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A rich hydrocarbon chemistry and high C to O ratio in the inner disk around a very low-mass star

Nature Astronomy volume 7, pages 805–814 (2023)Cite this article

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Abstract

Carbon is an essential element for life but how much can be delivered to young planets is still an open question. The chemical characterization of planet-forming disks is a crucial step in our understanding of the diversity and habitability of exoplanets. Very low-mass stars (less than 0.2 M⊙) are interesting targets because they host a rich population of terrestrial planets. Here we present the James Webb Space Telescope detection of abundant hydrocarbons in the disk of a very low-mass star obtained as part of the Mid-InfraRed Instrument mid-INfrared Disk Survey (MINDS). In addition to very strong and broad emission from C2H2 and its 13C12CH2 isotopologue, C4H2, benzene and possibly CH4 are identified, but water, polycyclic aromatic hydrocarbons and silicate features are weak or absent. The lack of small silicate grains indicates that we can look deep down into this disk. These detections testify to an active warm hydrocarbon chemistry with a high C/O ratio larger than unity in the inner 0.1 astronomical units (AU) of this disk, perhaps due to destruction of carbonaceous grains. The exceptionally high C2H2/CO2 and C2H2/H2O column density ratios indicate that oxygen is locked up in icy pebbles and planetesimals outside the water iceline. This, in turn, will have important consequences for the composition of forming exoplanets.

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Fig. 1: JWST–MIRI–MRS spectrum of J160532 showing prominent C2H2 emission.
Fig. 2: Detection of C6H6 (benzene), CO2 and C4H2.
Fig. 3: Column density ratios in the inner disk of J160532.
Fig. 4: The possible structure of the inner part of the J160532 disk.

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Data availability

The original data analysed in this work are part of the Guaranteed Time Observation-MIRI programme ‘MIRI EC Protoplanetary and Debris Disks Survey’ (ID 1282) with number 47 and will become public on 1 August 2023 on the MAST database https://archive.stsci.edu/. The continuum-subtracted spectra presented in Fig. 1 (right) and in Fig. 2 are available on Zenodo at https://zenodo.org/record/7850667. The spectroscopic data for all the species but benzene are available on the HITRAN database (https://hitran.org/). For benzene, the data will be shared on request to the corresponding author.

Code availability

The slab model used in this work is a private code developed by B.T. and collaborators. It is available from the corresponding author upon request. The synthetic spectra presented in this work can be reproduced using the slabspec code, which is publicly available at https://doi.org/10.5281/zenodo.4037306.

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Acknowledgements

The MIRI mid-INfrared Disk Survey team thank the entire MIRI European and United States instrument team. Support from StScI is also appreciated. The following National and International Funding Agencies funded and supported the MIRI development: NASA; ESA; Belgian Science Policy Office (BELSPO); Centre Nationale d’Etudes Spatiales (CNES); Danish National Space Centre; Deutsches Zentrum fur Luftund Raumfahrt; Enterprise Ireland; Ministerio De Economiá y Competividad; Netherlands Research School for Astronomy (NOVA); Netherlands Organisation for Scientific Research (NWO); Science and Technology Facilities Council; Swiss Space Office; Swedish National Space Agency and UK Space Agency. B.T. is a Laureate of the Paris Region fellowship programme, which is supported by the Ile-de-France Region and has received funding under Marie Sklodowska-Curie grant agreement no. 945298. B.T. acknowledges support from the Programme National ‘Physique et Chimie du Milieu Interstellaire’ (PCMI) of CNRS/INSU with INC/INP and cofunded by CNES. G.B. thanks the Deutsche Forschungsgemeinschaft (DFG), grant no. 325594231, FOR 2634/2. E.F.v.D. acknowledges support from the EU ERC grant no. 101019751 MOLDISK and the Danish National Research Foundation through the Center of Excellence ‘InterCat’ (DNRF150). D.G. thanks the Research Foundation Flanders for cofinancing the present research (grant no. V435622N). T.H. and K.S. acknowledge support from the ERC Advanced grant no. Origins 83 24 28. I.K., A.M.A. and E.F.v.D. acknowledge support from grant no. TOP-1614.001.751 from the Dutch Research Council (NWO). I.K. and J.K. acknowledge funding from H2020-MSCA-ITN- 2019, grant no. 860470 (CHAMELEON). O.A. and V.C. acknowledge funding from the Belgian F.R.S.-FNRS. I.A. and D.G. thank the European Space Agency (ESA) and the Belgian Federal Science Policy Office (BELSPO) for their support in the framework of the PRODEX Programme. D.B. has been funded by Spanish grant no. MCIN/AEI/10.13039/501100011033 grant nos. PID2019- 107061GB-C61 and no. MDM-2017-0737. A.C.G. has been supported by grant nos. PRIN-INAF MAIN-STREAM 2017 and PRIN-INAF 2019 (STRADE). T.P.R. acknowledges support from ERC grant no. 743029 EASY. D.R.L. acknowledges support from Science Foundation Ireland, grant no. 21/PATH-S/9339. L.C. acknowledges support by grant no. PIB2021-127718NB-I00, from the Spanish Ministry of Science and Innovation/State Agency of Research grant no. MCIN/AEI/10.13039/501100011033.

