Spitzer and Herschel multiwavelength characterization of the dust content of evolved HII regions

TL;DR

This study characterizes the dust content in 16 evolved HII regions using multiwavelength data, revealing cold and warm dust components, dust depletion mechanisms, and diagnostic IR colors for identifying such regions.

Contribution

It provides a detailed multiwavelength analysis of dust in evolved HII regions, introducing a two-component grey-body model and identifying IR color diagnostics for these regions.

Findings

Cold dust temperature range 20-30 K

Warm dust temperature range 50-90 K

Dust depletion likely driven by radiation pressure

Abstract

We have analyzed a uniform sample of 16 evolved HII regions located in a 2 deg X 2 deg Galactic field centered at (l,b) = (30 deg, 0 deg) and observed as part of the Herschel Hi-GAL survey. The evolutionary stage of these HII regions was established using ancillary radio continuum data. By combining Hi-GAL PACS (70 micron, 160 micron) and SPIRE (250 micron, 350 micron and 500 micron) measurements with MIPSGAL 24 micron data, we built Spectral Energy Distributions (SEDs) of the sources and showed that a 2-component grey-body model is a good representation of the data. In particular, wavelengths > 70 micron 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 lambda < 70 micron, the data indicated 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 HII regions, although likely not in a large amount. In addition, the data appear to corroborate the hypothesis that the main mechanism responsible for the (partial) depletion of dust in HII regions is radiation-pressure-driven drift. In this framework, we speculated that the 24 micron emission which spatially correlates with ionized gas might be associated with either Very Small Grain (VSG) or BG replenishment, as recently proposed for the case of Wind-Blown Bubbles (WBB). Finally, we found that evolved HII regions are characterized by distinctive far-IR and sub-mm colors, which can be used as diagnostics for their identification in unresolved Galactic and extragalactic regions.