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Session 20 - Molecular Clouds, Dense Cores, and Protostars.
Display session, Monday, January 15
North Banquet Hall, Convention Center
We have obtained narrow-band images of M17 SW and the Orion Bar in the PAH emission bands (at 3.29 and 3.42 \mum) and plateau (at 3.36 \mum). The emission bands at 3.3 and 3.4 \mum arise from the photodissociation regions (PDRs) between ionized gas and neutral gas. In both the Orion Bar and M17 SW, the PDRs present nearly edge-on geometry, providing excellent opportunities for comparing models of PDRs with observations. The 3.29 and 3.36 \mum images correlate very well, suggesting a common carrier for both the main PAH band and underlying plateau. We observe an exponential drop in the strength of the 3.3 \mum emission with a 1/e folding distance of 9\arcsec in Orion and 5\arcsec in M17 SW, confirming previous imaging by Giard et al. (1994, Aamp;A, \bf291, 239). These results imply that the mean density is 2.4 times higher in the Orion Bar than in M17 SW. Other investigators using PDR models and observations of H_2 and CO find that the UV field in the Orion Bar has a 1/e folding distance of only 3\arcsec. The discrepancy with our value can be resolved if the PDR has a clumpy distribution. The PAHs trace the UV field directly, but the H_2 and CO arise from the shielded regions. The interclump medium contains processed PAHs with feature strength ratios (3.4/3.3 \mum) of only \sim 0.14, while the clumps contain material with strength ratios as high as unity. These results support the conclusion of Geballe et al. (1994, ApJ, 434, L15) that the 3.4 \mum feature arises from a C-H stretch in aliphatic sidegroups attached to the PAH molecules. We suggest the PAHs within the clumps represent primitive material shielded from the harsh UV environment of the Orion region which has stripped the sidegroups away from the PAHs in the interclump volume.
