5.3. Advection-dominated Accretion Flows
Provided the accretion rate is not too high, it is possible for the surface density of the disk to be so low that Coulomb interactions alone are not sufficient to maintain thermal coupling between ions and electrons on the infall timescale. Assuming nothing else couples these particles together, then the thermal energy of the ions will not be radiated away and instead may simply be advected into the central black hole. If the gravitational energy released is channeled primarily into the ions, rather than into the electrons, then the ions can become very hot and produce a gas pressure-supported disk with a very low radiative efficiency (Ichimaru 1977; Rees et al. 1982). Such advection-dominated accretion flows (ADAFs) have received considerable attention in recent years, thanks to the seminal work of Abramowicz et al. (1995) and Narayan & Yi (1995), who showed them to be thermally and viscously stable. The connection between the thin disk, slim disk, and two-temperature ADAF solutions as a function of black hole mass, accretion rate, radius, and viscosity parameter has been discussed by Chen et al. (1995).
It is not clear yet whether the two requirements for an ADAF (low thermal coupling between the ions and electrons and predominately ion heating rather than electron heating) can truly be satisfied in a realistic flow. Begelman & Chiueh (1988) have identified unstable plasma modes that could significantly heat electrons in regions of the flow in which a high level of small-scale magnetohydrodynamic turbulence is present. Such modes may destroy the two-temperature structure of an ADAF, especially in AGNs, if collisionless shocks exist in the flow with sufficiently high filling factor (Narayan & Yi 1995). In addition, the magnetohydrodynamic turbulence that drives accretion primarily heats the electrons unless the magnetic field is significantly below equipartition strength (Quataert 1998; Gruzinov 1998; Quataert & Gruzinov 1998).
Two-temperature ADAFs are by definition very inefficient and are therefore most applicable to low-luminosity AGNs. The most successful application of the theory is perhaps to explaining the broadband emission from the Galactic center (Narayan, Yi, & Mahadevan 1995). ADAFs may also serve as useful models of X-ray-emitting coronae in brighter AGNs.