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Topic: Accretion (science)

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	Accretion - Wikipedia, the free encyclopedia
		Accretion (geology), an increase of land by the deposit of waterborne sediment
		Accretion (science), increase in size by gradual addition of smaller parts
		Raindrop accretion, in meteorology, is the process by which water vapor in clouds forms water droplets around nucleation sites, then falls to the surface as precipitation.
	en.wikipedia.org /wiki/Accretion (149 words)

	Accretion (science) - Wikipedia, the free encyclopedia
		Accretion is a process in which the size of something gradually increases by steady addition of smaller parts.
		In atmospheric science for growth of a precipitation particle by the collision of an ice crystal or snowflake with a supercooled liquid droplet that freezes upon impact.
		Accretion disks are common around smaller stars or stellar remnants in a close binary, or fl holes in the centers of spiral galaxies.
	en.wikipedia.org /wiki/Accretion_(science) (317 words)

	Full Record & Graph Of A Top 20 Paper in Comets
		The simulations combine three elements: (1) three-body accretion cross sections of solids onto an isolated planetary embryo, (2) a stellar evolution code for the planet's gaseous envelope, and (3) a planetesimal dissolution code within the envelope, used to evaluate the planet's effective capture radius and the energy deposition profile of accreted material.
		It is engendered by a strong positive feedback on the gas accretion rates, driven by the rapid contraction of the gaseous envelope and the rapid expansion of the outer boundary, which depends on the planet's total mass.
		Within the context of the adopted model of planetesimal accretion, the joint constraints of the time scale for dissipation of the solar nebula and the current high-Z masses of the giant planets lead to estimates of the initial surface density (sigma(init)) of planetesimals in the outer region of the solar nebula.
	www.esi-topics.com /comets/interviews/paper2.html (671 words)

	NASA's Solar System Exploration: Science & Technology: Ring Around the Black Hole (Site not responding. Last check: 2007-10-12)
		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)

	Science and Reason
		Accretion disks of matter form around even ordinary stars, and eventually, in many if not most cases, result in the creation of planetary systems like our own.
		Only in the presence of a very intense gravitational field is the matter of the accretion disk heated sufficiently to produce the radiation emitted by quasars or fl holes that are supernova remnants.
		Two of these involve heating of matter in the accretion disk at a considerable distance from the center -- either by intense friction of turbulent motion in the matter or by the radiation coming from the gravitationally compressed matter near the Schwarzschild radius.
	scienceandreason.blogspot.com (9878 words)

	Planet Formation on the Fast Track: Science News Online, Jan. 25, 2003 (Site not responding. Last check: 2007-10-12)
		In the accretion model, these so-called gas giants may take as much as 10 million years to form.
		Even if there are no other hot stars around, recent simulations show that when molecular clouds fragment into individual stars, the gravitational tug of war between neighbors can lop off the gaseous, outer parts of protoplanetary disks in 100,000 years or less.
		In the Nov. 29, 2002 Science, he and his colleagues describe the results of an extensive simulation based on the instability model.
	www.sciencenews.org /20030125/bob9.asp (1747 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)

	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)

	Quiescent Accretion -- 288 (5473): 1929 -- Science (Site not responding. Last check: 2007-10-12)
		The formation of accretion disks around a massive object requires the dissipation of angular momentum of the gas and therefore some process that gives the gas viscosity.
		For close binary systems where the disks are weakly ionized and quiescent, the process of accretion does not follow the standard model of self-sustained magnetohydrodynamical turbulence.
		2022) shows that an anticorrelation between the recurrence of outbursts and the mass ratio of the two objects leads to spiral waves, or to shock waves tidally induced by the companion, that ensures accretion during quiescent periods.
	www.sciencemag.org /cgi/content/summary/288/5473/1929b (197 words)

	The Humming Black Hole
		The disk, called an "accretion disk", becomes so hot and glows so brightly at x-ray wavelengths that it's visible to Earth-orbiting x-ray telescopes from 10,000 or more light years away.
		Gas from an evolved star feeds the accretion disk of an orbiting fl hole or neutron star.
		It might be the inner boundary of the disk, or perhaps a transition region between two different parts of the disk.
	science.nasa.gov /headlines/y2000/ast15may_1m.htm?fp (1271 words)

	Science and Reason: How do stars form?
		In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass (~0.5 solar masses), and that final stellar masses are determined by the subsequent accretion of unbound gas from the clump.
		Competitive accretion models explain brown dwarfs and free-floating planets as protostars ejected from star-forming clumps before accreting much mass, predicting that they should lack disks, have high velocity dispersions, and form more frequently in denser clumps.
		Here we derive a simple estimate for the rate of competitive accretion as a function of the star-forming environment, based partly on simulations, and determine in what types of environments competitive accretion can occur.
	scienceandreason.blogspot.com /2005/10/how-do-stars-form.html (334 words)

