PDRs4All

Context . Infrared emission from polycyclic aromatic hydrocarbons (PAHs) plays a major role in determining the charge balance of their host environments that include photodissociation regions (PDRs) in galaxies, planetary nebulae, and rims of molecular clouds. Aims . We aim to investigate the distribution and sizes of charged PAHs across the key zones of the Orion Bar PDR, i.e., the ionization front, the atomic PDR, and the dissociation fronts. Methods . We employed JWST MIRI-MRS observations of the Orion Bar from the Early Release Science program “PDRs4All” and synthetic images in the JWST MIRI filters. We investigated the spatial morphology of the aromatic infrared bands (AIBs) at 6.2, 7.7, 8.6, and 11.0 μm (commonly tracing PAH cations) and the neutral PAH-tracing 11.2 μm AIB, their (relative) correlations, and their relationship with existing empirical prescriptions for AIBs. Results . The 6.2, 7.7, 8.6, 11.0, and 11.2 μm AIBs are similar in spatial morphology on larger scales. Aside from the 11.0 μm AIB, these AIBs exhibit enhanced intensities at the dissociation fronts. Analyzing three-feature intensity correlations, two distinct groups emerge: the 8.6 and 11.0 μm AIBs versus the 6.2 and 7.7 μm AIBs. We attribute these correlations to PAH size. The 6.2 and 7.7 μm AIBs trace cationic, medium-sized PAHs. Quantum chemical calculations reveal that the 8.6 μm AIB is carried by large, compact, cationic PAHs, and the 11.0 μm AIB's correlation with it implies that this band is as well. The 6.2/8.6 and 7.7/8.6 PAH band ratios thus probe PAH size. We conclude that the 6.2/11.2 AIB ratio is the most reliable proxy for charged PAHs within the cohort. We outline JWST MIRI imaging prescriptions that serve as effective tracers of the PAH ionization fraction, as traced by the 7.7/11.2 PAH emission. Conclusions . This study showcases the efficacy of the 6−9 μm AIBs in probing the charge state and size distribution of emitting PAHs, offering insights into the physical conditions of their host environments. JWST MIRI photometry offers a viable alternative to IFU spectroscopy for characterizing this emission in extended objects.