Magnetic field limitations on advection-dominated flows

Recent papers discussing advection-dominated accretion flows (ADAF) as a so lution for astrophysical accretion problems should be treated with some cau tion because of their uncertain physical basis. The suggestions underlying ADAF involve ignoring the magnetic held reconnection in heating of the plas ma flow, assuming electron heating due only to binary Coulomb collisions wi th ions. Here we analyze the physical processes in optically thin accretion hows at low accretion rates including the influence of an equipartition ra ndom magnetic held and heating of electrons due to magnetic held reconnecti on. The important role of the magnetic field pointed out by Shvartsman come s about because the magnetic energy density, E-m, increases more rapidly wi th decreasing distance than the kinetic energy density, E-k (or thermal ene rgy 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 reconne ction. For the smaller distances it is likely that there is approximate equ ipartition, E-m approximate to E-k. Dissipation of magnetic energy is assoc iated with the destruction of magnetic flux. We discuss reasons for believi ng that the held annihilation leads to appreciable electron heating. Such h eating significantly restricts the applicability of ADAF solutions, and it leads to a radiative efficiency of the flows of similar to 25% of the stand ard accretion disk value.