PDRs4All: XVIII. The evolution of the PAH ionisation and PAH size distribution across the Orion Bar
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2026
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| author | Maragkoudakis, Alexandros Boersma, Christiaan Peeters, Els Allamandola, Louis J. Temi, Pasquale Esposito, Vincent J. Bregman, Jesse D. Ricca, Alessandra Alarcón, Felipe Berné, Olivier Buragohain, Mridusmita Cami, Jan Canin, Amélie Chown, Ryan Dartois, Emmanuel Fuente, Asunción Goicoechea, Javier R. Habart, Emilie Kannavou, Olga Khan, Baria Lai, Thomas S. -Y. Onaka, Takashi Van De Putte, Dries Schroetter, Ilane Sidhu, Ameek Tielens, Alexander G. G. M. Trahin, Boris Zhang, Yong |
| author_facet | Maragkoudakis, Alexandros Boersma, Christiaan Peeters, Els Allamandola, Louis J. Temi, Pasquale Esposito, Vincent J. Bregman, Jesse D. Ricca, Alessandra Alarcón, Felipe Berné, Olivier Buragohain, Mridusmita Cami, Jan Canin, Amélie Chown, Ryan Dartois, Emmanuel Fuente, Asunción Goicoechea, Javier R. Habart, Emilie Kannavou, Olga Khan, Baria Lai, Thomas S. -Y. Onaka, Takashi Van De Putte, Dries Schroetter, Ilane Sidhu, Ameek Tielens, Alexander G. G. M. Trahin, Boris Zhang, Yong |
| contents | We investigate the evolution of the PAH population's charge state and size across key physical zones in the Orion Bar, which include the HII region, the atomic PDR (APDR), and three HI/H2 dissociation fronts (DF1, DF2, and DF3). 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" ERS Program. 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 have minimal contribution. Emission from neutral PAHs peaks in the HII region that consists of emission from a face-on PDR associated to the background OMC-1 molecular cloud, and in the molecular cloud regions past DF2. PAH anions are observed deep within the DF2 and DF3 zones. The average PAH size ranges between ~$60-74$ Nc. 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 ~$2-9 x 10^4$. Intensity ratios tracing PAH ionisation scale well with $γ$ in regions encompassing edge-on or face-on PDR emission, but their correlation weakens within the molecular cloud zone. Modelling of the $5-15$ $μ$m PAH spectrum with pyPAHdb achieves comprehensive characterization 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2601_23282 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | PDRs4All: XVIII. The evolution of the PAH ionisation and PAH size distribution across the Orion Bar Maragkoudakis, Alexandros Boersma, Christiaan Peeters, Els Allamandola, Louis J. Temi, Pasquale Esposito, Vincent J. Bregman, Jesse D. Ricca, Alessandra Alarcón, Felipe Berné, Olivier Buragohain, Mridusmita Cami, Jan Canin, Amélie Chown, Ryan Dartois, Emmanuel Fuente, Asunción Goicoechea, Javier R. Habart, Emilie Kannavou, Olga Khan, Baria Lai, Thomas S. -Y. Onaka, Takashi Van De Putte, Dries Schroetter, Ilane Sidhu, Ameek Tielens, Alexander G. G. M. Trahin, Boris Zhang, Yong Astrophysics of Galaxies Solar and Stellar Astrophysics We investigate the evolution of the PAH population's charge state and size across key physical zones in the Orion Bar, which include the HII region, the atomic PDR (APDR), and three HI/H2 dissociation fronts (DF1, DF2, and DF3). 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" ERS Program. 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 have minimal contribution. Emission from neutral PAHs peaks in the HII region that consists of emission from a face-on PDR associated to the background OMC-1 molecular cloud, and in the molecular cloud regions past DF2. PAH anions are observed deep within the DF2 and DF3 zones. The average PAH size ranges between ~$60-74$ Nc. 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 ~$2-9 x 10^4$. Intensity ratios tracing PAH ionisation scale well with $γ$ in regions encompassing edge-on or face-on PDR emission, but their correlation weakens within the molecular cloud zone. Modelling of the $5-15$ $μ$m PAH spectrum with pyPAHdb achieves comprehensive characterization 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. |
| title | PDRs4All: XVIII. The evolution of the PAH ionisation and PAH size distribution across the Orion Bar |
| topic | Astrophysics of Galaxies Solar and Stellar Astrophysics |
| url | https://arxiv.org/abs/2601.23282 |