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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For thin disks, where the advection of internal energy (or entropy) can be neglected, and for changes that happen on time scales longer than the dynamical time Ω−1 , the left-hand side is small compared with the terms on the right-hand side. 53) where Ts is the surface temperature of the disk, σr is Stefan-Boltzmann’s radiation constant σr = ar c/4, and the factor 2 comes about because the disk has two radiating surfaces (assumed to radiate like blackbodies). Thus, the energy balance is local (for such slow changes): what is generated by viscous dissipation inside the disk at any radius r is also radiated away from the surface at that position.

It is not possible, either, when the difference in rotation velocity is too small to put the accreting mass on an escape orbit. This was realized early on (Syunyaev and Shakura, 1977). Instead of being flung out, accretion is halted and mass accumulates in the disk. The buildup of mass in the disk then leads to a following episode of accretion. In this phase the magnetosphere is compressed such that ri < rco . Instead of propellering, the result is cyclic accretion. 3. The phase with ri > rco , during which accretion is halted, is called the “dead disk” phase in Syunyaev and Shakura (1977).

This idea is called propellering (Illarionov and Sunyaev, 1975). The term is then used as synonymous with the condition ri > rco . Although a process like this is likely to happen when the rotation rate of the star is sufficiently large (the CV AE Aqr being an example), it is not necessary. It is not possible, either, when the difference in rotation velocity is too small to put the accreting mass on an escape orbit. This was realized early on (Syunyaev and Shakura, 1977). Instead of being flung out, accretion is halted and mass accumulates in the disk.

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