Magnetic field limitations on advection-dominated flows

Recent papers discussing advection-dominated accretion flows (ADAF) as a solution for astrophysical accretion problems should be treated with some caution because of their uncertain physical basis. The suggestions underlying ADAF involve ignoring the magnetic field reconnection in heating of the plasma flow, assuming electron heating due only to binary Coulomb collisions with ions. Here we analyze the physical processes in optically thin accretion hows at low accretion rates including the influence of an equipartition random magnetic field and heating of electrons due to magnetic field reconnection. The important role of the magnetic field pointed out by Shvartsman comes about because the magnetic energy density, E-m, increases more rapidly with decreasing distance than the kinetic energy density, E-k (or thermal energy density). Once E-m grows to a value of order E-k, further accretion to smaller distances is possible only if magnetic flux is destroyed by reconnection. For the smaller distances it is likely that there is approximate equipartition, E-m approximate to E-k. Dissipation of magnetic energy is associated with the destruction of magnetic flux. We discuss reasons for believing that the held annihilation leads to appreciable electron heating. Such heating significantly restricts the applicability of ADAF solutions, and it leads to a radiative efficiency of the flows of similar to 25% of the standard accretion disk value.