Abstract
Forty years ago, it was proposed that gas-phase organic chemistry in the interstellar medium can be initiated by the methyl cation CH3+ (refs. 1,2,3), but so far it has not been observed outside the Solar System4,5. Alternative routes involving processes on grain surfaces have been invoked6,7. Here we report James Webb Space Telescope observations of CH3+ in a protoplanetary disk in the Orion star-forming region. We find that gas-phase organic chemistry is activated by ultraviolet irradiation.
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Data availability
The JWST data presented in this paper are publicly available through the MAST online archive (http://mast.stsci.edu) using the PID 1288. The MIRI spectra presented in Fig. 2 and Extended Data Figs. 1 and 2 are available in ASCII format at https://doi.org/10.5281/zenodo.7989669 (ref. 59). The PGOPHER files to create the model spectra of CH3+ are available via https://doi.org/10.5281/zenodo.7993330 (ref. 60). Source data are provided with this paper.
Code availability
The JWST pipeline used to produce the final data products presented in this article is available at https://github.com/spacetelescope/jwst. The MEUDON PDR code is publicly available at https://ism.obspm.fr/pdr_download.html.
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Acknowledgements
O.B. is funded by a CNES APR programme. MIRI data reduction is performed at the French MIRI centre of expertise with the support of CNES and the ANR-labcom INCLASS between IAS and the company ACRI-ST. Part of this work was supported by the Programme National Physique et Chimie du Milieu Interstellaire (PCMI) of CNRS/INSU with INC/INP cofunded by CEA and CNES. Quantum chemical calculations were performed using HPC resources from the ‘Mésocentre’ computing centre of CentraleSupélec and École Normale Supérieure Paris-Saclay supported by CNRS and Région Ile-de-France (http://mesocentre.centralesupelec.fr/). J.R.G. and S.C. thank the Spanish MCINN for funding support under grant no. PID2019-106110GB-I00. J.C and E.P. acknowledge support from the University of Western Ontario, the Institute for Earth and Space Exploration, the Canadian Space Agency and the Natural Sciences and Engineering Research Council of Canada. The Cologne spectroscopy group acknowledges funding by the Deutsche Forschungsgemeinschaft DFG (CRC956, subproject B2, ID no. 184018867) and the ERC AdG Missions (ID no. 101020583). Work by Y.O. and M. Röllig is carried out within the Collaborative Research Centre 956, subproject C1, funded by the DFG - project ID no. 184018867. C.B. is grateful for an appointment at NASA Ames Research Center through the San José State University Research Foundation (grant no. 80NSSC22M0107). T.O. acknowledges support from JSPS Bilateral Programme, grant no. 120219939. A.F. thanks Spanish MICIN for funding support from PID2019-106235GB-I00.
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O.B. found the signal in the data and led the analysis of the data and write-up of the article. M.-A.M.-D., I.S., U.J., B.G., E.D., L.H.C., E.B., F.A., J.C., E.R., J.H.B., O.A., C.J., S.S., S.T., J.C., M.G., A.T., T.O. and M.Z. conducted the spectroscopic analysis and participated in the write-up. M.-A.M.-D. created Fig. 3, and Extended Data Figs. 4 and 5. I.S. created Figs. 1 and 2 and Extended Data Figs. 1–3. I.S. and O.B. created Extended Data Fig. 6. J.R.G. performed the chemical models, created Extended Data Figs. 7 and 8, and participated in the write-up. O.B., E.H. and E.P. led the JWST observing programme. I.S., J.R.G., E.D., E. Bergin, F.A., J.C., A.C., B.T., C.J., A.T., M.Z., A.A., J.B.-S., C.B., E. Bron, R.C., S.C., D.D., M.E., A.F., K.D.G., L.I., O.K., B.K., O.L., D.L., R.L.G., A.M., R.M., Y.O., T.O., S.P., M.W.P., M. Robberto, M. Röllig, B.S., T.S., A.S., B.T., D.V.D.P., S.V. and M.G.W. contributed to the observing programme with JWST. I.S., A.C., R.C., A.S., B.T., F.A., D.V.P. reduced the data. E.D., M.-A.M.-D., L.H.C., J.R.G. and O.B. conducted the column density analysis. J.H.B. wrote the section on the excitation of CH3+. M.G.W. and J.H.B. corrected the English throughout the manuscript. All authors contributed to the discussions and provided feedback on the manuscript.
