PDRs4All

XX. The 6–9 μm region as a probe of PAH charge and size in the Orion Bar

¹ Department of Physics & Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
² Universidad Nacional de Colombia, Ave Cra 30 # 45-3, Bogotá, Colombia
³ Institute for Earth and Space Exploration, The University of Western Ontario, London, ON N6A 3K7, Canada
⁴ Carl Sagan Center, SETI Institute, 339 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA
⁵ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁶ Astronomy Department, University of Maryland, College Park, MD 20742, USA
⁷ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan
⁸ NASA Ames Research Center, MS 245-6, Moffett Field, CA 940351000, USA
⁹ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
¹⁰ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
¹¹ Astronomy Department, Ohio State University, Columbus, OH 43210, USA
¹² Institut des Sciences Moléculaires d'Orsay, Université Paris-Saclay, CNRS, Bâtiment 520, 91405 Orsay Cedex, France
¹³ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
¹⁴ Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
¹⁵ Department of Physics, Institute of Science, Banaras Hindu University (BHU), Varanasi 221005, India
¹⁶ Astrophysical Institute and University Observatory, Schillergässchen 2-3, 07745 Jena, Germany
¹⁷ Instituto de Matemática, Estatística e Física, Universidade Federal do Rio Grande, 96201-900, Rio Grande, RS, Brazil
¹⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁹ School of Physics and Astronomy, Sun Yat-sen University, 2 Da Xue Road, Tangjia, Zhuhai 519000, Guangdong Province, China
²⁰ Atomic Physics Division, Stockholm University, 106 91 Stockholm, Sweden

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 2 October 2025
Accepted: 5 February 2026

Abstract

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.