Rotating Accretion Flows: From Infinity to the Black Hole
by Li, Jason and Ostriker, Jeremiah and Sunyaev, Rashid
18 pages, 9 figures, submitted to ApJ
Accretion onto a supermassive black hole of a rotating inflow is a particularly difficult problem to study because of the wide range of length scales involved. There has been some analytic and numerical treatment of the global properties of accretion flows, but detailed numerical simulations are required to address certain critical aspects. We use the ZEUS code to run hydrodynamical simulations of rotating, axisymmetric accretion flows with Bremsstrahlung cooling, considering solutions with and without viscous angular momentum transport, and also electron thermal conduction. Infalling gas is followed from well beyond R_Bondi down to the vicinity of the black hole. Absent viscous transport, when the centrifugal balance radius significantly exceeds R_Schwarzschild, the accretion rate is zero and the flow approaches a stationary solution in which pressure impedes inflow from large radii. With viscosity, we find two general classes of solutions: low inflow rate, hot, vertically extended disks with very low accretion and disk and conical wind outflows near R_Bondi, and strong inflow solutions which have cold, geometrically thin disks accreting at close to Mdot_Edd. We produce a continuum of solutions with respect to the Eddington ratio Mdot_Bondi/Mdot_Edd, and there is a sharp transition between the two general classes of solutions at Eddington ratio ~ few x 10^(-2). The low accretion inflow-outflow solutions are of two types. Equatorial outflow dominates when viscosity is larger than thermal conductivity, but polar outflow can be significant when the Prandtl number ~ 0.05. Our simulations have converged with respect to spatial resolution and temporal duration, and they do not depend strongly on our choice of boundary conditions. We also note the possibility that radiative feedback loops can cause the flow to switch between the hot and cold disk states, with potential applications to quasars.