Three‐dimensional Magnetohydrodynamic Simulations of Spherical Accretion

We present three-dimensional numerical magnetohydrodynamic simulations ofradiatively inefficient spherical accretion onto a black hole. The simulationsare initialized with a Bondi flow, and with a weak, dynamically unimportant,large-scale magnetic field. The magnetic field is amplified as the gas flowsin. When the magnetic pressure approaches equipartition with the gas pressure,the field begins to reconnect and the gas is heated up. The heated gas isbuoyant and moves outward, causing line stretching of the frozen-in magneticfield. This leads to further reconnection, and more heating andbuoyancy-induced motions, so that the flow makes a transition to a state ofself-sustained convection. The radial structure of the flow changesdramatically from its initial Bondi profile, and the mass accretion rate ontothe black hole decreases significantly. Motivated by the numerical results, wedevelop a simplified analytical model of a radiatively inefficient sphericalflow in which convective transport of energy to large radii plays an importantrole. In this "convection-dominated Bondi flow" the accretion velocity ishighly subsonic and the density varies with radius as ~R^{-1/2} rather than thestandard Bondi scaling ~R^{-3/2}. We estimate that the mass accretion rate ontothe black hole is significantly less than the Bondi accretion rate.Convection-dominated Bondi flows may be relevant for understanding manyastrophysical phenomena, e.g. post-supernova fallback and radiativelyinefficient accretion onto supermassive black holes, stellar-mass black holesand neutron stars.Comment: 23 pages, 6 figures, submitted to Ap