ON RADIATION PRESSURE IN STATIC, DUSTY H II REGIONS

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: beta, gamma, and the product Q(0)n(rms); beta characterizes the stellar spectrum, gamma characterizes the dust/gas ratio, Q(0) is the stellar ionizing output (photons/s), and n(rms) is the rms density within the ionized region. Adopting standard values for beta and gamma, varying Q(0)n(rms) generates a one-parameter family of density profiles, ranging from nearly uniform density (small Q(0)n(rms)) to shell-like (large Q(0)n(rms)). When Q(0)n(rms) greater than or similar to 10(52) cm(-3) s(-1), dusty H II regions have conspicuous central cavities, even if no stellar wind is present. For given beta, gamma, and Q(0)n(rms), a fourth quantity, which can be Q(0), 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 beta, gamma, and Q(0)n(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.