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Topic: Accretion

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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.
		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_disc (390 words)

	The Accretion Theory (Site not responding. Last check: 2007-10-16)
		In the accretion method of building a pyramid, a solid central core was constructed, then this was expanded outwards on all four sides by the addition of successive accretion layers (or concentric shells) of masonry ranging from five to fifteen feet in thickness.
		A variation of this approach was to form each accretion layer with two kinds of blocks: an outer face formed of well-squared blocks of larger dimensions that served as both a casing for the layer as well as a retaining wall for an inner portion of roughly-shaped blocks.
		The idea that the Egyptians continued building pyramids with accretion layers to the end of Dynasty VI can be traced to the 19th Century Egyptologist, Richard Lepsius, who suggested that it provided a way for a king to add to his tomb over the course of a long reign (Edwards 1993:273).
	www.catchpenny.org /accretion.html (3070 words)

	Black Holes
		As the matter in the accretion disk loses energy and spirals downward into the fl hole it is heated to very high temperatures and emits X-rays.
		X-rays from the accretion disk of the binary knock electrons off atoms in a volume of space that may be light years in diameter.
		Accretion into fl holes is not limited to binary star systems.
	csep10.phys.utk.edu /astr162/lect/blackhole/blackhole.html (1902 words)

	Selected results - ADAF theory (Site not responding. Last check: 2007-10-16)
		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)

	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)

	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 the high-viscosity flows the small-scale convective instability is suppressed, and the accretion flows are accomanied by powerful outflows and/or global meridional circulations of matter depending on both the strength of viscosity and the adiabatic index.
		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.
	www.tac.dk /meetings/discs2000/talks.html (4450 words)

	SUB Göttingen - Dissertationen - Schreiber, Matthias: Disc accretion onto white dwarfs
		Thermal instabilities in the accretion disc associated with the ionisation of hydrogen can lead to a limit-cycle behaviour in which the disc switches quasi-periodically between high and low accretion states.
		The process of disc accretion in non-magnetic CVs is subject to a number of external conditions, namely mass transfer variations of the secondary star, stream overflow and irradiation by the white dwarf.
		The accretion disc is mostly influenced by the white dwarf irradiation.
	webdoc.sub.gwdg.de /diss/2001/schreiber (541 words)

	Basics of accretion (Site not responding. Last check: 2007-10-16)
		The first suggestion that X-ray binaries were powered by accretion onto a compact object was made by Shklovsky in 1967, five years after the discovery of Sco X-1, and three years after Salpeter's original recognition of the possible importance of accretion as an astrophysical energy source (in quasars).
		Accretion of matter in the strong gravitational field of these objects is in most cases responsible for the efficient production of radiative energy.
		The accretion mechanism of Be stars works more efficiently when the system eccentricity is low as the NS passes closer to the Be star and in a denser region around it.
	www.mporzio.astro.it /~gianluca/phdthesis/node10.html (3135 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)

	Daniel Proga's JILA Home Page
		We consider inviscid accretion flows with a spherically symmetric density distribution at the outer boundary, but with spherical symmetry broken by the introduction of a small, latitude-dependent angular momentum.
		Our main result is that the properties of the accretion flow do not depend as much on the outer boundary conditions (i.e., the amount as well as distribution of the angular momentum) as on the geometry of the non-accreting matter.
		Consequently, the geometry of the polar region, where material is accreted (the funnel), and the mass accretion rate through it are constrained by the size and shape of the torus.
	rocinante.colorado.edu /~proga (2252 words)

	Accreting Binaries
		Because angular momentum must be conserved, accretion from one star in a binary to the other often involves the accreting matter going into orbit around the other.
		As the matter in the accretion disk undergoes collisions and interactions it is heated and loses energy by radiating light, with the wavelength depending on the temperature of the disk.
		Accretion in binary systems can also take the form of a wind from the surface of one star, as opposed to a thin accretion stream flowing through the inner Lagrange point.
	csep10.phys.utk.edu /astr162/lect/binaries/accreting.html (651 words)

