Three-Dimensional Simulations of Spherical Accretion Flows with Small-Scale Magnetic Fields

Spherical (nonrotating) accretion flows with small-scale magnetic fields have been investigated using three-dimensional, time-dependent MHD simulations. These simulations have been designed to model high-resolution (quasi) steady accretion flows in a wedge computational domain that represents a small fraction of the full spherical domain. Subsonic and supersonic (super-fast-magnetosonic) accretion flows have been considered. Two accretion regimes have been studied: conservative, or radiatively inefficient, and nonconservative, in which the heat released in magnetic reconnections is completely lost. The flows in both regimes are turbulent. They show the flattened radial density profiles and reduction of the accretion velocities and mass accretion rates in comparison with hydrodynamic Bondi flows. In the conservative regime, the turbulence is more intensive and supported mostly by thermal convection. In the nonconservative regime, the turbulence is less intensive and supported by magnetic buoyancy and various magnetic interactions. We have concluded that steady, supersonic spherical accretion cannot be developed in the presence of small-scale magnetic fields. Subject headings: accretion, accretion disks — black hole physics — convection — MHD — turbulence