The gas dynamics around OB association. II - Dependence on the initial ambient density

One- and two-dimensional calculations of the gasdynamical evolution of H II regions excited by an evolving OB association are presented. The various phases (classical expansion, recombination, reionization and full recombination, phases I-IV, respectively) are studied and found to depend not only on the behaviour of the stellar photon flux, but also on the initial value of the ambient density. Phase II, (recombination) leads to the formation of a large expanding thick neutral shell. However, for large values of the ambient density, phase II can be inhibited and the H II region will continue to expand according to the classical formulation, even though the ionizing photon flux from the source continues to decay. The latter evolution will make the H II region appear as a ring-like nebula for a considerable period of time. Many of the galactic and extragalactic ring-like nebulae and expanding neutral shells, usually interpreted as the result of supernova explosions or stellar winds, can thus be explained with the present calculations.