J/A+A/709/A38 PAH across the Orion Bar (Maragkoudakis+, 2026)
PDRs4All. XIX. The evolution of the PAH ionisation and PAH size distribution
across the Orion Bar.
Maragkoudakis A., Boersma C., Peeters E., Allamandola J.L., Temi P.,
Esposito J.V., Bregman D.J., Ricca A., Alarcon F., Berne O., Buragohain M.,
Cami J., Canin A., Chown R., Dartois E., Fuente F., Goicoechea J.R.,
Habart E., Kannavou O., Khan B., Lai T.S.-Y., Onaka T., Van De Putte D.,
Schroetter I., Sidhu A., Tielens A.G.G.M., Trahin B., Zhang Y.
<Astron. Astrophys. 709, A38 (2026)>
=2026A&A...709A..38M 2026A&A...709A..38M (SIMBAD/NED BibCode)
ADC_Keywords: H II regions ; Interstellar medium ; Infrared
Keywords: techniques: spectroscopic - HII regions -
photon-dominated region (PDR) - infrared: ISM -
planetary nebulae: individual: Orion Bar
Abstract:
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 PDR scales, properties closely linked to physical conditions
of their inhabiting environments.
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 bright HI/H2
dissociation fronts (DF1, DF2, and DF3). We connect changes in PAH
charge and size as probed by empirical emission proxies with the
varying physical properties of their surrounding environments.
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 was performed on the
5-15um spectrum across the entire JWST mosaic, as well as on the
weighted average spectra of the five key physical zones.
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. 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 ∼60-74NC . 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 gamma
ranges between ∼2-9x104. Intensity ratios which are empirical
tracers of PAH ionisation (I6.2 /I11.2, I7.7 /I11.2, I8.6
/I11.2) scale well with gamma in regions encompassing edge-on or
face-on PDR emission, but their correlation weakens within the
molecular cloud zone.
Modelling of the 5-15um 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.
Description:
PAH charge breakdown maps of the PAH ionisation parameter, maps of the
6.2um 7.7um, 8.6um, and 11.2um PAH flux, PAH band intensity ratio maps
and maps of the PAH ionisation parameter.
File Summary:
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Acknowledgements:
Alexandros Maragkoudakis, maragkoudakis.alex(at)gmail.com
License: CC-BY-4.0 [see https://spdx.org/licenses/]
References:
Berne et al., Paper I 2022PASP..134e4301B 2022PASP..134e4301B
Habart, et al., Paper II 2024A&A...685A..73H 2024A&A...685A..73H
Peeters et al., Paper III 2024A&A...685A..74P 2024A&A...685A..74P, Cat. J/A+A/685/A74
Chown et al., Paper IV 2024A&A...685A..75C 2024A&A...685A..75C, Cat. J/A+A/685/A75
Elyajouri et al., Paper V 2024A&A...685A..76E 2024A&A...685A..76E
Pasquini et al., Paper VI 2024A&A...685A..77P 2024A&A...685A..77P
Schroetter et al. Paper VII 2024A&A...685A..78S 2024A&A...685A..78S
Van De Putte et al., Paper VIII 2024A&A...687A..86V 2024A&A...687A..86V
Fuente et al., Paper IX 2024A&A...687A..87F 2024A&A...687A..87F
Goicoechea et al., Paper X 2024A&A...689L...4G 2024A&A...689L...4G
Zannese et al., Paper XI 2025A&A...696A..99Z 2025A&A...696A..99Z
Goicoechea et al., Paper XII 2025A&A...696A.100G 2025A&A...696A.100G
Chown et al., Paper XIIII 2025A&A...698A..86C 2025A&A...698A..86C
Khan et al., Paper XIV 2025A&A...699A.133K 2025A&A...699A.133K
Schroetter et al., Paper XV 2025A&A...699L..13S 2025A&A...699L..13S
Van De Putte et al., Paper XVI 2025A&A...701A.111V 2025A&A...701A.111V, Cat. J/A+A/701/A111
Zannese et al., Paper XVII 2026A&A...705A.128Z 2026A&A...705A.128Z
Amio et al., Paper XVIII 2026A&A...708A.111A 2026A&A...708A.111A, Cat. J/A+A/708/A111
(End) Patricia Vannier [CDS] 21-Apr-2026