Structure and dynamics of H II regions

The early evolution of H II regions, through the first few times 105 years, is investigated under conditions of enhanced central density and increasing cluster luminosity. It it found that a D-type ionization front can overtake the precursor shock under conditions of a fairly steep density gradient such as may exist just beyond a stellar dust cocoon, or as new stars turn on within the H II region. The shock is converted into a strong, outward moving pressure pulse as it is ionized. The resulting large-scale mass motions produce splitting of such emission lines as [O III] 52 μm, while lines such as H109α become significantly broadened. Rayleigh-Taylor instability in the shocked layer occurs as the ionization front approaches the shock, but the neutral globules so produced have a comparatively small and short-term effect on nebular dynamics. The evolutionary effects of different recombination rates within the ionization front and different cooling rates within the shocked layer are investigated and found to be small. Possible modifications which the presence of dust or stellar winds could produce are briefly considered