PDRs4All
Authors: Van De Putte Dries , Meshaka Raphael , Trahin Boris , Habart Emilie , Peeters Els , Berné Olivier , Alarcón Felipe , Canin Amélie , Chown Ryan , Schroetter Ilane , Sidhu Ameek , Boersma Christiaan , Bron Emeric , Dartois Emmanuel , Goicoechea Javier R. , Gordon Karl D. , Onaka Takashi , Tielens Alexander G. G. M. , Verstraete Laurent , Wolfire Mark G. , Abergel Alain , Bergin Edwin A. , Bernard-Salas Jeronimo , Cami Jan , Cuadrado Sara , Dicken Daniel , Elyajouri Meriem , Fuente Asunción , Joblin Christine , Khan Baria , Lacinbala Ozan , Languignon David , Le Gal Romane , Maragkoudakis Alexandros , Okada Yoko , Pasquini Sofia , Pound Marc W. , Robberto Massimo , Röllig Markus , Schefter Bethany , Schirmer Thiébaut , Tabone Benoit , Vicente Sílvia , Zannese Marion , Colgan Sean W. J. , He Jinhua , Rouillé Gaël , Togi Aditya , Aleman Isabel , Auchettl Rebecca , Baratta Giuseppe Antonio , Bejaoui Salma , Bera Partha P. , Black John H. , Boulanger Francois , Bouwman Jordy , Brandl Bernhard , Brechignac Philippe , Brünken Sandra , Buragohain Mridusmita , Burkhardt Andrew , Candian Alessandra , Cazaux Stéphanie , Cernicharo Jose , Chabot Marin , Chakraborty Shubhadip , Champion Jason , Cooke Ilsa R. , Coutens Audrey , Cox Nick L. J. , Demyk Karine , Meyer Jennifer Donovan , Foschino Sacha , García-Lario Pedro , Gerin Maryvonne , Gottlieb Carl A. , Guillard Pierre , Gusdorf Antoine , Hartigan Patrick , Herbst Eric , Hornekaer Liv , Issa Lina , Jäger Cornelia , Janot-Pacheco Eduardo , Kannavou Olga , Kaufman Michael , Kemper Francisca , Kendrew Sarah , Kirsanova Maria S. , Klaassen Pamela , Kwok Sun , Labiano Álvaro , Lai Thomas S.-Y. , Le Floch Bertrand , Le Petit Franck , Li Aigen , Linz Hendrik , Mackie Cameron J. , Madden Suzanne C. , Mascetti Joëlle , McGuire Brett A. , Merino Pablo , Micelotta Elisabetta R. , 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 , Rho Jeonghee , Ricca Alessandra , Roman-Duval Julia , Roser Joseph , Roueff Evelyne , Salama Farid , Sales Dinalva A. , Sandstrom Karin , Sarre Peter , Sciamma-O’Brien Ella , Sellgren Kris , Shenoy Sachindev S. , Teyssier David , Thomas Richard D. , Witt Adolf N. , Wootten Alwyn , Ysard Nathalie , Zettergren Henning , Zhang Yong , Zhang Ziwei E. , Zhen Junfeng 
Published: May 2024

Context.Mid-infrared emission features are important probes of the properties of ionized gas and hot or warm molecular gas, which are difficult to probe at other wavelengths. The Orion Bar photodissociation region (PDR) is a bright, nearby, and frequently studied target containing large amounts of gas under these conditions. Under the “PDRs4All” Early Release Science Program for JWST, a part of the Orion Bar was observed with MIRI integral field unit (IFU) spectroscopy, and these high-sensitivity IR spectroscopic images of very high angular resolution (0.2″) provide a rich observational inventory of the mid-infrared (MIR) emission lines, while resolving the HIIregion, the ionization front, and multiple dissociation fronts.

Aims.We list, identify, and measure the most prominent gas emission lines in the Orion Bar using the new MIRI IFU data. An initial analysis summarizes the physical conditions of the gas and demonstrates the potential of these new data and future IFU observations with JWST.

Methods.The MIRI IFU mosaic spatially resolves the substructure of the PDR, its footprint cutting perpendicularly across the ionization front and three dissociation fronts. We performed an up-to-date data reduction, and extracted five spectra that represent the ionized, atomic, and molecular gas layers. We identified the observed lines through a comparison with theoretical line lists derived from atomic data and simulated PDR models. The identified species and transitions are summarized in the main table of this work, with measurements of the line intensities and central wavelengths.

Results.We identified around 100 lines and report an additional 18 lines that remain unidentified. The majority consists of HIrecombination lines arising from the ionized gas layer bordering the PDR. The HIline ratios are well matched by emissivity coefficients from H recombination theory, but deviate by up to 10% because of contamination by HeIlines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni. We show how the NeIII/NeII, SIV/SIII, and ArIII/ArIIratios trace the conditions in the ionized layer bordering the PDR, while FeIII/FeIIand NiIII/NiIIexhibit a different behavior, as there are significant contributions to FeIIand NiIIfrom the neutral PDR gas. We observe the pure-rotational H2lines in the vibrational ground state from 0–0S(1) to 0–0S(8), and in the first vibrationally excited state from 1–1S(5) to 1–1 S(9). We derive H2excitation diagrams, and for the three observed dissociation fronts, the rotational excitation can be approximated with one thermal (~700 K) component representative of an average gas temperature, and one nonthermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model of the Orion Bar PDR, and find that the predicted excitation matches the data qualitatively, while adjustments to the parameters of the PDR model are required to reproduce the intensity of the 0–0 S (6) to S (8) lines.