Thermal instability of a radiation-pressure-dominated alpha disk
= Thermal instability of a radiation-pressure-dominated alpha disk
At fixed surface density, a total-pressure alpha disk supported by radiation pressure has viscous heating $Q^+\propto T_c^8$ but radiative cooling $Q^-\propto T_c^4$ when electron scattering supplies a nearly constant opacity. Heating therefore wins after a positive temperature perturbation.