Oblique refraction of an ionisation front

Abstract

GAS dynamical effects arising from the absorption of ionising radiation from an O or B group star embedded in interstellar gas have been studied by many authors, and both similarity and numerical solutions have been obtained for the unsteady expansion of the region of almost fully ionised hydrogen gas (HII region) surrounding the star^1,2. The boundary between the HII region and the neutral hydrogen comprising the HI region is sharp in comparison with the scale of the motion and is termed an ionisation front. A problem, of interest to both the astronomer and theoretical astrophysicist, concerns the interaction between an ionisation front and a cloud of high density molecular gas, as strong observational evidence indicates that dense pockets of molecular gas exist in the vicinity of some O and B stars³. This paper describes an investigation of the interaction between a planar ionisation front and a planar cloud of high density molecular gas. It is found that, in order to determine a unique solution, an extra constraint must be applied which involves the absorption of radiation within the HII region. In two-dimensional flows, however, this condition is limited to flow geometries in which the reflected shock does not interfere with the flow along lines of sight normal to the incident ionisation front. This is analogous to the refraction of shock waves where, beyond a certain angle of incidence, regular refraction will cease and the flow pattern will become irregular^4,5.