= Thermal stability of an accretion disk
\b[Thermal stability of an accretion disk] asks whether a small temperature disturbance decays while the local <surface density> is effectively fixed. Write net heating as $Q^+-Q^-$; with positive effective <heat capacity>, stability requires $\partial_T(Q^+-Q^-)<0$ at equilibrium. A hotter patch must gain a restoring excess of cooling. The comparison belongs on the <thermal timescale of an accretion disk>, before slower radial redistribution changes the column. Its outcome depends on the stress and <opacity> prescriptions. The standard total-pressure <alpha disk> closure predicts instability in a radiation-supported, electron-scattering regime.<Dynamics of Astrophysical Discs by Gordon I. Ogilvie>[Lecture9, section9.1: fixed-column perturbations and heating/cooling response]{ref}
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