Spatial Analysis of the Polycyclic Aromatic Hydrocarbon Features Southeast of the Orion Bar

Abstract

Polycyclic aromatic hydrocarbon (PAH) and dust emission features between 10 and 37 μm, observed with Spitzer at 11 positions southeast of the Bright Bar (BB) in Orion, are analyzed and connected to atomic and H₂ lines reported earlier. Variations at these positions indicate changes in local conditions and materials sampled. The major findings are: (1) PAH erosion and destruction are important from the BB out to about 5'. (2) The ionized PAH fraction, inferred from the 11.0 μm PAH band, increases from the BB out to 6farcm5. This counterintuitive behavior is linked to PAH dehydrogenation. (3) The "11.2" μm PAH band profile shifts from class A _11.2 to A(B)_11.2 between 9' and 10', indicating these lines-of-sight probe a different environment, likely shielded molecular cloud material. (4) The different spatial behavior of the PAH bands and the 10-15 μm plateau supports the view that the plateau originates in a separate carrier. (5) The fullerene/PAH band strength ratio decreases out to about 7', increases between 9' and 10' and drops at 12'. The first region is where PAHs are dehydrogenated and eroded whereas the second, shielded zone, is where the "11.2" μm profile shifts and PAH erosion is unlikely. This suggests fullerenes are intimately mixed with PAHs in shielded regions. Taken together, the observations suggest three different regimes are sampled: (1) the H II region-photodissociation region (PDR) interface directly southeast of the BB, (2) shielded molecular cloud material farther out, and (3) the H II region-PDR interface seen limb brightened at the outermost position.