Spectral imaging of the Orion Bar at 3.3, 8.4, and 11.3 microns: Comparison with a fluorescent polycyclic aromatic hydrocarbon model

Abstract

Spectral images were obtained of the Orion Bar which sample polycyclic aromatic hydrocarbon (PAH) emission at 3.3, 8.4, and 11.3 μm. The images are strikingly different even though they all sample PAH emission. In particular, the 3.3 and 11.3 μm images sample PAH emission from C-H bonds, yet the 3.3 μm image contains many small bright knots while the 11.3 μm image is much more uniform. For comparison with a fluorescent PAH model, a data set was created from the measured intensities of 250 locations in each image. From the comparison, we conclude that: (1) the size distribution of PAHs varies within the Bar, with the bright 3.3 μm knots containing the largest proportion of small PAHs; (2) the points along the front of the Bar have emission cross sections characteristic of neutral PAHs while within the Bar, the emission cross sections are different, consistent with the PAHs being charged; (3) the PAHs along the front of the Bar are larger than average for the Bar; (4) emission along the back of the Bar is consistent with PAH emission in an attenuated UV radiation field; (5) there is no evidence for PAH dehydrogenation.