Transition Radius from Cooling-Dominated to Advection-Dominated Regimes in Two Temperature Disks

Optically thin advection-dominated flows are known to be unable to extend outward beyond a certain critical (transition) radius in disk accretion. Outside of the radius, the disks are cooling-dominated. The advection-dominated self-similar steady flows cannot join thermally to the cooling-dominated steady standard disks, unless there is some external heating at the transition zone. We examine, under the condition of no such heating, the radius r_tr of the transition analytically when the advection-dominated inner flows are two-temperature ones. The results are almost the same as those obtained by Honma (1996, AAA 65.067.241) to a one-temperature disk: i.e., r_tr/r_g ~ 5.5times 10(2(alpha ^4/{dot ) m}(2)) , where alpha is the viscosity parameter and {dot m} is the mass accretion rate normalized by the critical accretion rate defined by the Eddington luminosity. This coincidence with the one-temperature case comes from the fact that the transition occurs after the ion temperature in the advection-dominated region decreases sharply outwards.