PDRs4All: IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar
Journal article, 2024
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 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 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 B11.2 in the molecular PDR to class A11.2 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’.
ISM: individual objects: Orion Bar
Infrared: ISM
Astrochemistry
Photon-dominated region (PDR)
Techniques: spectroscopic
ISM: molecules
Author
Ryan Chown
Western University
Ameek Sidhu
Western University
Els Peeters
Western University
A.G.G.M. Tielens
Leiden University
Jan Cami
Western University
O. Berné
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Emilie Habart
Institut d'Astrophysique Spatiale
Felipe Alarcon
University of Michigan
Amelie Canin
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Ilane Schroetter
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Boris Trahin
Institut d'Astrophysique Spatiale
Dries Van De Putte
Space Telescope Science Institute (STScI)
Alain Abergel
Institut d'Astrophysique Spatiale
E. A. Bergin
University of Michigan
J. Bernard-Salas
Observatoire de la Cote d'Azur
Christiaan Boersma
NASA Ames Research Center
E. Bron
Paris Observatory
S. Cuadrado
CSIC - Instituto de Fisica Fundamental (IFF)
E. Dartois
University Paris-Saclay
D. Dicken
Royal Observatory
Meriem El-Yajouri
Institut d'Astrophysique Spatiale
A. Fuente
Spanish National Observatory (OAN)
J.R. Goicoechea
CSIC - Instituto de Fisica Fundamental (IFF)
K. Gordon
Space Telescope Science Institute (STScI)
Lina Issa
Institut de Recherche en Astrophysique et Planétologie (IRAP)
C. Joblin
Leiden University
Olga Kannavou
Institut d'Astrophysique Spatiale
Baria Khan
Western University
Ozan Lacinbala
KU Leuven
David Languignon
Paris Observatory
R. Le Gal
Institut de Planétologie et d’Astrophysique de Grenoble (IPAG)
Alexandros Maragkoudakis
NASA Ames Research Center
Raphael Meshaka
Institut d'Astrophysique Spatiale
Yoko Okada
University of Cologne
Takashi Onaka
University of Tokyo
Sofia Pasquini
Western University
Marc W. Pound
University of Maryland
Massimo Robberto
Space Telescope Science Institute (STScI)
M. Rollig
Physikalischer Verein
Bethany Schefter
Western University
Thiébaut Schirmer
Institut d'Astrophysique Spatiale
S. Vicente
Institute of Astrophysics and Space Sciences (IA)
M.G. Wolfire
University of Maryland
Marion Zannese
Institut d'Astrophysique Spatiale
I. Aleman
Federal University of Itajubá
Louis Allamandola
NASA Ames Research Center
Rebecca Auchettl
Australian Synchrotron (ANSTO)
Giuseppe Antonio Baratta
Istituto nazionale di astrofisica (INAF)
Salma Bejaoui
NASA Ames Research Center
Partha P. Bera
NASA Ames Research Center
John H Black
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
F. Boulanger
Laboratoire de Physique de l’Ecole Normale Supérieure
Jordy Bouwman
University of Colorado at Boulder
B. R. Brandl
Leiden University
Philippe Brechignac
University Paris-Saclay
S. Brünken
Radboud University
Mridusmita Buragohain
University of Hyderabad
Andrew Burkhardt
Environment Environment
Alessandra Candian
Anton Pannekoek Institute for Astronomy
S. Cazaux
Delft University of Technology
Jose Cernicharo
CSIC - Instituto de Fisica Fundamental (IFF)
Marin Chabot
University Paris-Saclay
Shubhadip Chakraborty
Gitam School of Science
Jason Champion
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Sean W. J. Colgan
NASA Ames Research Center
Ilsa R. Cooke
