Influence of Ohmic Heating on Advection-dominated Accretion Flows

Advection-dominated, high-temperature, quasi-spherical accretion flow onto acompact object, recently considered by a number of authors, assume that thedissipation of turbulent energy of the flow heats the ions and that thedissipated energy is advected inward. It is suggested that the efficiency ofconversion of accretion energy to radiation can be very much smaller thanunity. However, it is likely that the flows have an equipartition magneticfield with the result that dissipation of magnetic energy at a rate comparableto that for the turbulence must occur by Ohmic heating. We argue that thisheating occurs as a result of plasma instabilities and that the relevantinstabilities are current driven in response to the strong electric fieldsparallel to the magnetic field. We argue further that these instabilities heatpredominantly the electrons. We conclude that the efficiency of conversion ofaccretion energy to radiation can be much smaller than unity only for theunlikely condition that the Ohmic heating of the electrons is negligible.Comment: 6 pages, AAS LateX macros 4.0, Submitted to Ap. J. Letter