The Orion HII Region and the Orion Bar in the Mid-Infrared

Abstract

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 $\simeq 0.06$~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.

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