PDRs4All
Authors: Peeters Els , Habart Emilie , Berné Olivier , Sidhu Ameek , Chown Ryan , Van De Putte Dries , Trahin Boris , Schroetter Ilane , Canin Amélie , Alarcón Felipe , Schefter Bethany , Khan Baria , Pasquini Sofia , Tielens Alexander G. G. M. , Wolfire Mark G. , Dartois Emmanuel , Goicoechea Javier R. , Maragkoudakis Alexandros , Onaka Takashi , Pound Marc W. , Vicente Sílvia , Abergel Alain , Bergin Edwin A. , Bernard-Salas Jeronimo , Boersma Christiaan , Bron Emeric , Cami Jan , Cuadrado Sara , Dicken Daniel , Elyajouri Meriem , Fuente Asunción , Gordon Karl D. , Issa Lina , Joblin Christine , Kannavou Olga , Lacinbala Ozan , Languignon David , Le Gal Romane , Meshaka Raphael , Okada Yoko , Robberto Massimo , Röllig Markus , Schirmer Thiébaut , Tabone Benoit , Zannese Marion , Aleman Isabel , Allamandola Louis , 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 , Colgan Sean W. J. , 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 , He Jinhua , Herbst Eric , Hornekaer Liv , Jäger Cornelia , Janot-Pacheco Eduardo , Kaufman Michael , 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 , 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 , Ysard Nathalie , Zettergren Henning , Zhang Yong , Zhang Ziwei E. , Zhen Junfeng 
Published: March 2024

Context. JWST has taken the sharpest and most sensitive infrared (IR) spectral imaging observations ever of the Orion Bar photodis-sociation region (PDR), which is part of the nearest massive star-forming region the Orion Nebula, and often considered to be the ‘prototypical’ strongly illuminated PDR.

Aims. We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the H II region to the atomic PDR – crossing the ionisation front (IF) –, and the subsequent transition to the molecular PDR – crossing the dissociation front (DF). Given the prevalence of PDRs in the interstellar medium and their dominant contribution to IR radiation, understanding the response of the PDR gas to far-ultraviolet (FUV) photons and the associated physical and chemical processes is fundamental to our understanding of star and planet formation and for the interpretation of any unresolved PDR as seen by JWST.

Methods. We used high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science programme. We constructed a 3″ × 25″’ spatio-spectral mosaic covering 0.97– 5.27 μm at a spectral resolution R of ~2700 and an angular resolution of 0.075″–0.173″. To study the properties of key regions captured in this mosaic, we extracted five template spectra in apertures centred on the three H2 dissociation fronts, the atomic PDR, and the H II region. This wealth of detailed spatial-spectral information was analysed in terms of variations in the physical conditions-incident UV field, density, and temperature – of the PDR gas.

Results. The NIRSpec data reveal a forest of lines including, but not limited to, He I, H I , and C I recombination lines; ionic lines (e.g. Fe III and Fe II); O I and N I fluorescence lines; aromatic infrared bands (AIBs, including aromatic CH, aliphatic CH, and their CD counterparts); pure rotational and ro-vibrational lines from H2; and ro-vibrational lines from HD, CO, and CH+, with most of them having been detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. In addition, we observed numerous smaller-scale structures whose typical size decreases with distance from θ1 Ori C and IR lines from C I , if solely arising from radiative recombination and cascade, reveal very high gas temperatures (a few 1000 K) consistent with the hot irradiated surface of small-scale dense clumps inside the PDR. The morphology of the Bar, in particular that of the H2 lines, reveals multiple prominent filaments that exhibit different characteristics. This leaves the impression of a ‘terraced’ transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. We attribute the different characteristics of the H2 filaments to their varying depth into the PDR and, in some cases, not reaching the C+/C/CO transition. These observations thus reveal what local conditions are required to drive the physical and chemical processes needed to explain the different characteristics of the DFs and the photochemical evolution of the AIB carriers.

Conclusions. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star and planet formation as well as galaxy evolution.