PDRs4All XIX. The 6 to 9 $\mu$m region as a probe of PAH charge and size in the Orion Bar

Abstract

Infrared emission from polycyclic aromatic hydrocarbons (PAHs) play a major role in determining the charge balance of their host environments that include photo-dissociation regions (PDRs) in galaxies, planetary nebulae, and rims of molecular clouds. We aim to investigate the distribution and sizes of charged PAHs across the key zones of the Orion Bar PDR. We employ JWST MIRI-MRS observations of the Orion Bar from the Early Release Science program ''PDRs4All'' and synthetic images in the JWST MIRI filters. We investigate the spatial morphology of the AIBs at 6.2, 7.7, 8.6, and 11.0 $\mu$m that commonly trace PAH cations, and the neutral PAH-tracing 11.2 $\mu$m AIB, their (relative) correlations, and the relationship with existing empirical prescriptions for AIBs. The 6.2. 7.7, 8.6, 11.0, and 11.2 $\mu$m AIBs are similar in spatial morphology, on larger scales. Analyzing three-feature intensity correlations, two distinct groups emerge: the 8.6 and 11.0 $\mu$m vs. the 6.2 and 7.7 $\mu$m AIBs. We attribute these correlations to PAH size. The 6.2 and 7.7 $\mu$m AIBs trace cationic, medium-sized PAHs. Quantum chemical calculations reveal that the 8.6 $\mu$m AIB is carried by large, compact, cationic PAHs, and the 11.0 $\mu$m AIB's correlation to it implies, so is this band. 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. This study showcases the efficacy of the 6-9 $\mu$m AIBs to probe the charge state and size distribution of the 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.