PDRs4All

XIV. Probing CH out-of-plane bending modes of PAH molecules in the Orion Bar with JWST

¹ Department of Physics & Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
² Department of Physics, Durham University, Durham DH1 3LE, UK
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 94035-1000, USA
⁹ Institut des Sciences Moléculaires d’Orsay, Université Paris-Saclay, CNRS, Bâtiment 520, 91405 Orsay Cedex, France
¹⁰ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² School of Physics, University of Hyderabad, Hyderabad, Telangana 500046, India
¹³ Anton Pannekoek Institute for Astronomy, University of Amsterdam, Amsterdam, The Netherlands
¹⁴ Telespazio UK for ESA, ESAC, E-28692 Villanueva de la Cañada, Madrid, Spain
¹⁵ IPAC, California Institute of Technology, Pasadena, CA, USA
¹⁶ Instituto de Matemática, Estatística e Física, Universidade Federal do Rio Grande, 96201-900 Rio Grande, RS, Brazil
¹⁷ School of Physics and Astronomy, Sun Yat-sen University, 2 Da Xue Road, Tangjia, Zhuhai 519000, Guangdong Province, China
¹⁸ Astronomy Department, Ohio State University, Columbus, OH 43210, USA
¹⁹ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
²⁰ Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
²¹ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA

^★ Corresponding author: bkhan95@uwo.ca

Received: 10 February 2025
Accepted: 3 April 2025

Abstract

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.