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Session 106 - HII Regions and Ultracompact HII Regions.
Display session, Thursday, January 16
Metropolitan Ballroom,
We present long-slit spectroscopy of the Orion Bar at 8--13 \mum, obtained 1996 September 28-30 at the 5-m Hale telescope of Palomar Observatory, using Spectro-Cam 10. The Bar is a photo-dissociation region viewed edge-on, so the layering produced from the attenuation of the UV radiation field is easily distinguishable to the observer. We oriented the spectrometer slit perpendicular to the Bar in order to trace the behavior of the PAH emission features at 8.6, 11.3, and 12.7 \mum and their dependence on the UV field. The slit position used is identical with the 3 \mum data published by Sloan et al. (1997, ApJ, 474, in press), allowing us to directly compare the behavior of the PAH features in these two spectral regions.
The 11.3 \mum emission band dominates the mid-infrared spectrum, just as the 3.29 \mum band dominates the 3 \mum spectrum. Both are produced from aromatic C--H bonds, but the intensity of the 11.3 \mum band decreases more slowly into the neutral region. Its behavior resembles that of the 3.40 and 3.51 \mum bands and the 3 \mum PAH plateau, suggesting a common carrier for all of these bands. This finding is consistent with the hypotheses that larger and more stable PAHs emit more strongly at 11.3 \mum and that they also produce the 3 \mum PAH plateau. Compared to the 8.6 \mum feature, the 11.3 \mum feature survives further into the ionized region. This behavior is opposite to that expected from ionized PAHs, which would show a reduction in the strength of the 11.3 \mum feature relative to the 8.6 \mum feature.
