Compact H II Regions: What Lies Within?

This paper presents both stellar mass and H II region diagnostics based on dusty, radiation-pressure-dominated photoionization models for compact and ultracompact H II regions, and compares these with observational constraints. These models successfully reproduce the observed relationship between the density and the thickness of the ionized layer. The absorption of ionizing photons in the dusty ionized plasma makes denser ionized regions thinner than simple photoionization models would predict, improving the fit with the observations. The models provide a good fit to observed diagnostic plots involving ratios of infrared emission lines, all accessible with the IRS instrument of the Spitzer Space Telescope. These give the effective temperature to an accuracy of about 2500 K and the mass of the ionizing star to a precision of about +-30%. The S IV/S III ratio is sensitive to foreground extinction as well as to stellar effective temperature or mass. From this ratio, we determine that the mean extinction to observed compact H II regions is typically AV ~ 30 mag. The electron temperature depends on the chemical abundances, the pressure, and the effective temperature of the exciting star. We use these models to rederive the slope of the Galactic abundance gradient, with the result that d log(O/H)/dRG = 0.06 +- 0.01 dex kpc-1, bringing the Galactic abundance gradient derived from compact H II regions into closer agreement with those based on other techniques. The shape of the far-IR SED of compact H II regions can be used to constrain the mean pressure or density in the H II region. The Spitzer MIPS instrument should be very helpful in this regard.