PDRs4All

XIX. The evolution of the PAH ionisation and PAH size distribution across the Orion Bar

¹ NASA Ames Research Center, MS 245-6, Moffett Field, CA 940351000, USA
² Department of Physics & Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
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 Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
⁶ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
⁷ School of Physics, University of Hyderabad, Hyderabad, Telangana 500046, India
⁸ Faculty of Computer Science & Technology, Algoma University, Sault Ste. Marie, ON P6A 2G4, Canada
⁹ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
¹⁰ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
¹¹ IPAC, California Institute of Technology, Pasadena, CA, USA
¹² Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan
¹³ Astronomy Department, University of Maryland, College Park, MD 20742, USA
¹⁴ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁵ School of Physics and Astronomy, Sun Yat-sen University, 2 Da Xue Road, Tangjia, Zhuhai 519000, Guangdong Province, China

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

Received: 25 August 2025
Accepted: 29 January 2026

Abstract

Context. JWST observations of the Orion Bar have revealed rich and diverse polycyclic aromatic hydrocarbon (PAH) emission. These observations allow for the first time a comprehensive characterisation of the charge state and size of the PAH population on morphologically resolved photodissociation regions (PDR) scales, properties closely linked to physical conditions of their inhabiting environments.

Aims. We investigate the evolution of the PAH population’s charge state and size across key physical zones in the Orion Bar, which include the H ii region, the atomic PDR (APDR), and three bright H I/H₂ dissociation fronts (DF1, DF2, and DF3). We connect changes in the PAH charge and size as probed by empirical emission proxies with the varying physical properties of their surrounding environments.

Methods. Utilising the NASA Ames PAH Infrared Spectroscopic Database (PAHdb) and the pyPAHdb spectral modelling tool, we analysed the MIRI-MRS observations of the Orion Bar from the ‘PDRs4All’ JWST Early Release Science Program. Decomposition and modelling were performed on the 5−15 μm spectrum across the entire JWST mosaic, as well as on the weighted average spectra of the five key physical zones.

Results. pyPAHdb modelling reveals the fractional contribution of the different PAH charge states and sizes to the total PAH emission across the Orion Bar. Cationic PAH emission peaks in the APDR region, where neutral PAHs make a minimal contribution. Emission from neutral PAHs peaks in the H ii region that consists of emission from a face-on PDR associated with the background OMC-1 molecular cloud, and in the molecular cloud regions past DF2. The PAH anions are observed deep within the DF2 and DF3 zones. Small and medium-sized PAHs make up ∼ 70% of the PAH emission across the mosaic, with the peak of the small PAH emission found between the DF2 and DF3 zones. The average PAH size in the Orion Bar ranges between ∼ 60−74 N_C. The modelling reveals regions of top-down PAH formation at the ionisation front, and bottom-up PAH formation within the molecular cloud region. The PAH ionisation parameter, γ, ranges between ∼ 2−9 × 10⁴. Intensity ratios that are empirical tracers of PAH ionisation (I_6.2/I_11.2, I_7.7/I_11.2, I_8.6/I_11.2) scale well with γ in regions encompassing edge-on or face-on PDR emission, but their correlation weakens within the molecular cloud zone.

Conclusions. Modelling of the 5−15 μm PAH spectrum with pyPAHdb achieves a comprehensive characterisation of the net contribution of neutral and cationic PAHs across different environments, whereas empirical PAH proxy intensity ratio tracers can be highly variable and unreliable outside regions dominated by PDR emission. The derived average PAH size in the different physical zones is consistent with a view of PAHs being more extensively subjected to ultraviolet processing closer to the ionisation front, and less affected within the molecular cloud.