The Orion Nebula in the Mid-Infrared

We present two wide-field (≈5′ × 3.′5), diffraction-limited (λ/D ≃ 0.″5 at 10 μm), broadband 10 and 20 μm images of the Orion Nebula, plus six 7–13 μm narrowband (λ/Δλ ≃ 1) images of the BN/KL complex taken at the 3.8 m UKIRT telescope with the MPIA MAX camera. The wide-field images, centered on the Trapezium and BN/KL regions, are mosaics of 35″ × 35″ frames obtained with standard chopping and nodding techniques and reconstructed using a new restoration method developed for this project. They show the filamentary structure of the dust emission from the walls of the H II region and reveal a new remarkable group of arclike structures ≈1′ to the south of the Trapezium. The morphology of the Ney-Allen Nebula, produced by wind-wind interaction in the vicinity of the Trapezium stars, suggests a complex kinematical structure at the center of the cluster. We find indications that one of the most massive members of the cluster, the B0.5 V star θ1 Ori D, is surrounded by a photoevaporated circumstellar disk. Among the four historic Trapezium OB stars, this is the only one without a binary companion, suggesting that stellar multiplicity and the presence of massive circumstellar disks may be mutually exclusive. In what concerns the BN/KL complex, we find evidence for extended optically thin silicate emission on top of the deep 10 μm absorption feature. Assuming a simple two-component model, we map with ≃0.″5 spatial resolution the foreground optical depth, color temperature, and mid-IR luminosity of the embedded sources. We resolve a conspicuous point source at the location of the IRc2-A knot, approximately 0.″5 north of the deeply embedded H II region "I." We analyze the spectral profile of the 10 μm silicate absorption feature and find indication for grain crystallization in the harsh nebular environment. In the OMC-1 South region, we detect several point sources and discuss their association with the mass-loss phenomenology observed at optical and millimeter wavelengths. Finally, we list the position and photometry of 177 point sources, the large majority of which are detected for the first time in the mid-IR. Twenty-two of them lack a counterpart at shorter wavelengths and are therefore candidates for deeply embedded protostars. The comparison of photometric data obtained at two different epochs reveals that source variability at 10 μm is present up to a level of ≈1 mag on a timescale of ∼2 yr. With the possible exception of a pair of OB stars, all point sources detected at shorter wavelengths display 10 μm emission well above the photospheric level, which we attribute to disk circumstellar emission. The recent model of Robberto et al. provides the simplest explanation for the observed mid-IR excess.