Download Accretion Processes in Astrophysics (Canary Islands Winter by Martinez-Pais I.G., Shahbaz T., Velazquez J.C. (eds.) PDF

By Martinez-Pais I.G., Shahbaz T., Velazquez J.C. (eds.)

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The cooling time is therefore H E = 3τ . 97) tc = F c This is to be compared with the accretion time, which can be written in terms of the mass in the disk at radius r, of the order 2πr2 Σ, and the accretion time: tacc ≈ 2πr2 Σ . 99) (where a factor 3/2 H/r ∼ O(1) has been neglected). Since r > R, this shows that accretion has to be around the Eddington rate or larger in order to be both radiation and advection dominated. , Rees, 1978). Equating tacc and tc yields rt ≈ 4m. 100) R Inside rt , the flow is advection dominated: the radiation field produced by viscous dissipation stays trapped inside the flow, instead of being radiated from the disk, as happens in a standard thin disk.

82) The factor in brackets measures the kinetic energy of the matter at the inner edge of the disk (r = R), in the frame of the stellar surface. Because of this dissipation, the disk inflates into a “belt” at the equator of the star, of thickness D and radial extent of the same order. 59. 55, ignoring the factor f ). In practice, this works out to a factor of a few. The surface of the belt is therefore not very hot. The situation is quite different if the boundary layer is not optically thick (Pringle and Savonije, 1979).

They show how with increasing Accretion disks 29 accretion rate, a standard thin Shakura-Sunyaev disk turns into a radiation-supported advection flow. , 1991). , 1999). 2 Super-Eddington accretion onto neutron stars In the case of accretion onto a neutron star, the energy trapped in the flow, plus the remaining orbital energy, settles onto its surface. If the accretion rate is below M˙ E , the energy can be radiated away by the surface, and steady accretion is possible. A secondary star providing the mass may, under some circumstances, transfer more than M˙ E , since it does not know about the neutron star’s Eddington value.

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