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

G.C. Sloan, J.D. Bregman (NASA Ames), T.R. Geballe (JAC), L.J. Allamandola (NASA Ames), C.E. Woodward (Wyoming)

1997, ApJ, 474, 735

Long-slit spectra across the Orion Bar reveal significant differences in the spatial behavior of the components of the 3 µm 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 either in attached methyl sidegroups or super-hydrogenated PAHs (H-PAHs), or perhaps both, could produce the complicated spectral and spatial structure at 3.40 µm. The distribution of these fragile PAH-based species traces the physical and chemical conditions in the interface between the ionized region and molecular cloud.

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