Author information

Authors and Affiliations

  1. Université Paris-Saclay, CNRS, Institut d’Astrophysique Spatiale, Orsay, France

    B. Tabone & A. Abergel

  2. Leiden Observatory, Leiden University, Leiden, the Netherlands

    B. Tabone, G. Bettoni & E. F. van Dishoeck

  3. Max-Planck Institute for Extraterrestrial Physics (MPE), Garching, Germany

    G. Bettoni, E. F. van Dishoeck & S. Grant

  4. Kapteyn Astronomical Institute, University of Groningen, Groningen, the Netherlands

    A. M. Arabhavi, I. Kamp, M. Mueller & J. Kanwar

  5. Institute of Astronomy, KU Leuven, Leuven, Belgium

    D. Gasman, B. Vandenbussche, I. Argyriou & C. Waelkens

  6. Max-Planck Institute for Astronomy (MPIA), Heidelberg, Germany

    Th. Henning, M. Güdel, J. Bouwman, S. Scheithauer, G. Perotti, M. Samland, J. Schreiber & K. Schwarz

  7. Institute for Particle Physics and Astrophysics, ETH Zürich, Zürich, Switzerland

    M. Güdel & A. M. Glauser

  8. Department of Astrophysics, University of Vienna, Vienna, Austria

    M. Güdel, R. Guadarrama & N. Pawellek

  9. Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, Gif-sur-Yvette, France

    P. O. Lagage, C. Nehmé & E. Pantin

  10. Dublin Institute for Advanced Studies, Dublin, Ireland

    T. Ray, A. Caratti o Garatti & D. Rodgers-Lee

  11. STAR Institute, University of Liège, Liège, Belgium

    O. Absil & V. Christiaens

  12. Centre for Astrobiology (CAB), CSIC-INTA, Madrid, Spain

    D. Barrado & M. Morales-Calderón

  13. LESIA, Observatoire de Paris, University PSL, CNRS, Sorbonne University, University of Paris, Meudon, France

    A. Boccaletti

  14. INAF—Astronomical Observatory of Capodimonte, Napoli, Italy

    A. Caratti o Garatti

  15. UK Astronomy Technology Centre, Royal Observatory Edinburgh, Edinburgh, UK

    V. Geers & G. Wright

  16. Department of Space, Earth and Environment, Onsala Space Observatory, Chalmers University of Technology, Onsala, Sweden

    K. Justannont & J. H. Black

  17. SRON Netherlands Institute for Space Research, Groningen, the Netherlands

    F. Lahuis

  18. Department of Astronomy, Stockholm University, AlbaNova University Center, Stockholm, Sweden

    G. Olofsson & G. Östlin

  19. Department of Astrophysics/IMAPP, Radboud University, Nijmegen, the Netherlands

    L. B. F. M. Waters & H. Jang

  20. SRON Netherlands Institute for Space Research, Leiden, the Netherlands

    L. B. F. M. Waters

  21. Space Research Institute, Austrian Academy of Sciences, Graz, Austria

    J. Kanwar

  22. Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Budapest, Hungary

    N. Pawellek

  23. Laboratoire de Météorologie Dynamique/IPSL, CNRS, Ecole Polytechnique, Institut Polytechnique de Paris, Sorbonne Université, PSL Research University, Palaiseau, France

    A. Perrin

  24. Centre for Astrobiology (CAB, CSIC-INTA), Carretera de Ajalvir, Madrid, Spain

    L. Colina

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  1. B. Tabone
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Contributions

B.T. and G.B. did the analysis using molecular data files created by A.M.A. and A.P. and a model developed by B.T. and J.H.B. G.B., S.G. and D.G. performed the data reduction, supported by I.A., J.S., M.S., G.P., V.C. and J.B. E.F.v.D., B.T. and G.B. wrote the manuscript. T.H. and I.K. planned and co-led the MIRI guaranteed time project on disks. All authors participated in either the development and testing of the MIRI instrument and its data reduction, in the discussion of the results and/or commented on the manuscript.

Corresponding author

Correspondence to B. Tabone.

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The authors declare no competing interests.

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Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Comparison between the Spitzer-IRS spectrum and the MIRI-MRS spectrum and baseline fits of the MIRI-MRS spectrum.