	Accretion biography .ms (Site not responding. Last check: 2007-10-16)
		In astrophysics, accretion is the gravitational process by which bodies such as planets and stars form from gas and dust.
		In atmospheric science, accretion is the growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact.
		In finance, accretion is the change in the price of a bond bought at a discount between the original price and the par value of the bond.
	accretion.biography.ms (84 words)

	The Role of Accretion
		The sub-Eddington (and possibly, advective) nature of the accretion will ensure that such sources are of relatively low luminosity, and of low radio power, and such objects might be identifiable with the FR I radio sources as is indeed postulated in the unified models of radio sources (Urry and Padovani 1995).
		During the accretion process, the molecular clouds are presumably being converted rapidly to stars in a massive nuclear starburst, so that the growth of the BH is determined by the competition between inflow, accretion, and star formation.
		This accretion timescale is related to the free-fall timescale from the point of origin of the infalling gas to the mass centre by a scaling factor:
	www.atnf.csiro.au /pasa/14_3/dopita/paper/node2.html (1838 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)

	1997 NSIF Proceedings
		Protein accretion curves for four genotypes of terminal cross barrows representing high (A), medium-high (C and D) and medium (B) lean growth genotypes are presented in figure 1.
		This causes a rapid increase in lipid accretion and the ratio of lipid to protein accretion (Figure 4).
		Protein accretion curves can be used to estimate the magnitude in which environmental conditions (health status, facilities, stressors) limit the expression of the pigs genetic potential for lean growth.
	mark.asci.ncsu.edu /nsif/97proc/schinckel.htm (2167 words)

	Jose Gracia @ Accretion Disks
		The gravitational binding energy liberated during the accretion process is believed to be the driving force behind a vast class of objects containing fl holes as active galactic nuclei (AGN), galactic micro-Quasars and high-mass X-Ray binaries (HMXB).
		One short-coming of all previous studies on the dynamical evolution of bimodal accretion flows is the improper treatment of the thermal coupling between electrons and ions.
		To clarify this fundamental issue of accretion disk theory it, this essential to gain better understanding of the development and evolution of the magnetised turbulent flow near the fl hole.
	www.phys.uoa.gr /~jgracia/accretion.html (1464 words)

	accretion and Stock Trading at TradeStars + Stock Trading (Site not responding. Last check: 2007-10-16)
		accretion is extremely risky and can result in substantial financial losses in a U.S. Senate accretion subcommittee, former SEC Chairman Arthur Levitt testified that, I am concerned that some stock traders dont fully understand the accretion level of risk they are assuming.
		I am concerned that some people may be lured North American accretion Securities Administrators Association, Inc. issued a report that describes the major problems and abuses in the accretion industry and summarizes the legal actions brought to date.
		The NASDs release of July 29, 1999, describes a accretion NASD rule proposal that was approved by the SEC on July 10, 2000.
	www.tradestars.com /content/accretion.asp (267 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)

	The mass of accretion disks
		Quasars and active galactic nuclei (AGN) are likely powered by matter accretion onto a super-massive fl hole located at their center.
		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.
		When this mass exceeds a fraction of the central mass (about 10% typically), then the departure to the keplerian rotation law is significant: the centrifugal force is no more compensated by the central attraction only but by the combined gravitational attraction of the fl hole and of the disc.
	www.obspm.fr /actual/nouvelle/dec02/ngc1068.en.shtml (1074 words)

	Amazon.com: Accretion Power in Astrophysics (Cambridge Astrophysics): Books: Juhan Frank,Andrew R. King,Derek J. ... (Site not responding. Last check: 2007-10-16)
		Accretion is recognised as a phenomenon of fundamental importance in astrophysics.
		Accretion Power in Astrophysics examines accretion as a source of energy in binary star systems containing compact objects and in active galactic nuclei.
		Accretion Power in Astrophysics examines accretion as a source of energy in both binary star systems containing compact objects, and in active galactic nuclei.
	www.amazon.com /exec/obidos/tg/detail/-/0521408636?v=glance (628 words)