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Extended data figures and tables
Extended Data Fig. 1 ON and OFF spectra of d203-506 over the full MIRI-MRS spectral range.
The ON-OFF spectrum is also shown. Main atomic and H2 lines are indicated.
Extended Data Fig. 2 ON-OFF spectrum of d203-506 over the full MIRI-MRS spectral range.
Main atomic and H2 lines are indicated.
Extended Data Fig. 3 H2 excitation diagram derived from the line intensities in Extended Data Table 2 using the H2 toolbox (see Methods for details).
Error bars result from the propagation of the absolute calibration error of MIRI, which we take from ref. 58.
Extended Data Fig. 4 Spectroscopic models I and II.
a, Model I, with zoom on the strongest lines (b). c, Model II, with zoom on the strongest lines (d). For these models, the excitation temperature is T = 400 K, and we use a Gaussian profile of 0.35 cm−1 full-width-at-half-maximum.
Extended Data Fig. 5 Spectroscopic models III and IV.
a, Model III, with zoom on the strongest lines (b). c, Model IV, with zoom on the strongest lines (d). For these models, the excitation temperature is T = 400 K, and we use a Gaussian profile of 0.35 cm−1 full-width-at-half-maximum.
Extended Data Fig. 6 NIRSpec spectrum of d203-506. The spectrum is shown in gray, the shaded regions is the ±3 sigma error interval of the data.
This includes the error provided by the JWST pipeline, and error ν3 band of CH3+ in the NIRSpec spectrum of d203-506. Model of the OH emission (blue), H2 emission (orange), CH3+ emission (green), and sum of these three (red). Beyond 3.22 μm, emission due to the wings of the Aromatic Infrared Band at 3.3 μm is seen, affecting the baseline of the NIRSpec spectrum. The OH spectrum is computed with an LTE model at a temperature of 800 K. A detailed model of the OH emission will be presented in a forthcoming paper (Zannese et al. in prep). The H2 lines are fitted individually. The CH3+ model used here was computed using the constants for v = 0 and v3 = 1 from Extended Data Table 3, at 400 K.
Extended Data Fig. 7 Photochemical model results for d203-506 adopting G0 = 4 × 104 and different gas densities (nH) and dust grain properties.
Upper panels: Density and gas temperature structure as a function of visual extinction (AV) from the wind surface. The gray curve shows the density of vibrationally excited H2*(v > 0). Lower panels: Abundance profiles with respect to H nuclei. The pink dotted curves show the molecular fraction \({f}_{{H}_{2}}\) profile. Dashed curves in model a) refer to a model with the same gas density but G0 lower by a factor 104.
Extended Data Fig. 8 Dominant CH3+ formation and destruction reactions at the CH3+ abundance peak predicted by the photochemical model shown in Fig. 7.
This reaction network also leads to abundant HCO+ in FUV-irradiated gas layers where x(C+) > x(CO). Red arrows show endoergic reactions when H2 is in the ground-vibrational state v = 0. These reactions become fast only in disk layers where the gas temperature is high (several hundred K) and/or significant vibrationally excited H2* (v ≥ 0) exists. The formation of CH3+ from methane will only be relevant if very high CH4 and H+ abundances coexist in the gas.
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Berné, O., Martin-Drumel, MA., Schroetter, I. et al. Formation of the methyl cation by photochemistry in a protoplanetary disk. Nature 621, 56–59 (2023). https://doi.org/10.1038/s41586-023-06307-x
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DOI: https://doi.org/10.1038/s41586-023-06307-x
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