PDRs4All
Authors: Chown Ryan , Sidhu Ameek , Peeters Els , Tielens Alexander G. G. M. , Cami Jan , Berné Olivier , Habart Emilie , Alarcón Felipe , Canin Amélie , Schroetter Ilane , Trahin Boris , Van De Putte Dries , Abergel Alain , Bergin Edwin A. , Bernard-Salas Jeronimo , Boersma Christiaan , Bron Emeric , Cuadrado Sara , Dartois Emmanuel , Dicken Daniel , El-Yajouri Meriem , Fuente Asunción , Goicoechea Javier R. , Gordon Karl D. , Issa Lina , Joblin Christine , Kannavou Olga , Khan Baria , Lacinbala Ozan , Languignon David , Le Gal Romane , Maragkoudakis Alexandros , Meshaka Raphael , Okada Yoko , Onaka Takashi , Pasquini Sofia , Pound Marc W. , Robberto Massimo , Röllig Markus , Schefter Bethany , Schirmer Thiébaut , Vicente Sílvia , Wolfire Mark G. , Zannese Marion , Aleman Isabel , Allamandola Louis , Auchettl Rebecca , Baratta Giuseppe Antonio , Bejaoui Salma , Bera Partha P. , Black John H. , Boulanger François , Bouwman Jordy , Brandl Bernhard , Brechignac Philippe , Brünken Sandra , Buragohain Mridusmita , Burkhardt Andrew , Candian Alessandra , Cazaux Stéphanie , Cernicharo Jose , Chabot Marin , Chakraborty Shubhadip , Champion Jason , Colgan Sean W. J. , Cooke Ilsa R. , Coutens Audrey , Cox Nick L. J. , Demyk Karine , Meyer Jennifer Donovan , Foschino Sacha , García-Lario Pedro , Gavilan Lisseth , Gerin Maryvonne , Gottlieb Carl A. , Guillard Pierre , Gusdorf Antoine , Hartigan Patrick , He Jinhua , Herbst Eric , Hornekaer Liv , Jäger Cornelia , Janot-Pacheco Eduardo , Kaufman Michael , Kemper Francisca , Kendrew Sarah , Kirsanova Maria S. , Klaassen Pamela , Kwok Sun , Labiano Álvaro , Lai Thomas S.-Y. , Lee Timothy J. , Lefloch Bertrand , Le Petit Franck , Li Aigen , Linz Hendrik , Mackie Cameron J. , Madden Suzanne C. , Mascetti Joëlle , McGuire Brett A. , Merino Pablo , Micelotta Elisabetta R. , Misselt Karl , Morse Jon A. , Mulas Giacomo , Neelamkodan Naslim , Ohsawa Ryou , Omont Alain , Paladini Roberta , Palumbo Maria Elisabetta , Pathak Amit , Pendleton Yvonne J. , Petrignani Annemieke , Pino Thomas , Puga Elena , Rangwala Naseem , Rapacioli Mathias , Ricca Alessandra , Roman-Duval Julia , Roser Joseph , Roueff Evelyne , Rouillé Gaël , Salama Farid , Sales Dinalva A. , Sandstrom Karin , Sarre Peter , Sciamma-O’Brien Ella , Sellgren Kris , Shenoy Sachindev S. , Teyssier David , Thomas Richard D. , Togi Aditya , Verstraete Laurent , Witt Adolf N. , Wootten Alwyn , Zettergren Henning , Zhang Yong , Zhang Ziwei E. , Zhen Junfeng 
Published: August 2023

Context. Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 µm. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. These high-quality data allow for an unprecedentedly detailed view of AIBs.

Aims. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR (i.e. the three H2dissociation fronts), the atomic PDR, and the HIIregion.

Methods. We used JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extracted five template spectra to represent the morphology and environment of the Orion Bar PDR. We investigated and characterised the AIBs in these template spectra. We describe the variations among them here.

Results. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. The Orion Bar spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 µm with well-defined profiles. In addition, the spectra display a wealth of weaker features and sub-components. The widths of many AIBs show clear and systematic variations, being narrowest in the atomic PDR template, but showing a clear broadening in the HIIregion template while the broadest bands are found in the three dissociation front templates. In addition, the relative strengths of AIB (sub-)components vary among the template spectra as well. All AIB profiles are characteristic of class A sources as designated by Peeters (2022, A&A, 390, 1089), except for the 11.2 µm AIB profile deep in the molecular zone, which belongs to class B11.2. Furthermore, the observations show that the sub-components that contribute to the 5.75, 7.7, and 11.2 µm AIBs become much weaker in the PDR surface layers. We attribute this to the presence of small, more labile carriers in the deeper PDR layers that are photolysed away in the harsh radiation field near the surface. The 3.3/11.2 AIB intensity ratio decreases by about 40% between the dissociation fronts and the HIIregion, indicating a shift in the polycyclic aromatic hydrocarbon (PAH) size distribution to larger PAHs in the PDR surface layers, also likely due to the effects of photochemistry. The observed broadening of the bands in the molecular PDR is consistent with an enhanced importance of smaller PAHs since smaller PAHs attain a higher internal excitation energy at a fixed photon energy.

Conclusions. Spectral-imaging observations of the Orion Bar using JWST yield key insights into the photochemical evolution of PAHs, such as the evolution responsible for the shift of 11.2 µm AIB emission from class B11.2in the molecular PDR to class A11.2in the PDR surface layers. This photochemical evolution is driven by the increased importance of FUV processing in the PDR surface layers, resulting in a “weeding out” of the weakest links of the PAH family in these layers. For now, these JWST observations are consistent with a model in which the underlying PAH family is composed of a few species: the so-called ‘grandPAHs’.