Variations in the 3 micron spectrum across the Orion Bar: Polycyclic aromatic hydrocarbons and related molecules

Long-slit spectra across the Orion Bar reveal significant differences in the spatial behavior of the components of the 3 mu m polycyclic aromatic hydrocarbon (PAH) spectrum. The strong PAH band at 3.29 mu m generally decreases exponentially with distance from the ionization front into the molecular cloud (scale height similar to 12 ''), although excesses appear similar to 10 '' and similar to 20 '' behind the ionization front, close to layers of H-2 and CO emission, respectively. The 3.40 mu m PAH feature separates into two components with very different spatial distributions. The main component (at 3.395 mu m), along with the 3.51 mu m band and the PAH plateau (3.3-3.6 mu m), shows excess emission similar to 10 '' and similar to 20 '' behind the ionization front, stronger than the excesses in the 3.29 mu m band. The extra component of the 3.40 mu m band, which peaks at similar to 3.405 mu m, has a spatial distribution very similar to the H-2 emission. Aromatic C-H stretches in PAHs most likely produce the 3.29 mu m feature. Aliphatic C-H stretches in either attached methyl side-groups or superhydrogenated PAHs, or perhaps both, could produce the complicated spectral and spatial structure at 3.40 mu m.