Numerical simulation of nonaxisymmetric adiabatic accretion flow

The hydrodynamics of gas flow past a finite-sized gravitating central object is investigated in the adiabatic (γ = 4/3) approximation in two spatial dimensions. Specific attention is focused on the description of the nonaxisymmetric case associated with a density gradient transverse to the mean flow. It is found that the flow morphology depends on the ratio ɛ of the axisymmetric accretion radius to the length scale of the density variation. For small to intermediate values of ɛ (0.005 < ɛ < 0.25) the flows are highly time dependent. The wake region behind the central object becomes unstable. The evolution of the instability into the nonlinear regime results in the temporary formation of an accretion disk which can circulate in either the direct or counter sense. For large ɛ (≡1) a quasi-steady state is found in which an accretion disk rotating in only the direct sense is reached. The application of these results to X-ray binaries is discussed.