A Model for Quasi-Spherical Magnetized Accretion Flow

Abstract

A model for axisymmetric magnetized accretion flow is proposed. The dominant mechanism of energy dissipation is assumed to be the magnetic diffusivity due to turbulence in the accretion flow. In analogy to the advection-dominated accretion flow (ADAF) solutions, a constant fraction of the resistively dissipated energy is stored in the accreting gas and the rest is radiated. The general self-similar solutions which describe a resistive and {\it nonrotating} flow with purely poloidal magnetic field is first introduced. The radial dependence of physical quantities is identical to viscous ADAF solutions. Although the main focus of this study is nonrotating magnetized accretion flow, for rotating flow with both poloidal and toroidal components of magnetic field we find the radial scaling of solutions similar to the nonrotating case. We show that the accretion and the rotation velocities are both below the Keplerian rate, irrespective of the amount of cooling. We show that the set of the equations is reduced to a second order differential equation for a nonrotating flow. The geometrical shape of the disk changes depending on the fraction of the resistively dissipated energy which is stored in the accreting gas. However, there is a hot low-density gas above the disk in almost all cases. The net accretion rate is calculated for a set of illustrative parameters.