photons is calculated assuming that the disk is emitting in an optically thick line; Using G. Rybicki's adaptation of photon-escape probability formalism, we can understand the dark stripes as directions where photons from inside the disk are scattered out of the line of sight by surface material.
	www.physics.utah.edu /~bromley/disk_images.html (547 words)

	Stone's Accretion Disk Research
		Accretion flows are ubiquitous in astrophysics: they occur around protostars, accreting compact objects in binary systems, and supermassive fl holes at the cores of galaxies.
		Angular momentum transport in accretion flows is mediated by MHD turbulence driven by the Balbus-Hawley (or magnetorotational) instability (MRI).
		While understanding the properties of hydrodynamical accretion flows is important, it is generally agreed that angular momentum transport is in fact mediated by magnetic stresses.
	www.astro.princeton.edu /~jstone/disks.html (1039 words)

	NASA - Top Story - neutron star explosion reveals inner accretion disk
		They could see details as fine as the neutron star's accretion disk, a ring of gas swirling around and flowing onto the neutron star, as the disk buckled from the explosion and then slowly recovered its original form after approximately 1,000 seconds.
		"Accretion disks are known to flow around many objects in the Universe, from newly forming stars to the giant fl holes in distant quasars.
		The scientists said the bursting neutron stars serve as a laboratory to study accretion disks, which are seen (but in less detail) throughout the Universe around nearby stellar fl holes and exceedingly distant quasar galaxies.
	www.nasa.gov /centers/goddard/news/topstory/2004/0220stardisk.html (1278 words)

	Precession of an Accretion Disk Lesson Plan
		Due to the tilt and precession of its rotational axis, the disk may periodically come between the neutron star and the observer, causing fluctuations in the number of X-ray photons received.
		The orbital plane of the system may be so little inclined to the line of sight that the neutron star, along with its accretion disk may pass behind the companion star, giving rise to dramatic periodic variations in the light curve.
		The total light curve is then modulated by the combined periods of the rotation of the neutron star, the precession of the accretion disk and the occultation of the neuron star by its companion.
	heasarc.gsfc.nasa.gov /docs/xte/learning_center/precess_accretion.html (853 words)

	PhD
		With this instability accretion energy and angular momentum are transported by waves and do not heat up the disk except by the mean of spiral shocks.
		We obtain in the inner region of the disk a cavity similar to the electromagnetic one.
		Throught this mecanism the accretion energy and angular momentum removed from the inner part of the disk are send toward the corona, where it might power a wind or a jet.
	www.pas.rochester.edu /~pvarni/phd.html (790 words)

	Accretion Disk Polar Jetting, Binary Star Formation and Nemesis
		The chance of a collision from an incoming particle is related proportionally with the density and thickness of the accretion disk in its path as well as the speed of transit through the disk.
		Consequently, orbits are longer lived as their planes approach a right angle to that of the accretion disk and/or the eccentricity of the elliptical orbit increases.
		Near the core of the forming star in the accretion disk an eye exists where dropping particles are sufficient in number to hamper close orbits.
	www.aplg.com /nemesis.htm (996 words)

	3. Formation of an Accretion Disk.
		Bonnet-Bidaud and van der Klis (1979) showed that even if the accretion is from the stellar wind, the mass accretion rate will be large enough to require an accretion disk to remove angular momentum from infalling material.
		The existence of an accretion disk is also implied by the presence of 40 mHz quasi-periodic oscillations (QPO).
		These are believed to be due to the interaction between the accretion disk and the magnetosphere.
	lheawww.gsfc.nasa.gov /users/audley/diss/node52.html (250 words)

	Suzan Edwards
		At first the circumstellar disk is in a phase called an accretion disk, where mass is transported inward toward the star and angular momentum is transported outward.
		The accretion phase is intriguing because it is always accompanied by the simultaneous presence of a high velocity ejection of material into collimated, bipolar jets that emerge perpendicular to the plane of the disk.
		This might be on the stellar surface, at the truncation zone in the accretion disk, or further out in the disk beyond the reach of the stellar magnetic field.
	maven.smith.edu /~edwards/edwards.htm (824 words)

	Accretion Disk + Black Hole in the Core of Galaxy NGC 4261 (SEDS HST Archive 46 of 135) (Site not responding. Last check: 2007-11-07)
		HST Image of NGC 4261: A giant disk of cold gas and dust fuels a possible fl hole at the core of the galaxy.
		Estimated to be 300 light-years across, the disk is tipped enough (about 60 degrees) to provide astronomers with a clear view of the bright hub, which presumably harbors the fl hole.
		The dark, dusty disk represents a cold outer region which extends inwards to an ultra-hot accretion disk with a few hundred million miles from the suspected fl hole.
	www.seds.org /hst/ngc4261.html (180 words)

	Help for the Accretion Disk Model Database (Site not responding. Last check: 2007-11-07)
		Inner radius of the disk in stellar radii.
		This value is calculated integrating the Surface density in the disk from the inner radius to the outer radius.
		cosine of the inclination angle of the disk.
	cfa-www.harvard.edu /youngstars/dalessio/help.html (878 words)

	[No title] (Site not responding. Last check: 2007-11-07)
		According to the core accretion model planetesimals build up from collisions of dust particles in the dust subdisk—the layer in the accretion disk to which the dusty condense.
		The disk instability method operates in disks that are typically on the high end of observed disk masses.
		Core accretion would need to speed up by an order of magnitude to operate within the lifetime of the disk, but simulations have shown that disk instability can operate in a marginally unstable disk with a mass near that of the largest observed disks.
	argyre.colorado.edu /life/astrobiology5830-fall-04/proposal_K.doc (3232 words)