	SPACE.com -- Strange New World Unlike Any Other
		But how it grew such a massive core is beyond the ability of current theories to explain, according to one expert who does not agree that the isolated discovery proves anything.
		With the standard core accretion model, as it is called, dust around a newborn star gathers into clumps, which become asteroids, comets and protoplanets.
		"This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance," said Greg Henry, an astronomer at Tennessee State University, Nashville.
	www.space.com /scienceastronomy/050630_planet_core.html (729 words)

	APOD: 2005 March 12 - Accretion Disk Simulation (Site not responding. Last check: 2007-10-12)
		Such accretion disks power bright x-ray sources within our own galaxy.
		They form in binary star systems which consist of a donor star (not shown above), supplying the accreting material, and a compact object whose strong gravity ultimately draws the material towards its surface.
		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).
	antwrp.gsfc.nasa.gov /apod/ap050312.html (211 words)

	nuclear reactions in AGN accretion disk
		As I understand it, it's all atomic material in the accretion disk of a big fl hole, with very high orbital velocities and high density.
		However, the the disk is quite flat, and cooler than one would think with the velocities involved, because the material is swirling in smoothly, with individual particles having fairly tame velocities relative to nearby particles.
		It would depend on the size of the fl hole and the density of the material in the accretion disc.
	www.physicsforums.com /showthread.php?threadid=114151 (432 words)

	ScienceDaily: Chandra X-ray Observatory Pinpoints Edge Of Accretion Disk Around Black Hole
		Matter stripped from a companion star by a fl hole can form a flat, pancake-like structure, called an "accretion disk." As material spirals toward the inner edge of the accretion disk, it is heated by the immense gravity of the fl hole, which causes it to radiate in X-rays.
		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.sciencedaily.com /releases/2001/05/010508082955.htm (1957 words)

	ESA Science & Technology: Physics of Astrophysical Outflows and Accretion Disks
		The program is designed to further recent theoretical progress in our understanding of the fundamental physics of accretion flows, outflows and jets, and the dynamical and thermodynamical connection between them.
		Much of this progress has been driven by large scale numerical simulations, but there has also been increased analytic and numerical understanding of the microphysics of these flows.
		the physics of accretion disks, ranging from young stellar objects to white dwarfs, neutron stars, and fl holes
	sci.esa.int /science-e/www/object/index.cfm?fobjectid=36414 (167 words)

	ScienceDaily: Scientists Observe Light Fighting To Escape Black Hole's Pull
		The flares that produced these hotspots may be due to a reconnection in the fl hole's magnetic field, creating sparks in the accretion disk.
		The energies of the hotspots are a reflection of their distance from the fl holes and the fl hole spin rate.
		If one can accurately measure the tilt of the accretion disk from the broad iron K line, this would be a direct measure of fl hole spin.
	www.sciencedaily.com /releases/2002/06/020627004618.htm (2354 words)

	John Dewey [Internet Encyclopedia of Philosophy]
		For one, Hegelian idealism was not conducive to accommodating the methodologies and results of experimental science which he accepted and admired.
		Dewey himself had attempted to effect such an accommodation between experimental psychology and idealism in his early Psychology (1887), but the publication of William James' Principles of Psychology (1891), written from a more thoroughgoing naturalistic stance, suggested the superfluity of idealist principles in the treatment of the subject.
		Dewey later began to suspect that the issues surrounding the conditions of truth, as well as knowledge, were hopelessly obscured by the accretion of traditional, and in his view misguided, meanings to the terms, resulting in confusing ambiguity.
	www.utm.edu /research/iep/d/dewey.htm (5925 words)

	Science Jamboree 2000: (Site not responding. Last check: 2007-10-12)
		The primary goal of this work is to understand stellar evolution in the upper HRD, from the past history of mass loss of these stars, by studying the morphology, kinematics and chemical composition of their nebulae.
		Recent observations show that there is no correlation between the orientation of the accretion disk, traced by the position angle of radio jets, and the host galaxy disk in Seyfert galaxies.
		This result is counter to our intuition, since most of the gas in a spiral is in the plane of the galaxy, and one would expect the central accretion disk angular momentum vector to be aligned with that of the galaxy.
	www.stsci.edu:8083 /institute/sd/archive/jamboree2000.html (5651 words)

