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Astronomy and Astrophysics, vol. 696, pp. A99
Abstract
Context . The methylidyne cation (CH + ) and the methyl cation (CH 3 + ) are building blocks of organic molecules in the ultraviolet (UV) irradiated gas, yet their coupled formation and excitation mechanisms mostly remain unprobed. The James Webb Space Telescope (JWST), with its high spatial resolution and good spectral resolution, provides unique access to the detection of these molecules. Aims . Our goal is to use the first detection of CH + and CH 3 + infrared rovibrational emission in the Orion Bar and in the protoplanetary disk d203-506 to probe their formation and excitation mechanisms and constrain the physico-chemical conditions of the environment. Methods . We used spectro-imaging acquired using both the NIRSpec and MIRI-MRS instruments on board JWST to study the infrared CH + and CH 3 + spatial distribution at very small scales (down to 0.1′′) and compared it to excited H 2 emission. We studied their excitation in detail, and in the case of CH + , we compared the observed line intensities with chemical formation pumping models based on recent quantum dynamical calculations. Throughout this study, we compare the emission of these molecules in two environments: the Bar a photodissociation region – and a protoplanetary disk (d203-506), both of which are irradiated by the Trapezium cluster. Results . We detected CH + and CH 3 + vibrationally excited emission both in the Bar and d203-506. These emissions originate from the same region as highly excited H 2 (high rotational and rovibrational levels) and correlate less with the lower rotational levels of H 2 ( J ′ < 5) or the emission of aromatic and aliphatic infrared bands. Our comparison between the Bar and d203-506 revealed that both CH + and CH 3 + excitation and/or formation are highly dependent on gas density. The excitation temperature of the observed CH + and CH 3 + rovibrational lines is around T ∼ 1500 K in the Bar and T ∼ 800 K in d203-506. Moreover, the column densities derived from the rovibrational emission are less than 0.1% of the total known (CH + ) and expected (CH 3 + ) column densities. These different results show that CH + and CH 3 + level populations strongly deviate from local thermodynamical equilibrium. The CH + rovibrational supra-thermal emission ( v = 1 and v = 2) can be explained by chemical formation pumping with excited H 2 via C + + H 2 * = CH + + H. The difference in the population distribution of the H 2 * energy levels between the Orion Bar and d203-506 then result in different excitation temperatures. These results support a gas phase formation pathway of CH + and CH 3 + via successive hydrogen abstraction reactions. However, we do not find any evidence of CH 3 + emission in the JWST spectrum, which may be explained by the fact its spectroscopic signatures could be spread in the JWST spectra. Finally, the observed CH + intensities coupled with a chemical formation pumping model provide a diagnostic tool to trace the local density. Conclusions . Line emission from vibrationally excited CH + and CH 3 + provides new insight into the first steps of hydrocarbon gas-phase chemistry in action. This study highlights the need for extended molecular data of detectable molecules in the interstellar medium in order to analyze the JWST observations.
Authors 22
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut d'Astrophysique Spatiale
Affiliation as printed
Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut d'Astrophysique Spatiale
Affiliation as printed
Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut d'Astrophysique Spatiale
Affiliation as printed
Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut des Sciences Moléculaires d'Orsay
Affiliation as printed
Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91400 Orsay, France
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Instituto de Física Fundamental
Affiliation as printed
Instituto de Física Fundamental (CSIC), Calle Serrano 121–123, 28006 Madrid, Spain
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut des Sciences Moléculaires d'Orsay
Affiliation as printed
Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91400 Orsay, France
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut des Sciences Moléculaires d'Orsay
Affiliation as printed
Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91400 Orsay, France
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut des Sciences Moléculaires d'Orsay
Affiliation as printed
Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91400 Orsay, France
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut des Sciences Moléculaires d'Orsay
Affiliation as printed
Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91400 Orsay, France
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Centre National de la Recherche Scientifique · Institut de Planétologie et d'Astrophysique de Grenoble · Université Grenoble Alpes
Affiliation as printed
Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, CNRS, 38000 Grenoble, France
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Université Paris Sciences et Lettres · Observatoire de Paris · Sorbonne Université · Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique et Atmosphères
Affiliation as printed
Observatoire de Paris, Université PSL, Sorbonne Université, LERMA, 75014 Paris, France
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Leiden University · Leiden Observatory · University of Maryland, College Park
Affiliation as printed
Astronomy Department, University of Maryland, College Park, MD 20742, USA
Leiden Observatory, Leiden University, 2300 RA Leiden, The Netherlands
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Centre National de la Recherche Scientifique · Institut de Radioastronomie Millimétrique · Institut de Planétologie et d'Astrophysique de Grenoble · Université Grenoble Alpes
Affiliation as printed
Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, CNRS, 38000 Grenoble, France
Institut de Radioastronomie Millimétrique (IRAM), 300 Rue de la Piscine, F-38406 Saint-Martin d’Hères, France
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Chalmers University of Technology
Affiliation as printed
Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
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Institute of Astrophysics and Space Sciences
Affiliation as printed
Instituto de Astrofísica e Ciências do Espaço, Tapada da Ajuda, Edifício Leste, 2∘ Piso, 1349-018 Lisboa, Portugal
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Centre National de la Recherche Scientifique · Université Toulouse III - Paul Sabatier · Institut de Recherche en Astrophysique et Planétologie
Affiliation as printed
Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
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Western University · Space Telescope Science Institute
Affiliation as printed
Department of Physics & Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
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Affiliation as printed
Department of Physics & Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
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Affiliation as printed
Department of Physics & Astronomy, The University of Western Ontario, London, ON N6A 3K7, Canada
Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
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Centre National de la Recherche Scientifique · Université Toulouse III - Paul Sabatier · Institut de Recherche en Astrophysique et Planétologie
Affiliation as printed
Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
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Centre National de la Recherche Scientifique · Université Toulouse III - Paul Sabatier · Institut de Recherche en Astrophysique et Planétologie
Affiliation as printed
Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
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Centre National de la Recherche Scientifique · Université Paris-Saclay · Institut d'Astrophysique Spatiale
Affiliation as printed
Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, 91405 Orsay, France
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