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circumstellar matter—dust, extinction—galaxies: starburst—H II regions—infrared: ISM—techniques: spectroscopic
Abstract:
This paper presents both stellar mass and H II region diagnostics based on dusty, radiation-pressure dominated photoionization models for compact and ultra-compact H II regions, and compares these with observational constraints. These models, based upon the TLUSTY model stellar atmospheres, 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 should predict, improving the fit with te observations. The models provide a good fit to observed diagnostic plots involving rations of infrared emission lines, all accessible with the IRS instrument of the Spitzer Space Telescope. These are excellent diagnostics for the effective temperature, or for the mass of the ionizing star in the case of H II regions excited by a single star, although somewhat sensitive to chemical abundances, they allow the effective temerature to be determined to an accuracy of about 2500K, and the mass to a precision of about +/- 30%. The S IV line lies in the silicate absorption feature, making the [S IV]/[S III] ratio sensitiv 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 ~ 30mag. The electron temperature depends upon the chemical abundances, the pressure and the effective temperature of the exciting star. We use this model to re-derive the slope of the galactic abundance gradient, with the result that dlog (O/H)/dRg = 0.06 +/- 0.01dex kpc-1. This brings the galactic abundance gradient derived from compact H II regions into closer agreement with other techniques. Finally, we find that the shape of the SED of compact H II regions can be used to constrain the mean pressure or density in the H II region, and that the MIPS instrument of the Spitzer Space Telescope should be very helpful in this regard.