University of British Columbia (UBC)
A. Coutens
Institut de Recherche en Astrophysique et Planétologie (IRAP)
N. L. J. Cox
Observatoire de la Cote d'Azur
K. Demyk
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Jennifer Donovan Meyer
National Radio Astronomy Observatory
Sacha Foschino
Institut de Recherche en Astrophysique et Planétologie (IRAP)
P. Garcia-Lario
European Space Astronomy Centre (ESAC)
Lisseth Gavilan
NASA Ames Research Center
M. Gerin
Paris Observatory
C. A. Gottlieb
Harvard-Smithsonian Center for Astrophysics
P. Guillard
Institut d 'Astrophysique de Paris
Antoine Gusdorf
Laboratoire de Physique de l’Ecole Normale Supérieure
Patrick Hartigan
Rice University
J. H. He
Yunnan Observatories
E Herbst
University of Virginia
Liv Hornekaer
Aarhus University
Cornelia Jäger
Max Planck Society
Eduardo Janot-Pacheco
University of Sao Paulo (USP)
M. Kaufman
San Jose State University
F. Kemper
Institute of Space Sciences (ICE) - CSIC
S. Kendrew
European Space Agency (ESA)
M. S. Kirsanova
Russian Academy of Sciences
Pamela Klaassen
Royal Observatory
Sun Kwok
University of British Columbia (UBC)
A. Labiano
European Space Astronomy Centre (ESAC)
Thomas Lai
California Institute of Technology (Caltech)
Timothy J. Lee
NASA Ames Research Center
B. Lefloch
Institut de Planétologie et d’Astrophysique de Grenoble (IPAG)
Franck Le Petit
Paris Observatory
Aigen Li
University of Missouri
H. Linz
Max Planck Society
Cameron J. MacKie
Lawrence Berkeley National Laboratory
S. C. Madden
University Paris-Saclay
Joëlle Mascetti
Institut des Sciences Moleculaires, Talence
Brett McGuire
National Radio Astronomy Observatory
Pablo Merino
CSIC - Instituto de Ciencia de Materiales de Madrid (ICMM)
Elisabetta R. Micelotta
University of Helsinki
K. A. Misselt
University of Arizona
Jon A. Morse
BoldlyGo Institute
Giacomo Mulas
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Naslim Neelamkodan
United Arab Emirates University
Ryou Ohsawa
National Astronomical Observatory of Japan
A.A. Omont
Institut d 'Astrophysique de Paris
R. Paladini
California Institute of Technology (Caltech)
Maria Elisabetta Palumbo
Istituto nazionale di astrofisica (INAF)
Amit Pathak
Banaras Hindu University
Yvonne J. Pendleton
University of Central Florida
Annemieke Petrignani
University of Amsterdam
Thomas Pino
University Paris-Saclay
E. Puga
European Space Agency (ESA)
Naseem Rangwala
NASA Ames Research Center
Mathias Rapacioli
UMR5626
Alessandra Ricca
SETI Institute
J. Roman-Duval
Space Telescope Science Institute (STScI)
Joseph Roser
SETI Institute
Evelyne Roueff
Paris Observatory
Gaël Rouillé
Max Planck Society
Farid Salama
NASA Ames Research Center
Dinalva A. Sales
Estatística e Física
Karin Sandstrom
University of California at San Diego (UCSD)
Peter Sarre
University of Nottingham
Ella Sciamma-O'Brien
NASA Ames Research Center
Kris Sellgren
Ohio State University
Sachindev S. Shenoy
Space Science Institute
D. Teyssier
European Space Astronomy Centre (ESAC)
Richard D. Thomas
Stockholm University
Aditya Togi
Max Planck Society
Laurent Verstraete
Institut d'Astrophysique Spatiale
Adolf N. Witt
Texas State University
A. Wootten
National Radio Astronomy Observatory
H. Zettergren
Stockholm University
Yong Zhang
Sun Yat-Sen University
Ziwei E. Zhang
RIKEN
Junfeng Zhen
Deep Space Exploration Laboratory (DSEL)
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 685 A75Subject Categories
Astronomy, Astrophysics and Cosmology
DOI
10.1051/0004-6361/202346662