Schematic picture of our model. The bright star(s) is located in the middle, surrounded by the expanding H II region with radius . Outside the H II region is dust-cleansed neutral gas, surrounded by the dust front. The proportions in the figure are not realistic: the true thickness of the cleansed region is only a few percent of the radius of the H II region, and the thickness of the dust front is in turn about 1% of that of the cleansed region.