Spitzer and Herschel Multiwavelength Characterization of the Dust Content of Evolved H II Regions

We have analyzed a uniform sample of 16 evolved H II regions located in a 2° × 2° Galactic field centered at (l,b) = (30°, 0°) and observed as part of the Herschel Hi-GAL survey. The evolutionary stage of these H II regions was established using ancillary radio-continuum data. By combining Hi-GAL PACS (70 μm, 160 μm) and SPIRE (250 μm, 350 μm, and 500 μm) measurements with MIPSGAL 24 μm data, we built spectral energy distributions of the sources and showed that a two-component gray-body model is a good representation of the data. In particular, wavelengths >70 μm appear to trace a cold dust component, for which we estimated an equilibrium temperature of the big grains (BGs) in the range 20-30 K, while for λ < 70 μm, the data indicate the presence of a warm dust component at temperatures of the order of 50-90 K. This analysis also revealed that dust is present in the interior of H II regions, although likely not in a large amount. In addition, the data seem to corroborate the hypothesis that the main mechanism responsible for the (partial) depletion of dust in H II regions is radiation-pressure-driven drift. In this framework, we speculated that the 24 μm emission that spatially correlates with ionized gas might be associated with either very small grain or BG replenishment, as recently proposed for the case of wind-blown bubbles. Finally, we found that evolved H II regions are characterized by distinctive far-IR and submillimeter colors, which can be used as diagnostics for their identification in unresolved Galactic and extragalactic regions.