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A solar C/O ratio in planet-forming gas at 1 au in a highly irradiated disk

Nature Astronomy volume 9, pages 1326–1336 (2025)Cite this article

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Abstract

The chemical composition of exoplanets is thought to be influenced by the composition of the disks in which they form. JWST observations have unveiled a variety of chemical species in numerous nearby disks, which show substantial variations in the C/O abundance ratio. However, little is known about the composition and C/O ratio of disks around young stars in clusters exposed to strong ultraviolet radiation from nearby massive stars, which are representative of the environments where most planetary systems form, including ours. Here we present JWST spectroscopy of d203-504, a young 0.7 M⊙ star in the Orion nebula with a 30 au disk irradiated by nearby massive stars. These observations reveal spectroscopic signatures of CO, H2O, CH3+ and polycyclic aromatic hydrocarbons. Water and CO are detected in absorption in the inner disk (r ≲ 1 au), where the estimated gas-phase C/O ratio is 0.48, consistent with the solar value and that of the Orion nebula. By contrast, CH3+ and polycyclic aromatic hydrocarbons are found in the extended surface layers of the disk. These results suggest that gas in the inner disk is chemically shielded from ultraviolet radiation, whereas the surface layers of the disk experience ultraviolet-induced chemistry, potentially depleting their carbon content.

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Fig. 1: JWST NIRSpec/IFU and MIRI/MRS combined spectrum of the d203-504 proplyd.
Fig. 2: Analysis of PAH, CH3+ and H2 emissions from d203-504.
Fig. 3: Optical depth spectra of water and carbon monoxide absorption in the inner disk of d203-504.
Fig. 4: Chemical structure in the solar-type d203-504 proplyd.
Fig. 5: Fraction of the available cosmic carbon abundance locked in PAHs.

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Data availability

The JWST data presented in this paper are publicly available via the MAST online archive (http://mast.stsci.edu) using the programme ID 1288.

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 code used in this study is available from the corresponding author on reasonable request.

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Acknowledgements

This work is based in part on observations made with the NASA/ESA/CSA JWST. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with programme 1288. I.S. and O.B. are funded by the Centre National d’Etudes Spatiales through the APR programme. This research received funding from the programme ANR-22-EXOR-0001 Origins of the Institut National des Sciences de l’Univers, CNRS. We thank R. Le Gal for her comments on the paper. T.J.H. acknowledges funding from a Royal Society Dorothy Hodgkin Fellowship and UKRI guaranteed funding for a Horizon Europe ERC consolidator grant (EP/Y024710/1). J.R.G. thanks the Spanish MCINN for funding support (grant no. PID2023-146667NB-I00). This project is co-funded by the European Union (ERC, SUL4LIFE, grant agreement no. 101096293). A.F. thanks project PID2022-137980NB-I00 funded by the Spanish Ministry of Science and Innovation/State Agency of Research MCIN/AEI/10.13039/501100011033 and by ‘ERDF A way of making Europe’. T.O. acknowledges the support of the Japan Society for the Promotion of Science (KAKENHI grant no. JP24K07087). C.B. is grateful for an appointment at NASA Ames Research Center through the San José State University Research Foundation (grant no. 80NSSC22M0107).

Author information

Authors and Affiliations

  1. Institut de Recherche en Astrophysique et Planétologie, Université de Toulouse, CNRS, CNES, Toulouse, France

    Ilane Schroetter, Olivier Berné, Paul Amiot & Christine Joblin

  2. Laboratoire d’Etudes du Rayonnement et de la Matière, Observatoire de Paris, Université Paris Science et Lettres, Centre National de la Recherche Scientifique, Sorbonne Universités, Meudon, France

    Emeric Bron & Le Petit Franck

  3. Department of Astronomy, University of Michigan, Ann Arbor, MI, USA

    Felipe Alarcon & Edwin A. Bergin

  4. NASA Ames Research Center, Moffett Field, CA, USA

    Christiaan Boersma

  5. Department of Physics & Astronomy, The University of Western Ontario, London, Ontario, Canada

    Jan Cami & Els Peeters

  6. Institute for Earth and Space Exploration, The University of Western Ontario, London, Ontario, Canada

    Jan Cami & Els Peeters

  7. Carl Sagan Center, Search for ExtraTerrestrial Intelligence Institute, Mountain View, CA, USA

    Jan Cami & Els Peeters

  8. Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London, UK

    Gavin A. L. Coleman & Thomas J. Haworth

  9. Institut des Sciences Moléculaires d’Orsay, Université Paris-Saclay, Centre National de la Recherche Scientifique, Orsay, France