The Spitzer-IRS low-resolution spectrum58 has been shifted by 5 mJy to ease the comparison with the MIRI-MRS spectrum (top panel). Baseline fits used in the continuum subtracted spectrum presented in Fig. 1 and 2 in the Results section are shown in the middle and bottom panels, respectively. The blue dots represent the location where the continuum is evaluated. The red curve is the interpolated continuum used to produce continuum-subtracted spectra (in grey). The presence of warm dust is evidenced by the infrared continuum emission on either side of the two C2H2 bumps but no silicate feature is detected. HI and H2 lines are present in the spectrum and will be analysed in a next paper.

Extended Data Fig. 2 Constraints obtained from χ2 fits.

The χ2 maps for the fit of the 13.7 μm broad bump associated with C2H2 (left), and the CO2 (middle) and C4H2 (right) features are shown. The 1σ, 2σ, and 3σ confidence intervals are pictured in red, yellow, and orange, respectively. The best-fitting emitting radius R for each value of N and T is indicated as grey lines. In general, we find a degeneracy between a high T and low N solutions, and a low T and high N solutions. For CO2 the best fit corresponding to an emitting area of 0.033 au is chosen to alleviate the degeneracy and compare with the optically thick component of C2H2 (component I). We note that for R = 0.07 au, corresponding to component II, the CO2 feature can be fitted by either a hot and thin model or a cold and thick model. However, the thick solution over-predicts 13CO2 emission which is not detected. We therefore report in Table 1 the column density of the optically thin solution for component II.

Extended Data Fig. 3 Effect of line overlap on the main C2H2 feature at 13.7 μm.

Left: C2H2 emission as function of column density for T = 500 K and R = 0.1au. Note that the Q-branch becomes highly optically thick above N(C2H2) = 1020 cm−2 and flattens. The contrast between the amplitude of the narrow features on either side of the Q − branch and the continuum level decreases by increasing N(C2H2). A column density of at least N(C2H2) ≃ 1020 cm−2 is required to fit the observations. Right: Importance of line overlap in slab models. For highly optically thick lines that are close to each other such as in the Q − branch of C2H2, slab models neglecting line overlap overestimate the fluxes. For C2H2, this effect dominates for N ≳ 1019 cm−2.

Extended Data Fig. 4 Possible indication for CH4 emission in the 7.64-7.77 μm range.

CH4 emission could be present at 7.655 μm in addition to the many C2H2 lines in this region. The column density of CH4 is estimated assuming that the emission originates from component II (see main text, Table 1). The C2H2 model in purple corresponds to the component II for which the best-fit column density has been increased by a factor of 4 to better match the series of C2H2 lines in that specific spectral region.

Extended Data Fig. 5 Constraints of the amount of HCN in the 14 μm region.

The C2H2 model, including both component I and II is shown in red on top of the MIRI spectrum where the contribution of the C2H2 thick component is not subtracted. This figure shows that a maximum column density of HCN of N = 1.5 × 1017cm−2 can be hidden in the C2H2 line forest in this region assuming an origin in the optically thin component II (R = 0.07 au and T = 400K). HCN emission from the C2H2 thick component I would be highly masked by C2H2 and therefore its column density remains unconstrained.

Extended Data Fig. 6 Possible detection of weak H2O lines in the 17.2 μm and 6.5 μm regions.

The pure rotational lines at 16.5–18 μm can hide as much as N(H2O)=3 × 1018 cm−2 assuming a fixed temperature of 525 K and a characteristic emitting radius of R = 0.033au, corresponding to the optically thick C2H2 component I. These lines are not affected by masking of C2H2 since only very weak lines of C2H2 are present in these spectral ranges. Some lines in the 6.3-6.8 μm range are somewhat overestimated by our LTE model but non-LTE effects will tend to quench these lines compared to the pure rotational lines longward of ~ 12μm59.

Extended Data Fig. 7 J160532 line fluxes compared to other disks.

This figure presents a comparison of C2H2 versus H2O line flux scaled to 140 pc for a number of T Tauri disks observed with Spitzer and complied by ref. 60 and J160532 observed with JWST MIRI. The line fluxes for J160532 are consistently calculated by integrating the flux of the three water features at 17.12 μm, 17.22 μm, and 17.36 μm, and the C2H2 feature over a window between 13.65-13.72 μm as explained in ref. 60. Leftward (resp. downward) arrows represent upper limits on H2O (resp. C2H2) line flux.

Extended Data Table 1 Observed column density ratios of various species compared with disk models with solar C/O elemental ratio in the gas-phase
Full size table

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Tabone, B., Bettoni, G., van Dishoeck, E.F. et al. A rich hydrocarbon chemistry and high C to O ratio in the inner disk around a very low-mass star. Nat Astron 7, 805–814 (2023). https://doi.org/10.1038/s41550-023-01965-3

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