Spherically Symmetric Accretion Flows: Minimal Model with Magnetohydrodynamic Turbulence

Abstract

The first spherical accretion model was developed 55 years ago, but the theory is still far from being complete. The real accretion flow was found to be time dependent and turbulent. This paper presents the minimal magnetohydrodynamic (MHD) spherical accretion model that separately deals with turbulence. Treatment of turbulence is based on simulations of several regimes of collisional MHD. The effects of freezing-in amplification, dissipation, dynamo action, isotropization, and constant magnetic helicity are self-consistently included. The assumptions of equipartition and magnetic field-isotropy are released. The correct dynamics of magnetized flow is calculated. Diffusion, convection, and radiation are not accounted for. Two different types of radiatively inefficient accretion flows are found: (1) a transonic nonrotating flow and (2) a flow with effective transport of angular momentum outward. The nonrotating flow has an accretion rate several times smaller than Bondi rate, because turbulence inhibits accretion. The flow with angular momentum transport has an accretion rate about 10-100 times smaller than the Bondi rate. The effects of highly helical turbulence, states of outer magnetization, and different equations of state are discussed. The flows were found to be convectively stable on average, despite the fact that gas entropy increases inward. The proposed model has a small number of free parameters and the following attractive property. Inner density in the nonrotating magnetized flow was found to be several times lower than density in a nonmagnetized accretion. However, a density that is several times lower is still required to explain the observed low infrared luminosity and low Faraday rotation measure of accretion onto Sgr A*.