The influence of relaxation processes on acoustic perturbations in HII regions

The propagation of small-amplitude waves, taking into account the kinetics of doubly ionized oxygen, and the stability of acoustic perturbations in partially ionized hydrogen ale considered. The dispersion relation is analyzed using analytical and numerical methods. For typical HII regions, the presence of OIII ions does not strongly decrease wave damping; thus, previous conclusions that dissipation can considerably and ct the irregularity spectrum remain valid. The motion at lone wavelengths is not isothermal, and velocity irregularities should be accompanied by appreciable temperature variations. At short perturbation wavelengths, the predicted rms temperature fluctuations agree fairly well with the observed turbulent velocities in the Orion Nebula. Travelling-wave instabilities can develop in partially ionized hydrogen. This can strongly affect the dynamics of perturbations between the ionization front and the preceding shock wave.