Structure and evolution of irradiated accretion disks. I. Static thermal equilibrium structure

The thermal equilibrium structure of externally irradiated accretion disks is investigated. We assume that the irradiation flux is thermalized in the photosphere of the disk. The vertical structure of the disk can then be calculated by changing the surface boundary conditions. The response of the disk structure to the irradiation is totally different for radiative disks and for convective disks. For radiative disks, only the surface layer is heated by the irradiation, and there is practically no change in the internal structure. In the convective disks, however, the heat of irradiation can penetrate deeply into the disk, and thus the temperature increases at every depth. For a sufficiently strong irradiation with T_irr > 10,000 K, where the irradiation flux is F_irr ≡ σT_eff⁴, the disk is completely stabilized against thermal instabilities of the sort invoked to explain dwarf novae. For moderately strong irradiation T_irr ∼ 6000 K, however, there is still an unstable branch in the thermal equilibrium curve, although the difference in the temperature between the hot stable state and the cool stable state is less than that in nonirradiated disks. A simplified analytic model is presented to understand the effects of irradiation on the vertical structure of the disks. It is shown that in typical soft X-ray transients such as Aql X-1, Cen X-4, and A0620-00, irradiation can be completely neglected in quiescence; the disk is thus unstable against the dwarf-nova type instability. At maximum light, however, irradiation may suppress the instability. It is suggested that the 300 day modulation of the soft X-ray luminosity in Cyg X-1 might be caused by the thermal instability. Possible mechanisms to initiate the downward thermal transition are discussed.