Spatial Distribution of Dust Grains within HiiRegions

We discuss the dust distribution within photoionized regions. Assuming ageometry with a central dust cavity, which is strongly suggested by theliterature, we can estimate the cavity's radius from the ratio of the infraredand radio fluxes by using a simple transfer model of Lyman continuum photons.We apply the method to a sample of the Galactic H {\sc ii} regions. Theestimated typical radius of the dust cavity of the Galactic compact H {\sc ii}regions is about 30% of the Str{\"o}mgren radius. Taking account ofuncertainties both of the observational data and the model, we can reject adust distribution lacking a central cavity. Therefore, the dust cavity model issupported independently of the previous works. We discuss the formationmechanism of such a dust cavity and its detectability by present and futureinfrared facilities.Comment: 14 pages (emulateapj), 3 figures, accepted by Ap