Numerical simulations of two-dimensional and three-dimensional wind accretion flows of an isothermal gas

Abstract

We present the results of 2D and 3D numerical hydrodynamical calculations of accretion flows of an isothermal gas past a gravitating compact object. The calculations were performed both for a homogeneous medium and for a medium containing a transverse density or velocity gradient. We find that 2D isothermal flows exhibit the 'flip-flop' instability (previously seen in adiabatic calculations) both in the homogeneous and the inhomogeneous cases. In the 3D case, while some unsteadiness is observed, the instability is much less violent than in 2D. We calculate (for the first time with a 3D hydrocode) the rate of accretion of mass and angular momentum from an inhomogeneous medium. The mass accretion rate agrees quite well with the prediction of the Bondi-Hoyle theory. However, the rate of accretion of angular momentum is considerably lower than the rate at which angular momentum is deposited into the accretion cylinder. A possible consequence of our results for wind-fed X-ray binaries is discussed.