PDRs4All

IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar^★

¹ Department of Physics & Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
e-mail: rchown3@uwo.ca
² Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
³ Carl Sagan Center, SETI Institute, 339 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA
⁴ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁵ Astronomy Department, University of Maryland, College Park, MD 20742, 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 Institut d’Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
⁸ Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
⁹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹⁰ ACRI-ST, Centre d’Etudes et de Recherche de Grasse (CERGA), 10 Av. Nicolas Copernic, 06130 Grasse, France
¹¹ INCLASS Common Laboratory, 10 Av. Nicolas Copernic, 06130 Grasse, France
¹² NASA Ames Research Center, MS 245-6, Moffett Field, CA 94035-1000, USA
¹³ LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Université, 92190 Meudon, France
¹⁴ Instituto de Física Fundamental (CSIC), Calle Serrano 121-123, 28006 Madrid, Spain
¹⁵ Institut des Sciences Moléculaires d’Orsay, CNRS, Université Paris-Saclay, Bâtiment 520, 91405 Orsay Cedex, France
¹⁶ UK Astronomy Technology Centre, Royal Observatory Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
¹⁷ Observatorio Astronómico Nacional (OAN,IGN), Alfonso XII, 3, 28014 Madrid, Spain
¹⁸ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281/S9, 9000 Gent, Belgium
¹⁹ KU Leuven Quantum Solid State Physics (QSP), Celestijnenlaan 200d – PO Box 2414, 3001 Leuven, Belgium
²⁰ Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, CNRS, 38000 Grenoble, France
²¹ Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
²² I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
²³ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Bunkyo-ku, Tokyo 113-0033, Japan
²⁴ Department of Physics, Faculty of Science and Engineering, Meisei University, 2-1-1 Hodokubo, Hino, Tokyo 191-8506, Japan
²⁵ Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA
²⁶ Physikalischer Verein – Gesellschaft für Bildung und Wissenschaft, Robert-Mayer-Straße 2, 60325 Frankfurt am Main, Germany
²⁷ Institut für Angewandte Physik. Goethe-Universität Frankfurt, Max-von-Laue-Str. 1, 60438 Frankfurt am Main, Germany
²⁸ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
²⁹ Instituto de Astrofísica e Ciências do Espaço, Tapada da Ajuda, Edifício Leste, 2° Piso, 1349-018 Lisboa, Portugal
³⁰ Instituto de Física e Química, Universidade Federal de Itajubá, Av. BPS 1303, Pinheirinho, 37500-903, Itajubá, MG, Brazil
³¹ Bay Area Environmental Research Institute, Moffett Field, CA 94035, USA
³² Australian Synchrotron, Australian Nuclear Science and Technology Organisation (ANSTO), 800 Blackburn Rd, Clayton, Victoria 3168, Melbourne, Australia
³³ INAF – Osservatorio Astrofisico di Catania, Via Santa Sofia 78, 95123 Catania, Italy
³⁴ Laboratoire de Physique de l’École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, 75005 Paris, France
³⁵ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA
³⁶ Department of Chemistry, University of Colorado, Boulder, CO 80309, USA
³⁷ Institute for Modeling Plasma, Atmospheres, and Cosmic Dust (IMPACT), University of Colorado, Boulder, CO 80303, USA
³⁸ Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
³⁹ FELIX Laboratory, Institute for Molecules and Materials, Radboud University, Toernooiveld 7, 6525 ED Nijmegen, The Netherlands
⁴⁰ School of Physics, University of Hyderabad, Hyderabad, Telangana 500046, India
⁴¹ Department of Earth, Environment, and Physics Worcester State University, 486 Chandler St, Worcester, MA 01602, USA
⁴² Anton Pannekoek Institute for Astronomy (API), University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁴³ Delft University of Technology, Delft, Mekelweg 5, 2628 CD Delft, The Netherlands
⁴⁴ Laboratoire de Physique des deux infinis Irène Joliot-Curie, Université Paris-Saclay, CNRS/IN2P3, Bâtiment 104, 91405 Orsay Cedex, France
⁴⁵ Department of Chemistry, GITAM school of Science, GITAM Deemed to be University, NH 207, Nagadenehalli, Doddaballapur taluk, Bengaluru – 561203, Karnataka, India
⁴⁶ Institut de Physique de Rennes, UMR CNRS 6251, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France
⁴⁷ Department of Chemistry, The University of British Columbia, Vancouver, 2036 Main Mall, Vancouver, BC V6T 1Z1, Canada
⁴⁸ National Radio Astronomy Observatory (NRAO), 520 Edgemont Road, Charlottesville, VA 22903, USA
⁴⁹ European Space Astronomy Centre (ESAC/ESA), Villanueva de la Cañada, 28692 Madrid, Spain
⁵⁰ Observatoire de Paris, PSL University, Sorbonne Université, LERMA, 75014 Paris, France
⁵¹ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138, USA
⁵² Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis bd Arago, 75014 Paris, France
⁵³ Institut Universitaire de France, Ministère de l’Enseignement Supérieur et de la Recherche, 1 rue Descartes, 75231 Paris Cedex 05, France
