The Orion HII Region and the Orion Bar in the Mid-Infrared
  We present mid-infrared photometry of the Orion Bar obtained with FORCAST
aboard SOFIA at 6.4, 6.6, 7.7, 19.7, 31.5 and 37.1 \um. By complementing this
observations with archival FORCAST and \emph{Herschel}/PACS images we are able
to construct a complete infrared spectral energy distribution of the Huygens
region in the Orion nebula By comparing the infrared images with gas tracers,
we find that PACS maps trace the molecular cloud, while the FORCAST data trace
the photodissociation region (PDR) and HII region. Analysis of the energetics
of the region reveal that the PDR extends for 0.28~pc along the line-of-sight
and that the Bar is inclined at an angle of $4\degr$. The infrared and
submillimeter images reveal that the Orion Bar represents a swept up shell with
a thickness of 0.1~pc. The mass of the shell implies a shock velocity of
$\simeq 3$ km/s and an age of $\simeq 10^5$ yr for the HII region. Our analysis
shows that the UV and infrared dust opacities in the HII region and the PDR are
a factor 5 to 10 lower than in the diffuse interstellar medium. In the ionized
gas, Ly$\alpha$ photons are a major source of dust heating at distances larger
than $\simeq0.06$~pc from \toc. Dust temperatures can be explained if the size
of the grains is between 0.1 to 1~\um. We derive the photo-electric heating
efficiency of the atomic gas in the Orion Bar. The results are in good
qualitative agreement with models and The quantitative differences indicate a
decreased PAH abundance in this region.
