It is shown that, for accretion disks, the height scale is a constant whenever hydrostatic equilibrium and the subsonic turbulence regime hold in the disk. In order to have a variable height scale, processes are needed that contribute an extra term to the continuity equation. This contribution makes the viscosity parameter much greater in the outer region and much smaller in the inner region. Under these circumstances, turbulence is the presumable source of viscosity in the disk.
We have analyzed pair production in the innermost region of a two-temperature external soft photon Comptonized accretion disk. We have shown that, if the viscosity parameter is greater than a critical value α c , the solution to the disk equation is double valued: one, advection dominated, and the other, radiation dominated. When α α c , the accretion rate has to satisfyṁ 1 ṁ ṁ c in order to have two steady-state solutions. It is shown that these critical parametersṁ 1 ,ṁ c are functions of r, α, and θ e , and α c is a function of r and θ e . Depending on the combination of the parameters, the advection-dominated solution may not be physically consistent. It is also shown that the electronic temperature is maximum at the onset of the thermal instability, from which results this inner region. These solutions are stable against perturbations in the electron temperature and in the density of pairs.
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