Detecting stellar-wind bubbles through infrared arcs in HII regions
  Mid-infrared arcs of dust emission are often seen near ionizing stars within
HII regions. A possible explanations for these arcs is that they could show the
outer edges of asymmetric stellar wind bubbles. We use two-dimensional,
radiation-hydrodynamics simulations of wind bubbles within HII regions around
individual stars to predict the infrared emission properties of the dust within
the HII region. We assume that dust and gas are dynamically well-coupled and
that dust properties (composition, size distribution) are the same in the HII
region as outside it, and that the wind bubble contains no dust. We
post-process the simulations to make synthetic intensity maps at infrared
wavebands using the TORUS code. We find that the outer edge of a wind bubble
emits brightly at 24um through starlight absorbed by dust grains and
re-radiated thermally in the infrared. This produces a bright arc of emission
for slowly moving stars that have asymmetric wind bubbles, even for cases where
there is no bow shock or any corresponding feature in tracers of gas emission.
The 24um intensity decreases exponentially from the arc with increasing
distance from the star because the dust temperature decreases with distance.
The size distribution and composition of the dust grains has quantitative but
not qualitative effects on our results. Despite the simplifications of our
model, we find good qualitative agreement with observations of the HII region
RCW120, and can provide physical explanations for any quantitative differences.
Our model produces an infrared arc with the same shape and size as the arc
around CD -38 11636 in RCW120, and with comparable brightness. This suggests
that infrared arcs around O stars in HII regions may be revealing the extent of
stellar wind bubbles, although we have not excluded other explanations.
