SPECTRAL IMAGING OF THE ORION BAR AT 3.3, 8.4, AND 11.3 MICRONS - COMPARISON WITH A FLUORESCENT POLYCYCLIC AROMATIC HYDROCARBON MODEL

Spectral images were obtained of the Orion Bar which sample polycyclic aromatic hydrocarbon (PAH) emission at 3.3, 8.4, and 11.3 mum. The im ages are strikingly different even though they all sample PAH emission . In particular, the 3.3 and 11.3 mum images sample PAH emission from C-H bonds, yet the 3.3 mum image contains many small bright knots whil e the 11.3 mum image is much more uniform. For comparison with a fluor escent 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 br ight 3.3 mum knots containing the largest proportion of small PAHs; (2 ) the points along the front of the Bar have emission cross sections c haracteristic of neutral PAHs while within the Bar, the emission cross sections are different, consistent with the PAHs being charged; (3) t he 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 emissi on in an attenuated UV radiation field; (5) there is no evidence for P AH dehydrogenation.