Thermal Properties of Two‐dimensional Advection‐dominated Accretion Flow

We study the thermal structure of the widely adopted two-dimensionaladvection dominated accretion flow (ADAF) of Narayan & Yi (1995a). The criticalradius for a given mass accretion rate, outside of which the optically thin hotsolutions do not exist in the equatorial plane, agrees with one-dimensionalstudy. However, we find that, even within the critical radius, there alwaysexists a conical region of the flow, around the pole, which cannot maintain theassumed high electron temperature, regardless of the mass accretion rate, inthe absence of radiative heating. This could lead to torus-like advectioninflow shape since, in general, the ions too will cool down. We also find thatCompton preheating is generally important and, if the radiative efficiency,defined as the luminosity output divided by the mass accretion rate times thevelocity of light squared, is above sim 4x10^-3, the polar region of the flowis preheated above the virial temperature by Compton heating and it may resultin time-dependent behaviour or outflow while accretion continues in theequatorial plane. Thus, under most relevant circumstances, ADAF solutions maybe expected to be accompanied by polar outflow winds. While preheatinginstabilities exist in ADAF, as for spherical flows, the former are to someextent protected by their characteristically higher densities and highercooling rates, which reduce their susceptibility to Compton driven overheating.Comment: 18 pages including 4 figures. AASTEX. Submitted to Ap