Bimodal Accretion Disks: SSD-ADAF Transitions
  We show that, unlike the results presented previously in the literature, the transition from an outer Shakura-Sunyaev disk (SSD) to an advection-dominated accretion flow (ADAF) is possible for large values of the viscosity parameter $\alpha >0.5$. The transition is triggered by thermal instability of a radiation-pressure-supported SSD. The transition radius is close to the central black hole. We confirm our qualitative prediction by actually constructing global bimodal SSD-ADAF solutions. 
Wei-Min Gu and Ju-Fu Lu