2025A&A...699A.133K

Context. The infrared universe is dominated by emission from polycyclic aromatic hydrocarbons (PAHs) observed as aromatic infrared bands (AIBs). JWST has produced a rich trove of information on these PAH signatures.
Aims. We aim to investigate the photochemical evolution of PAHs in photodissociation regions (PDRs), focusing on their molecular edge structures across key zones, including the H II region, the ionization front, the atomic PDR, the dissociation front, and the molecular PDR.
Methods. We utilized JWST's MIRI-MRS observations of the Orion Bar for the PDRs4All JWST Early Release Science program. We investigated the spectral and spatial characteristics of 10-15 µm AIBs.
Results. The AIBs at 10.6, 10.8, 11.0, 11.2, 12.0, 12.7, 13.5, 14.0, and 14.2 µm share large-scale spatial morphologies, peaking in the atomic PDR and gradually declining with distance from the PDR surface. Correlations between the AIBs reveal that they are largely carried by PAHs. Profile variations and subcomponents of the 11.2 and 12.0 µm AIBs reveal a carrier that behaves independently of PAHs, which we attribute to very small grains (VSGs) and/or PAH clusters. We ascribe the 11.0 and 11.207 µm AIBs, part of the 12.0 µm AIB, and the 12.7, 13.5, and 14.2 µm AIBs to CH_oop modes and discuss their hydrogen-adjacency assignments. We propose that the 10.6, 10.8, and 14.0 µm AIBs do not arise from CH_oop modes. We derived the relative amounts of solo, duo, trio, and quartet CH groups to infer the molecular structures. These suggest that PAHs are dominated by solo and trio CH groups throughout the PDR. We attribute the decrease in duo and quartet CH groups relative to the solo CH groups toward the PDR surface to the effects of photolysis of the labile hydrogens.
Conclusions. The 10-15 µm AIBs are powerful probes of the PAH molecular structures. This study showcases the spatial and spectral variability in CH_oop features due to photochemical processing of PAHs, and the differentiated spectral characteristics of PAHs and VSGs, in a prototypical PDR.