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 μm polycyclic aromatic hydrocarbon (PAH) spectrum. The strong PAH band at 3.29 μm generally decreases exponentially with distance from the ionization front into the molecular cloud (scale height ∼12″), although excesses appear ∼10″ and ∼20″ behind the ionization front, close to layers of H₂ and CO emission, respectively. The 3.40 μm PAH feature separates into two components with very different spatial distributions. The main component (at 3.395 μm), along with the 3.51 μm band and the PAH plateau (3.3-3.6 μm), shows excess emission ∼10″ and ∼20″ behind the ionization front, stronger than the excesses in the 3.29 μm band. The extra component of the 3.40 μm band, which peaks at ∼ 3.405 μm, has a spatial distribution very similar to the H₂ emission. Aromatic C - H stretches in PAHs most likely produce the 3.29 μm 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 μm.