Herschel characterization of dust in Galactic evolved HII regions
Paladini, R., Veneziani, M., Umana, G., Noriega-Crespo, A., Anderson, L. D., Piacentini, F., Paradis, D., Bernard, J.-P., Natoli, P.
We have analyzed a uniform sample of 16 HII regions located in a 2×2 degree field observed as part of the Herschel Hi-Gal survey. The field, centered at (l, b) = (30°, 0°), is characterized by the presence of several HII regions, many of which are evolved. The sources have been selected based on the availability of ancillary radio data, which allow us to contrain their evolutionary stage, hence to guarantee uniformity of the sample. By combining Hi-Gal PACS (70 micron and 160 micron) and SPIRE (250 micron, 350 micron and 500 micron) measurements, with MIPS 24 micron and Bolocam 1.1 mm data, we build the Spectral Energy Distributions (SEDs) of the sources. The SED fits clearly show that, with respect to an isothermal greybody model, a 2-component model is a better representation of the data.
The analysis also evidences the clear presence of dust in the ionized gas associated to the HII regions. In particular, we obtain that wavelengths greater than 160 micron trace the cold dust component, for which we estimate an equilibrium temperature of the Big Grains (BGs) in the range 20−30 K. At wavelengths shorter than 160 micron, the data indicate either the presence of a warm dust component surrounding the cold component, or of a population of Very Small Grains stochastically heated at temperatures of order of 50 - 90 K, possibly spatially co-exhisting with the cold BGs population. Evidence for Polycyclic Aromatic Hydrocarbons (PAHs) depletion is also found in most of the sources in the sample. Finally, we find that evolved HII regions present distinctive far-IR and sub-mm colors, which can be used as diagnostics for their identification.