Global dynamics of radiatively inefficient accretion flows: advection versus convection

We obtain global solutions of radiatively inefficiently accretion flows around black holes. Whether and where convection develops in a flow are self-consistently determined with the mixing-length theory. The solutions can be divided into three types according to the strength of normal viscosity. Type I solution corresponds to large viscosity parameter $\alpha \ga 0.1$, which is purely advection-dominated and with no convection, and has been extensively studied in the literature. Type II solution is for moderate $\alpha \sim 0.01$, which has a three-zone structure. The inner zone is advection-dominated, the middle zone is convection-dominated and ranges from a few tens to a few thousands of gravitational radii, and the outer zone is convectively stable and matches outward a Keplerian disc. The net energy flux throughout the flow is inward as in type I solution. Type III solution which is for small $\alpha \la 0.001$ consists of two zones as Abramowicz et al. suggested previously: an inner advection-dominated zone and an outer convection-dominated zone, separated at a radius of a few tens of gravitational radii. This type of solution has an outward net energy flux. In both type II and III solutions the radial density profile is between the 1/2 law of self-similar convection-dominated accretion flow model and the 3/2 law of self-similar advection-dominated accretion flow model, and the efficiency of energy release is found to be extremely low. Our results are in good agreement with those of recent numerical simulations.