    Emmanuel Dartois

  10. Centro de Astrobiología (CAB), CSIC-INTA, Madrid, Spain

    Asuncion Fuente

  11. Instituto de Física Fundamental (Consejo Superior de Investigaciones Científica), Madrid, Spain

    Javier R. Goicoechea

  12. Institut d’Astrophysique Spatiale, Université Paris-Saclay, Centre National de la Recherche Scientifique, Orsay, France

    Emilie Habart & Marion Zannese

  13. Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo, Japan

    Takashi Onaka

  14. Physikalischer Verein - Gesellschaft für Bildung und Wissenschaft, Frankfurt, Germany

    Markus Rölling

  15. Astronomy Department, University of Maryland, College Park, MA, USA

    Alexander G. G. M. Tielens

  16. Leiden Observatory, Leiden University, Leiden, The Netherlands

    Alexander G. G. M. Tielens

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  1. Ilane Schroetter
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  2. Olivier Berné
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  3. Emeric Bron
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  4. Felipe Alarcon
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  5. Paul Amiot
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  6. Edwin A. Bergin
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  7. Christiaan Boersma
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  8. Jan Cami
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  9. Gavin A. L. Coleman
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  10. Emmanuel Dartois
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  11. Asuncion Fuente
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  12. Javier R. Goicoechea
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  13. Emilie Habart
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  14. Thomas J. Haworth
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  15. Christine Joblin
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  16. Le Petit Franck
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  17. Takashi Onaka
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  18. Els Peeters
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  19. Markus Rölling
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  20. Alexander G. G. M. Tielens
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  21. Marion Zannese
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Contributions

I.S. and O.B. wrote the paper with important input from E.B. I.S. did the line analysis with support from O.B. I.S., F.A. and the PDRs4All data reduction team reduced the data. E.H., E.P. and O.B. planned and co-led the PDRs4All early release science programme. O.B., E.B, F.A., P.A., E.A.B., C.B., J.C., G.A.L.C., E.D., A.F., J.R.G., E.H., T.J.H., C.J., L.P.F., T.O., E.P., M.R., A.G.G.M.T. and M.Z. contributed to the observing programme with JWST. All authors participated in the development and testing of the MIRI/MRS or NIRSpec instruments and their data reduction or in the discussion of the results or commented on the paper.

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Correspondence to Ilane Schroetter.

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Extended data

Extended Data Fig. 1 MUSE and NIRCam images of d203-504 environment.

Left: MUSE NFM composite image of both d203-506 (bottom left) and d203-504 (top). Red: the [C i](λ8727∘A) map showing disk contributions from both d203-506 and d203-504 objects. Green: the [Fe ii] (λ8617∘A) map corresponding to jets from both objects. Blue: the Hα emission map showing the ionization fronts (only d203-504 appears to have one). The glow on the top right corresponds to the Orion Bar ionization front. Right: JWST NIRCam image of the same system from the F210M filter.

Extended Data Fig. 2 ALMA 344 GHz emission map of d203-506 and d203-504.

Values are given in Jy/beam and the coordinates are givens as offsets in arcseconds with respect to the position of d203-504 (RA=5:35:20.268; DEC=-05:25:03.992).

Extended Data Fig. 3 Extended Data 3 ON and OFF extracted spectra and PAH emission.

Top panel: Observed spectrum of d203-504 (the ‘ON’ position) in black and the ‘OFF’ position in blue. Fitted continuums are shown in dotted black lines. Bottom panel: continuum subtracted spectrum of d203-504, ‘ON’ and ‘OFF’ positions in black and blue, respectively, and the disk PAH emission in green.

Extended Data Fig. 4 JWST NIRSpec/IFU and MIRI/MRS combined spectrum of the d203-504 proplyd.

The vertical lines indicate the position of HI (dashed), He I (dotted) and H2 (continuous) emission lines. Fine structure lines of atoms are labeled directly on the figure. This spectrum is in units of MJy/sr to emphasize the silicate emissions highlighted in green.

Extended Data Fig. 5 OH and CH+ emission.

Zooms on d203-504 NIRSpec spectrum, represented in black on both panels. On the upper panels, positions of OH emission lines are shown in blue and on the lower panels, positions of CH+ emission lines are shown in red.

Extended Data Fig. 6 MIRI water absorption models.

Spectroscopic models (Fmod(λ)) of d203-504 using Eq. 8 (blue) and Eq. 9 (red). Observed spectrum (Fobs(λ)) is in black.

Extended Data Table 1 Detected emission lines and their intensities
Full size table
Extended Data Table 2 Detected emission lines and their intensities
Full size table
Extended Data Table 3 H2 line detection properties
Full size table
Extended Data Table 4 Derived physical properties of considered species
Full size table

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Schroetter, I., Berné, O., Bron, E. et al. A solar C/O ratio in planet-forming gas at 1 au in a highly irradiated disk. Nat Astron 9, 1326–1336 (2025). https://doi.org/10.1038/s41550-025-02596-6

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