Evolution and saturation of Autowaves in photodissociation regions

Abstract

The propagation of plane, cylindrical, and spherical waves in a thermally unstable gas–dust medium has been simulated numerically. As applied to the photodissociation regions near O and B stars, we take into account the interaction of ultraviolet radiation with dust grains and large polycyclic aromatic hydrocarbon molecules as well as the gas cooling through the excitation of СII ions and OI atoms and the deexcitation of rotational levels of CO molecules. The instability regions have been determined. The perturbation growth times corresponding to them are ~10³−10⁵ yr. We show that wave breaking occurs irrespective of the geometry of motion, while a perturbation in the form of a single pulse gives rise to a sequence of shock waves. The post-shock gas velocity is approximately 0.1−0.5 of the sound velocity, so that the autowaves can contribute noticeably to the observed velocity dispersion of the gas near the boundaries of HII regions. Two-dimensional simulations suggest that the presence of multiple shocks in a thermally unstable medium can accelerate significantly the destruction of preexisting isolated condensations.