The theory of geometrically thick accretion disks in its current form is reviewed and preliminary calculations concerning their possible secular evolution are presented. These disks are now known, however, to be subject to global dynamical instabilities discovered by Papaloizou and Pringle. How far the theo.ry has to be modified to account for this, and indeed the very existence of thick disks themselves, are as yet unanswered questions. All work done on exploring the consequences of the instabilities and the many suggestions as to their physical cause(s) are reviewed here. The driving force of the instability appears to be a tapping of the shear energy of the differentially rotating fluid by modes which transport angular momentum outwards. In the most violent modes this is achieved through interactions between waves propagating around the rotation axis near the inner and outer radii of the torus. No general global stability criteria exist, but what can be gleaned from the theory of rotating stars is presented here. In particular the general limits on instability growth rates and "almost" corotation theorems are still valid, and both of these can be improved upon for particular classes of tori. In the special case of non-self-gravitating, constant specific angular momentum, homentropic slender tori one may calculate the full normal mode oscillation spectrum and this gives a complete analytic description of the instability for this case. This is in fact the most violent mode and is now known to be stabilized in Newtonian slender tori when the specific angular 2-13 momentum distribution is steeper than w2-3 . This result is generalized to pseudo-Newtonian tori, whicl1 could be of some practical significance if the less violent modes which exist beyond this point do not actually disrupt the torus. The surface interaction nature of the instability is demonstrated by studying two-dimensional annular flows. Pseudo-Newtonian cusps are found to be stabilizing in incompressible flows because they cannot support wave motion at the inner edge. The extra degrees of feedom existing in a compressible flow are, however, immediately destabilizing. Finally, the effects of accretion on the instability can be studied by constructing a continuous sequence of two-dimensional relativistic models going from a marginally bound annulus (which is unstable) to a pure radial infall from infinity (which is stable). The location of the marginal stability point is still unknown at the present time.
|Titolo:||The Stability of Thick Accretion Disks|
|Relatore/i esterni:||Abramowicz, M. A.|
|Data di pubblicazione:||10-set-1986|
|Appare nelle tipologie:||8.1 PhD thesis|