	Simulation of an Oscillating Accretion Disk (Site not responding. Last check: 2007-11-07)
		The radial coordinate is shown normalized to units of GM/c^2, where G is the gravitational coupling constant, M is the mass of the fl hole, and c is the speed of light.
		The vertical coordinate is everywhere normalized to one for the unperturbed disk.
		In reality we expect the vertical extent of the accretion disk to be a fraction of the shown radial extent.
	rocinante.colorado.edu /~mnowak/DISKO/movie.html (216 words)

	Radiation from an accretion disk with jets (Site not responding. Last check: 2007-11-07)
		9] have used such a model for an accretion disk with jets starting at the inner region of a disk, and were able to reproduce the UV bump in quasars.
		The disk/jet symbiosis is reflected mainly in a modified photon spectrum from the inner region of the accretion disk where the jet is anchored.
		The important result is that the spectrum from a Kerr accretion disk is cut off at high frequencies, from extreme UV to soft X-rays.
	www.physics.adelaide.edu.au /~adonea/paper1_html/node2.html (1265 words)

	Accretion Disk Physics (Site not responding. Last check: 2007-11-07)
		An accretion disk awakens a `retired' central star (a white dwarf) to new life.
		The disk itself is fed by the companion.
		It is this behavior which makes a dwarf nova disk an ideal object to verify accretion disk theory.
	www.aip.de /groups/turbulence/disk_t.html (159 words)

	Accretion Disks
		Accretion disks arise when material (usually gas) is being transferred from one celestial object to another.
		I will discuss an accretion disk in a binary star system, but the basic ideas are the same for all cases.
		The gas in the disk becomes very hot due to friction and being tugged on by the white dwarf and eventually loses angular momentum and falls onto the white dwarf.
	imagine.gsfc.nasa.gov /docs/ask_astro/answers/001106a.html (345 words)

	NASA's Solar System Exploration: Science & Technology: Ring Around the Black Hole
		These huge accretion disks, often spanning well over a light-year, vary greatly in density and temperature from the outer to inner edges.
		Her team found evidence of X-ray flaring in the accretion disk at distances of 35 and 175 times the fl-hole radius.
		These snaps stress the accretion disk, creating hot spots and ripples that perhaps ultimately churn to tidal waves in the disk, sending matter screaming into the fl hole.
	solarsystem.nasa.gov /scitech/display.cfm?ST_ID=265 (813 words)

	Visualization of the inner accretion disk around a Schwarzschild black hole
		During the sequence, the camera moves from a face-on (i=1 degree) view of the disk to an almost edge-on angle (i=80 degrees).
		The plot in the lower right shows the profile of line emission from the disk, assuming that the line is excited locally by the predicted disk emission.
		We compute the "emission" in the disk rest frame using several slightly different prescriptions, but in all cases the basic assumption is that the local magnetic stress traces regions of high dissipation.
	jilawww.colorado.edu /~pja/black_hole.html (497 words)

	Density and temperature scales (Site not responding. Last check: 2007-11-07)
		The range of densities and temperatures both within disks and from disk to disk is enormous, but it is possible to make some sensible estimates.
		However, the physics of the accretion process becomes complex very near the stellar surface, where such phenomena as standing shock waves are possible (see Shapiro and Teukolsky, 1983), and harder x-rays may originate in such processes.
		It is customary to assume that radiative transport diffuses heat from the disk mid-plane and then to adopt a model for the radiative opacity, with a degree of complication dictated by ones needs.
	astron.berkeley.edu /~jrg/ay202/node117.html (644 words)

	Introduction (Site not responding. Last check: 2007-11-07)
		Jets and accretion disks are strongly related through a symbiosis, and we postulate that there may also be a large scale symbiosis involving dust components.
		The intense emission observed during a flare could be also due to an increase in the accretion rate, in the bulk Lorentz factor of the emitting material in the jet, or in the efficiency of particle acceleration.
		In Section 2 we shall discuss the optical/UV emission from an accretion disk in symbiosis with the jet, and its effect on the BLR emission clouds (ionized by disk photons) and the torus (heating of dust).
	www.physics.adelaide.edu.au /~adonea/paper1_html/node1.html (1095 words)

	Chandra pinpoints edge of accretion disk around black hole
		Recent observations by a team of scientists led by a researcher from the Harvard-Smithsonian Center for Astrophysics show an "accretion disk" around the fl hole, which is matter stripped from a companion star by a fl hole can that forms a flat, pancake-like structure.
		Most astronomers agree that when material is transferred onto the fl hole at a high rate, then the accretion disk will reach to within about 25 miles of the event horizon -- the surface of “no return” for matter or light falling into a fl hole.
		However, scientists disagree on how close the accretion disk comes when the rate of transfer is much less.
	www.researchmatters.harvard.edu /story.php?article_id=256 (267 words)

	APOD: 2002 September 27 - Accretion Disk Simulation (Site not responding. Last check: 2007-11-07)
		Instead, the graphic shows spiral shock waves in a three dimensional simulation of an accretion disk -- material swirling onto a compact central object that could represent a
		For known x-ray binary systems the size of the accretion disk itself might fall somewhere between the diameter of the Sun (about 1,400,000 kilometers) and the diameter of the Moon's orbit (800,000 kilometers).
		disk develops instabilities which tend to smear out the pronounced spiral shocks.
	antwrp.gsfc.nasa.gov /apod/ap020927.html (208 words)

Try your search on: Qwika (all wikis)