⁵⁴ Department of Physics and Astronomy, Rice University, Houston TX, 77005-1892, USA
⁵⁵ Yunnan Observatories, Chinese Academy of Sciences, 396 Yangfangwang, Guandu District, Kunming 650216, PR China
⁵⁶ Chinese Academy of Sciences South America Center for Astronomy, National Astronomical Observatories, CAS, Beijing 100101, PR China
⁵⁷ Departments of Chemistry and Astronomy, University of Virginia, Charlottesville, Virginia 22904, USA
⁵⁸ InterCat and Dept. Physics and Astron., Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁵⁹ Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
⁶⁰ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, 05509-090 São Paulo, SP, Brazil
⁶¹ Department of Physics and Astronomy, San José State University, San Jose, CA 95192, USA
⁶² Institut de Ciencies de l’Espai (ICE, CSIC), Can Magrans, s/n, 08193 Bellaterra, Barcelona, Spain
⁶³ ICREA, Pg. Lluìs Companys 23, 08010 Barcelona, Spain
⁶⁴ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
⁶⁵ European Space Agency, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore MD 21218, USA
⁶⁶ Institute of Astronomy, Russian Academy of Sciences, 119017, Pyatnitskaya str. 48, Moscow, Russia
⁶⁷ Department of Earth, Ocean, & Atmospheric Sciences, University of British Columbia, British Columbia V6T 1Z4, Canada
⁶⁸ Telespazio UK for ESA, ESAC, 28692 Villanueva de la Cañada, Madrid, Spain
⁶⁹ IPAC, California Institute of Technology, Pasadena, CA, USA
⁷⁰ Department of Physics and Astronomy, University of Missouri, 701 S College Ave, Columbia, MO 65211, USA
⁷¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁷² Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
⁷³ Kenneth S. Pitzer Center for Theoretical Chemistry, Department of Chemistry, 420 Latimer Hall, University of California, Berkeley, CA 94720-1460, USA
⁷⁴ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
⁷⁵ Institut des Sciences Moléculaires, CNRS, Université de Bordeaux, 33405 Talence, France
⁷⁶ Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁷⁷ Instituto de Ciencia de Materiales de Madrid (CSIC), Sor Juana Ines de la Cruz 3, 28049, Madrid, Spain
⁷⁸ Department of Physics, PO Box 64, 00014 University of Helsinki, Helsinki, Finland
⁷⁹ Steward Observatory, University of Arizona, Tucson, AZ 85721-0065, USA
⁸⁰ BoldlyGo Institute, 31 W 34TH ST FL 7 STE 7159, New York, NY 10001, USA
⁸¹ Department of Physics, College of Science, United Arab Emirates University (UAEU), Al-Ain 15551, UAE
⁸² National Astronomical Observatory of Japan, National Institutes of Natural Science, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁸³ Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98bis bd Arago, 75014 Paris, France
⁸⁴ California Institute of Technology, IPAC, 770, S. Wilson Ave., Pasadena, CA 91125, USA
⁸⁵ Department of Physics, Institute of Science, Banaras Hindu University, Varanasi 221005, India
⁸⁶ University of Central Florida, Orlando, FL 32765, USA
⁸⁷ Van’t Hoff Institute for Molecular Sciences, University of Amsterdam, PO Box 94157, 1090 GD, Amsterdam, The Netherlands
⁸⁸ Laboratoire de Chimie et Physique Quantiques LCPQ/FERMI, UMR5626, Université de Toulouse (UPS) and CNRS, Toulouse, France
⁸⁹ Laboratory Astrophysics Group of the Max Planck Institute for Astronomy at the Friedrich Schiller University Jena, Institute of Solid State Physics, Helmholtzweg 3, 07743 Jena, Germany
⁹⁰ Instituto de Matemática, Estatística e Física, Universidade Federal do Rio Grande, 96201-900 Rio Grande, RS, Brazil
⁹¹ Center for Astrophysics and Space Sciences, Department of Physics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093, USA
⁹² School of Chemistry, The University of Nottingham, University Park, Nottingham NG7 2RD, UK
⁹³ Astronomy Department, Ohio State University, Columbus, OH 43210, USA
⁹⁴ Space Science Institute, 4765 Walnut St., R203, Boulder, CO 80301, USA
⁹⁵ Department of Physics, Stockholm University, 10691 Stockholm, Sweden
⁹⁶ Department of Physics, Texas State University, San Marcos, TX 78666, USA
⁹⁷ School of Physics and Astronomy, Sun Yat-sen University, 2 Da Xue Road, Tangjia, Zhuhai 519000, Guangdong Province, PR China
⁹⁸ Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, Hirosawa 2-1, Wako, Saitama 351-0198, Japan
⁹⁹ Institute of Deep Space Sciences, Deep Space Exploration Laboratory, Hefei 230026, PR China

Received: 14 April 2023
Accepted: 1 August 2023

Abstract

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 H₂ dissociation fronts), the atomic PDR, and the H II region.

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 H II region 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 B_11.2. 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 H II region, 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 B_11.2 in the molecular PDR to class A_11.2 in 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’.