Shadowing instabilities of ionization fronts

Weak upstream overdensities or underdensities lead to the corrugation of R-type ionization fronts. The propagation velocity of the R-type front varies as a result of the different line-of-sight emission measures, and is also decreased because of the obliqueness of the front. The transition to D type occurs earlier in some parts of the front, and the resulting non-linear instability of the front leads to the formation of dense clumps and tails within the H ii region. In this paper, we discuss criteria for the development of this instability. In developing numerical models of this process, we have encountered numerical artefacts that have to be treated with care in high-resolution hydrodynamical simulations of H ii regions. We discuss the application of our results to the structure and development of H ii regions.