	ScienceWeek
		Planetary Science: On Comets in the Asteroid Belt
		Social Science: On the Evolution of Human Cooperation
		Planetary Science: On the Moons of the Outer Planets
	www.scienceweek.com (557 words)

	New Scientist Premium- Science: Astronomers gain insight into black hole accretion - Science
		Science: Astronomers gain insight into fl hole accretion
		The source of the heat is simply friction between material in the inner and outer parts of the accretion disc which are forced to orbit at different speeds.
		This mechanism is very efficient at converting the gravitational energy of the infalling matter into heat.
	www.newscientist.com /article/mg12516982.000.html (282 words)

	Aristotle -- Metaphysics [Internet Encyclopedia of Philosophy]
		If form is the first principle of the science of physics, might it not be a first principle simply, behind which one cannot get, to which one may appeal for explanation but about which one cannot inquire?
		Strip away the accretion of mere facts, and what is left is that without which even those facts could not have gained admittance into the world: the forever vulnerable foundation of all that is in the world, the shaping, ruling form, the incessant maintenance of which is the only meaning of the phrase self-preservation.
		Indeed even the bodily material of the living thing is present in the world only as active, only as forming itself into none of the other things it might have been but just this one thoroughly defined animal or plant.
	www.iep.utm.edu /a/aris-met.htm (8228 words)

	Earth Science Picture of the Day (Site not responding. Last check: 2007-10-12)
		Concretions are hard masses of sedimentary and, more rarely, volcanic rock that form by the preferential precipitation of minerals (cementation) in localized portions of the rock.
		In this area, the rocks of the Dakota Formation also preserve numerous dinosaur track-ways in the rippled and petrified sands of an early Cretaceous shoreline.(see Earth Science Picture of the Day for May 12, 2003).
		The photo on the right was taken in June, 1984.
	epod.usra.edu /archive/epodviewer.php3?oid=149472 (342 words)

	Constellation-X :: Resources :: Science Papers
		Accretion Column Structure of Magnetic Cataclysmic Variables from X-Ray Spectroscopy
		Impact of reverberation in flared accretion discs on temporal characteristics of X-ray binaries
		Vignali, C., Brandt, W. N., Kaspi, S., Schneider, D. P., 2001, Two Years of Science with Chandra, held 5-7 Septemeber 2001 in Washington DC Exploratory Chandra Observations of the Highest-Redshift Quasars: X-Rays from the Dawn of the Modern Universe
	constellation.gsfc.nasa.gov /resources/papers/sci_papers.html (2111 words)

	The Nine Planets Glossary
		A more scholarly introduction the planetary science for those who want to dig a little deeper.
		French writer who is considered the founder of modern science fiction.
		Institute and Museum of History of Science, Florence ITALY (Biographies in the Galileo exhibit)
	www.nineplanets.org /help.html (4842 words)

	Marxism - Wikipedia, the free encyclopedia
		Finally, the enlightened despotism of Napoleon would be a synthesis of the two.
		This process, dialectic, sometimes involves gradual accretion but at other times requires discontinuous leaps -- violent upheavals of previously existing status quo.
		World-historical figures such as Napoleon Bonaparte are, on the Hegelian reading, servants of a World Spirit (Weltgeist) whose Freedom has reconciled with the Necessity of History.
	en.wikipedia.org /wiki/Marxism (5951 words)

	Disc-Disc Encounters between Low-Mass Protoplanetary Accretion Discs \| Jupiter Today - Your Daily Source of Jupiter News
		Simulations of the collapse and fragmentation of turbulent molecular clouds and dense young clusters show that encounters between disc-surrounded stars are relatively common events which should significantly influence the resulting disc structure.
		In turn this should alter the accretion rate of disc matter onto the star and the conditions under which planet formation occurs.
		This can lead to a reduction of the angular momentum in the entire disc and thus considerably increased accretion shortly after the encounter as well as in the long term.
	www.jupitertoday.com /news/viewsr.html?pid=16547 (606 words)

	Planets (Site not responding. Last check: 2007-10-12)
		The Solar System began forming about 4.6 billion years ago when a swirling cloud of gas began to spin and collapse inwards, flattening into a central mass with a surrounding 'accretion' disk.
		Scientists believe that the accretion disk took 100 million years to collapse from the moment the Sun ignited.
		The solar wind of the newly ignited Sun blew away leftover dust and gas in the vicinity of the inner condensations, leaving the rocky inner planets: Mercury, Venus, Earth and Mars.
	www.kosmoi.com /Science/Astronomy/Solar/Planets (1071 words)

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