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 an SLE disk to an 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.