ON THE STABILITY OF IRRADIATED ACCRETION DISKS

The effects of external heating on the stability of hot accretion disks are studied in some detail. It is known that geometrically thin, optically thick, non-irradiated accretion disks have two distinct branches in the surface density - mass-accretion rate plane: the upper branch is radiation pressure-dominated and is unstable against thermal and secular perturbations, while the lower one is gas pressure-dominated and is stable. We show quite generally that even when disks are strongly irradiated, the upper branch remains unstable and the lower branch remains stable; the lower branch, however, can become radiation pressure-dominated, if the irradiating flux, F(irr), is kept constant. A stable, radiation pressure-dominated state thus appears. If F(irr) changes in proportion to the mass-accretion rate through the disk, on the other hand, the instabilities associated with radiation pressure-dominated disks cannot be removed. Some observational implications are discussed in the context of long-term variations of low-mass X-ray binaries.