A Note on Bimodal Accretion Disks

The existence of bimodal disks is investigated. Following a simple argument based on energetic considerations we show that stationary, bimodal accretion disk models in which a Shakura–Sunyaev disk (SSD) at large radii matches an advection dominated accretion flow (ADAF) at smaller radii are never possible using the standard slim disk approach, unless some extra energy flux is present. The same argument, however, predicts the possibility of a transition from an outer Shapiro–Lightman–Eardley (SLE) disk to an ADAF, and from a SLE disk to a SSD. Both types of solutions have been found. We show that for any given accretion rate a whole family of bimodal SLE–ADAF and SLE–SSD solutions can be constructed, each model being characterized by the location of the transition radius. Subject headings: accretion, accretion disks — black hole physics — hydrodynamics