	Home Page (Site not responding. Last check: 2007-10-16)
		However, if the accretion disk has closely packed ions on its outer surface as suggested above, then light from the sides of the near jet and the hyperboloid ("torus") which beams back toward the disk should bounce back off as if the disk were a huge mirror because of the closely packed ions.
		The narrow absorption lines, I suspect, are created from this and maybe by the light of the inner opaque part of the accretion disc passing through hot gases in the plume at the end of the near jet.
		When the central accretion disc is visible it should appear larger than it is for the same reason as mentioned for the jet.
	www.geocities.com /isoptera.geo (5785 words)

	Selected results -3D MHD Simulations of Disk Accretion to an Inclined Rotator
		Thus, in addition to the main period associated with the rotation of a hot spot, an additional, shorter scale period may be observed.
		If accretion is not stationary, then intervals of quasi-variability may be observed connected with temporary precession of the streams.
		The accretion rate associated with pure hydrodynamic accretion is about 10 times smaller.
	www.astro.cornell.edu /us-rus/inclined.htm (1029 words)

	Cataclysmic Variables
		X-rays in these CVs are generally believed to come from where the accretion disk hits the white dwarf surface (known as the boundary layer), the details of which are currently a topic of active research.
		Thus the geometry of accretion is very different in these magnetic CVs; accretion disks are truncated or absent, "column" and "curtain" are two of the words used to describe the geometry near the surface.
		In these cases, accretion is closer to vertical, along the magnetic field lines, which results in a stronger shock and stronger X-ray emission than when the accretion is via a disk.
	imagine.gsfc.nasa.gov /docs/science/know_l2/cataclysmic_variables.html (930 words)

	accretion (Site not responding. Last check: 2007-10-16)
		Something contributing to such growth or increase: “the accretions of paint that had buried the door's details like snow” (Christopher Andreae).
		Accretion can occur through a company's internal development or by way to mergers and acquisitions.
		In the duration between the bond's issuance and maturity, no additional value is actually being accumulated within the bond but accretion occurs with the paper or implied capital gain.
	www.yourinsuranceleads.com /insurance-glossary/accretion.htm (177 words)

	Accretion Disks
		Accretion disks arise when material (usually gas) is being transferred from one celestial object to another.
		Two major places where astronomers see accretion disks are in binary star systems (two stars orbiting each other) and active galactic nuclei.
		I will discuss an accretion disk in a binary star system, but the basic ideas are the same for all cases.
	imagine.gsfc.nasa.gov /docs/ask_astro/answers/001106a.html (345 words)

	[No title] (Site not responding. Last check: 2007-10-16)
		Accretion is Mother Nature's little gift to a landowner.
		Example: if a brother and sister were supposed to divide a share of Dad's estate, but brother doesn't want it, then sister's share grows by accretion.
		3) in trusts, accretion occurs when a beneficiary gets a surprising increase in benefits due to an unexpected event.
	dictionary.law.com /definition2.asp?selected=2307&bold=\|\|\|\| (112 words)

	SPACE.com -- Planet Puzzle: Theorists Wrestle with How They're Built
		Core accretion requires bits of solid stuff to smash together to form bigger and bigger chunks.
		Accretion models require a core of at least 10 Earth masses, Boss said.
		Another problem is that core accretion requires several million years to make a gas giant, whereas gas disks around stars like the Sun — which feed gas giant growth — typically do not last that long.
	www.space.com /scienceastronomy/mystery_monday_050307.html (1389 words)

	1-ACCREATION Theory
		enlarged by gravitational accretion of extraterrestrial matter until it reached spherical shape, at which point gravity could omni-directionally focus total weight of the protoplanet on its exact center.
		This new H2O increases the total volume in the oceans, and the gases are either combined in solution with seawater or are released into the atmosphere, but mineral deposits deposited on the ocean bottom are subsequently covered by accretion of sediments consisting of dust (terrestrial and meteoric) and organic detritus from marine fauna and flora.
		accretion of extra-terrestrial dust and meteorites, and internal expansion of the molten core
	www.expanding-earth.org /page_1.htm (1121 words)

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