Hot Accretion With Saturated Conduction

Observations of the hot gas surrounding Sgr A* and a few other nearby galactic nuclei imply electron and proton mean free paths comparable to the gas capture radius: hot accretion likely proceeds under weakly-collisional conditions in these systems. As a result, thermal conduction, rather than convection, may be important on all scales and affect the global flow properties. The self-similar ADAF solution of Narayan & Yi (1994) is generalized to include a saturated form of thermal conduction, as is appropriate for the weakly-collisional regime of interest. Conduction provides extra heating and yet it reduces the free-free radiative efficiency of the accretion flow (by potentially large factors). These idealized solutions suggest that thermal conduction may be an important physical ingredient to understand hot accretion onto dim accreting black holes. Conduction could also play a role in reducing the rate at which black holes capture ambient gas and in providing an evaporation mechanism for an underlying cold thin disk.