THICKENING OF AXISYMMETRIC ACCRETION DISKS AND FLOWS

An absolute upper limit of 5.5 per cent is derived for the fraction of the total energy which can be extracted from accreting plasma inside the marginally stable circular orbit (r = rms) in Schwarzschild geometry without disrupting steady flow. On the basis of this we present rough upper limits on the thickening of thin accretion disks and flows under the action of a thermal Pringle-Rees (PR) instability. An estimate of the thickening timescale is also made. We find that there are many physically realistic situations involving radial steady-state flow where significant disk thickening can occur in the region of rms but that its evolution into a large spherical cloud would require large values of α the viscosity parameter, and Δϵ, the amount of energy dissipated in the unstable region - along with a small ur0/ur ratio, where ur0 is the radial velocity at r0, the radius at which the region of instability is entered and ur is the radial velocity at some r 0 © 1978 Royal Astronomical Society.