Accretion Basics

Accretion Basics - Accretion Basics By default accretion...

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Accretion Basics By default, accretion power is the only way to generate AGN luminosities in the required small volumes for interestingly long times. A fiducial value for the expected maximum luminosity from accretion is the Eddington limit . This is the luminosity at which, for uniform spherical accretion, the radiation pressure on accreting material balances the gravitational force and thus stops further accretion. In the case of zero angular momentum material falling at distance r onto a central object of mass M , the radiation pressure per unit infalling mass is κ L / 4 π r² c . Here κ is the absorption coefficient per unit mass; at high temperatures one takes the electron-scattering coefficient, and c accounts for momentum transfer. The Eddington luminosity is thus, in this regime, Thus, AGNs with bolometric luminosities up to 10 47 ergs/second may need central masses up to 10 9 solar masses. Some nearby weak AGN with dynamical evidence for large central masses must then by somewhere around 10 -4 of the Eddingtojn luminosity, a difference which has sometimes been invoked to distinguish objects in active accretion form those which may be
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This note was uploaded on 11/10/2011 for the course AST AST1002 taught by Professor Emilyhoward during the Fall '10 term at Broward College.

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Accretion Basics - Accretion Basics By default accretion...

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