Acoustic and Thermoreactive Instabilities in a Photoionized Multiphase Medium

We study thermoreactive and acoustic instabilities in a diffuse gas, photoionized and heated by a radiation field. The analysis of the thermal instability by Field (1965) is extended to include the effects of the hydrogen recombination reaction, which, in general, is found to be a stabilizing agent for the condensation mode. This effect is stronger when the mean photon energy is not much larger than the hydrogen ionization energy. In addition, there are thermoreactive unstable equilibria for which an isobaric transition to a stable phase is not possible and the system evolves toward a dynamic state characterized by large amplitude, nonlinear periodic oscillations of temperature, density and hydrogen ionization fraction. Acoustic waves are found to be unstable for some temperatures of both the cold and the warm phase of the ISM, also in regions where the gas is {\it thermally stable}, independently of the mean photon energy.