3.5, depending on the density. At $\log T_{\rm c}$ $\gta$ 3.7, ${\rm H^{-}}$ first becomes important. We define the domain between these two temperature regions as an intermediate state. The partially ionized region can reach well above $10^4$ K, depending on the density. In terms of the effective temperature, the hot state is roughly $\log T_{\rm eff}$ $\gta$ $10^{3.85}$.

Two regimes where changes in the physical conditions can affect the stability of the disk have been discussed in previous studies. One is the partially ionized, intermediate warm state centered at $\log T_{\rm eff}$ $\sim$ 3.8, or, equivalently, $\log T_{\rm c}$ $\sim$ 3.7–4.1 (Mineshige, Osaki 1985; Mineshige 1988; Kim et al. 1992). The other regime occurs in the temperature region centered at $\log T_{\rm eff}$ $\sim$ 3.4 (or $\log T_{\rm c}$ $\sim$ 3.4$-$3.7) in a marginally optically thin region (Cannizzo et al. 1982; Mineshige, Osaki 1983) dominated by molecular opacity, such as ${\rm {H_2O}}$ (Cannizzo, Wheeler 1984). These regions of variable opacity can lead to “metastable" or “stagnation" states in the evolution between the hot and cold stable states of the disk (discussed in subsection 4.2). Ionization stagnation is explicitly displayed in models for the outburst rise by Mineshige (1988) and 