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