Evaporation of accretion disks: mechanism and observational consequences Original Research Article

Observations of AGN show the coexistence of the hot optically thin plasma and cold optically thick gas in the vicinity of the black hole. We study how various assumptions about the heating mechanism influence the direction of the mass exchange through the boundary between the phases: evaporation, condensation or an equilibrium are possible, depending on the conditions. This basic criterion is applied to the formation of a disk/corona flow. We show that in the outermost part of the disk the two-temperature accreting corona forms through the partial evaporation of the disk. If the accretion rate is low the evaporation proceeds with an increasing efficiency at smaller radii and, at a certain radius, the disk finally disappears, replaced by a hot optically thin flow, possibly diminated by advection (ADAF). However, if the accretion rate is high the evaporation is replaced with condensation in the innermost part of the flow: the corona strength decreases inwards and the disk extends down to the marginally stable orbit. We discuss the value of the critical accretion rate dividing the models with and without inner ADAF flow in the context of observations. We also show that the considerable outflow is necessary to explain the stability of the sources in the high/soft state.