Magnetic Field Limitations on Advection-dominated Flows

Recent papers discussing advection-dominated accretion Ñows (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 Ðeld reconnection in heating of the plasma Ñow, assuming electron heating due only to binary Coulomb collisions with ions. Here we analyze the physical processes in optically thin accretion Ñows at low accretion rates including the inÑuence of an equipartition random magnetic Ðeld and heating of electrons due to magnetic Ðeld reconnection. The important role of the magnetic Ðeld pointed out by Shvartsman comes about because the magnetic energy density, increases more rapidly with decreasing distance than the kinetic energy density, E m , E k (or thermal energy density). Once grows to a value of order further accretion to smaller distances E m E k , is possible only if magnetic Ñux is destroyed by reconnection. For the smaller distances it is likely that there is approximate equipartition, Dissipation of magnetic energy is associated with the E m B E k . destruction of magnetic Ñux. We discuss reasons for believing that the Ðeld annihilation leads to appreciable electron heating. Such heating signiÐcantly restricts the applicability of ADAF solutions, and it leads to a radiative efficiency of the Ñows of D25% of the standard accretion disk value. Subject headings : accretion, accretion disks È galaxies : active È MHD È plasmas È stars : magnetic Ðelds È X-rays : stars