SIMBAD references

Radiation pressure acting on gas and dust causes H II regions to have central densities that are lower than the density near the ionized boundary. H II regions in static equilibrium comprise a family of similarity solutions with three parameters: β, γ, and the product Q₀n_rms; β characterizes the stellar spectrum, γ characterizes the dust/gas ratio, Q₀ is the stellar ionizing output (photons/s), and n_rmsis the rms density within the ionized region. Adopting standard values for β and γ, varying Q₀n_rms generates a one-parameter family of density profiles, ranging from nearly uniform density (small Q₀n_rms) to shell-like (large Q₀n_rms). When Q₀n_rms ≳ 10⁵²/cm3/s, dusty H II regions have conspicuous central cavities, even if no stellar wind is present. For given β, γ, and Q₀n_rms, a fourth quantity, which can be Q₀, determines the overall size and density of the H II region. Examples of density and emissivity profiles are given. We show how quantities of interest–such as the peak-to-central emission measure ratio, the rms-to-mean density ratio, the edge-to-rms density ratio, and the fraction of the ionizing photons absorbed by the gas–depend on β, γ, and Q₀n_rms. For dusty H II regions, compression of the gas and dust into an ionized shell results in a substantial increase in the fraction of the stellar photons that actually ionize H (relative to a uniform-density H II region with the same dust/gas ratio and density n = n_rms). We discuss the extent to which radial drift of dust grains in H II regions can alter the dust-to-gas ratio. The applicability of these solutions to real H II regions is discussed.