Thermal Instability in Radiation Hydrodynamics: Instability Mechanisms, Position-dependent S-curves, and Attenuation Curves

Abstract Local thermal instability can plausibly explain the formation of multiphase gas in many different astrophysical environments, but theory local TI is only well-understood optically thin limit equations radiation hydrodynamics (RHD). Here, we lay groundwork for transitioning from this to a full RHD treatment assuming gray opacity formalism. We consider situation where becomes thermally unstable due hardening field when main radiative processes are free–free cooling and Compton heating. identify two ways which happen: (i) temperature increases with time, through rise either intensity or energy hard X-ray component; (ii) attenuation reduces flux component such that depth slab. Both likely occur broad-line region active galactic nuclei columns be ionization-bounded. In instances significant, equilibrium solution curves become position-dependent it no longer suffices assess stability an irradiated column at all depths using single curve. demonstrate how analyze new curve—the curve—for purpose, show that, by Field’s criterion, negative slope along curve indicates constant-density slabs whenever depth.