H2 emission in the Orion Bar, NGC7023 and the Horsehead Nebula observed with the James Webb Space Telescope
Year: 2026
Type: preprint
Abstract: Molecular hydrogen (H2), the most abundant molecule in the interstellar medium, is a handy tool to study Photodissociation Regions. The JWST, with its high spatial resolution, sensitivity, and wavelength coverage, provides unique access to the detection and spatial morphology of the H2 rotational and rovibrational lines. Our goal is to analyze H2 line emission detected with JWST in several PDRs (the Orion Bar, NGC7023, and the Horsehead Nebula) to constrain the physical structure of dissociation front, and compare the impact of the ultraviolet field and the gas density across different regions. The analysis of H2 spatial morphology reveals similar filamentary structures across all regions. Spatial shifts between H2 lines (0.5") are observed and linked to differences in excitation mechanisms (thermalized vs FUV-pumped lines) and the temperature gradient. Despite differences in incident irradiation conditions, analysis of H2 rotational excitation yields high gas temperatures that are similar across the three PDRs. This is expected for a certain regime of excited PDRs (such as Orion and NGC7023), where the G0 at the DFs and the density are similar. However, it is surprising for less excited PDRs like the Horsehead Nebula. OPRs in the rotational and rovibrational levels differ across the three PDRs. We also used H2 rovibrational lines and the 0-0 S(3) line to estimate the visual extinction across the field of view, yielding markedly different results that constrain both the geometry and the grain composition. Due to the complex 3D geometry, H2 emission is observed throughout the FOV and is dominated by the thin UV illuminated layer, making H2 an unreliable tracer of gas temperature in deeper and coolers molecular layers of the PDR. Therefore, H2 emission is not sufficient to trace the possible temperature and density gradient across PDRs and must therefore be associated with other tracers.
Source: arXiv (Cornell University)
Authors: Zannese, M., Sidhu, A., Guerras, A., Pound, M., Wolfire, M., Tielens, A. G. G. M., Peeters, E., Habart, E., Abergel, A., Baes, M., Berné, O., Boersma, C., Bron, E., Cami, J., Chown, R., Dartois, E., Dell'Ova, P., Goicoechea, J. R., Gordon, K. D., Guillard, P., Kannavou, O., Misselt, K., Monnier, N., Noriega-Crespo, A., Onaka, T., Van de Putte, D., Verstraete, L., Witt, A. N.
Institutions: Centre National de la Recherche Scientifique, Consejo Superior de Investigaciones Científicas, Université Paris-Saclay, Instituto de Física Fundamental, Institut d'Astrophysique Spatiale, Western University, University of Michigan, University of Maryland, College Park, Leiden University, Leiden Observatory, Ghent University, Université Toulouse III - Paul Sabatier, Institut de Recherche en Astrophysique et Planétologie, Ames Research Center, Université Paris Sciences et Lettres, Observatoire de Paris, Sorbonne Université, The Ohio State University, Institut des Sciences Moléculaires d'Orsay, Space Telescope Science Institute, Institut d'Astrophysique de Paris, University of Arizona, The University of Tokyo, University of Toledo
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Funders: National Aeronautics and Space Administration, Space Telescope Science Institute, San José State University, Ames Research Center, Canadian Space Agency, Natural Sciences and Engineering Research Council of Canada, Japan Society for the Promotion of Science, Agencia Estatal de Investigación, European Social Fund
OpenAlex ID: W7214672191