Hydrogen-ionization disk instability

Partial hydrogen ionization changes disk cooling and permits cold and hot stable thermal branches separated by unstable equilibria. A dwarf nova can cycle between a cold matter-storing disk and a hot, rapidly accreting disk. The cycle is described by the evolution of local surface density and temperature, coupled to viscous evolution of an accretion disk.

At fixed disk radius, balancing local heating and cooling gives thermal equilibria parameterized by surface density and temperature. In the hydrogen-ionization disk instability, this locus has cold and hot stable branches joined by unstable equilibria. Crossing the turning points drives the state changes of a dwarf nova. Thermal stability of an accretion disk tests the local heating and cooling response, while viscous evolution of an accretion disk changes the surface density.