Outflows from Advection-Dominated Disks?

Narayan and Yi (1994, AAA 064.083) found that the Bernoulli parameter is a positive constant in advection-dominated accretion flows, thereby mass outflow being inevitable. We find, however, that their conclusion depends critically on their assumptions required to derive their self-similar solutions (i.e., substantial infall velocity, comparable to the rotational velocity, or high virial temperature at the outer boundary). Rather, positive Bernoulli parameters are not the universal feature of advection-dominated flow. If boundary conditions are properly set, the Bernoulli parameter is mostly negative, as long as the viscosity parameter alpha is not very large (alpha < 1) and the specific heat ratio gamma is 5/3. Our result shows that spontaneous outflows are not expected, even if accretion disks are advection-dominated. Still, outflow is possible if there exists some mechanism which puts most of the energy selectively into a small fraction of the disk material.