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Azimuthal C/O variations in a planet-forming disk

Nature Astronomy (2023)Cite this article

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An Author Correction to this article was published on 04 May 2023

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Abstract

The elemental carbon-to-oxygen ratio (C/O) in the atmosphere of a giant planet is a promising diagnostic of that planet’s formation history in a protoplanetary disk. Alongside efforts in the exoplanet community to measure the C/O ratio in planetary atmospheres, observational and theoretical studies of disks are increasingly focused on understanding how the gas-phase C/O ratio varies both with radial location and between disks. This is mostly tied to the icelines of major volatile carriers such as CO and H2O. Using ALMA observations of CS and SO, we have found evidence for an entirely unexpected type of C/O variation in the protoplanetary disk around HD 100546: an azimuthal variation from a typical, oxygen-dominated ratio (C/O ≈ 0.5) to a carbon-dominated ratio (C/O ≳ 1.0). We show that the spatial distribution and peculiar line kinematics of both CS and SO molecules can be well explained by azimuthal variations in the C/O ratio. We propose a shadowing mechanism that could lead to such a chemical dichotomy. Our results imply that tracing the formation history of giant exoplanets using their atmospheric C/O ratios will need to take into account time-dependent azimuthal C/O variations in a planet’s accretion zone.

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Fig. 1: Azimuthal disparity of SO and CS emission.
Fig. 2: Detected and modelled SO and CS emission in HD 100546.
Fig. 3: Spectral line profiles.
Fig. 4: Geometry of the HD 100546 disk model.
Fig. 5: HD 100546 model dust temperature map.
Fig. 6: Cooling, freeze-out and chemical timescales.

Data availability

The data presented here are from the ALMA Cycle 4 programme 2016.1.01339.S (principal investigator M. Kama). The raw data are publicly available from the ALMA archive. The reduced data and final imaging products are available upon reasonable request from the corresponding author.

Code availability

The ALMA data were reduced using CASA version 5.6.1-8, which is available at https://casa.nrao.edu/. Outputs from the DALI physical-chemical disk models are available at https://doi.org/10.5281/zenodo.7734194.

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Acknowledgements

We acknowledge D. Fedele for sharing the ALMA 870 μm continuum data. L.K. acknowledges funding via a Science and Technology Facilities Council (STFC) studentship. E.F.v.D. is supported by A-ERC grant agreement no. 101019751 MOLDISK. M.N.D. acknowledges the Swiss National Science Foundation (SNSF) Ambizione grant no. 180079, the Center for Space and Habitability (CSH) Fellowship, and the IAU Gruber Foundation Fellowship. C.W. acknowledges financial support from the University of Leeds, the Science and Technology Facilities Council, and UK Research and Innovation (grant numbers ST/T000287/1 and MR/T040726/1).

Author information

Authors and Affiliations

  1. Department of Physics and Astronomy, University College London, London, UK

    Luke Keyte, Mihkel Kama & Jonathan Rawlings

  2. Tartu Observatory, Tõravere, Estonia

    Mihkel Kama

  3. Leiden Observatory, Leiden University, Leiden, the Netherlands

    Alice S. Booth & Ewine F. van Dishoeck

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

    Edwin A. Bergin

  5. Department of Astronomy, University of Virginia, Charlottesville, VA, USA

    L. Ilsedore Cleeves

  6. Max-Planck Institut für Extraterrestrische Physik (MPE), Garching, Germany

    Ewine F. van Dishoeck

  7. Center for Space and Habitability, Universität Bern, Bern, Switzerland

    Maria N. Drozdovskaya

  8. National Astronomical Observatory of Japan, Mitaka, Japan

    Kenji Furuya

  9. Department of Earth Sciences and Institute of Astronomy, University of Cambridge, Cambridge, UK

    Oliver Shorttle

  10. School of Physics and Astronomy, University of Leeds, Leeds, UK

    Catherine Walsh

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Contributions

L.K. reduced the ACA CS data, ran the chemical models, performed analysis of both the data and models, and wrote the manuscript. M.K. contributed to the analysis of both the data and models, original research concepts and writing of the manuscript, and led the proposal for the ACA data. A.S.B. provided the ALMA SO data and contributed to the writing of the manuscript. E.A.B., L.I.C., E.F.v.D., M.N.D., K.F., J.R., O.S. and C.W. contributed to the writing of the manuscript.

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Correspondence to Luke Keyte.

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Nature Astronomy thanks Zhaohuan Zhu, Rebecca Nealon and Alison Young for their contribution to the peer review of this work.

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Extended data

Extended Data Fig. 1 Abundance maps and contribution functions for the modelled CS and SO emission in HD 100546.

Each panel shows an abundance map overlaid with contours representing 25% and 75% line emission (white). Top row: CS 7-6 emission from the C/O=0.5 region (left) and C/O>1 region (right). Bottom row: SO 77 − 66 + 78 − 67 emission from the C/O=0.5 region (left) and C/O>1 region (right).

Extended Data Fig. 2 Effect of varying the high-C/O wedge size and position on the modelled spectra.

Top row: SO 77 − 66 + 78 − 67 (left) and CS 7-6 (right) spectra for variations in wedge size (θ), centered on position ϕ = 0. Bottom panel: SO 77 − 66 + 78 − 67 (left and CS 7-6 (right) spectra for variations in wedge position (ϕ), for a fixed angular size θ = 60∘.

Extended Data Fig. 3 Temperature and density maps for the baseline HD 100546 disk model (C/O=0.5).

Top left: Gas number density. Bottom left: Dust number density. Top right: Gas temperature. Bottom right: Dust temperature.

Supplementary information

Supplementary Information

Supplementary Figs. 1–4, Tables 1–4 and Discussions 1 and 2.

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Keyte, L., Kama, M., Booth, A.S. et al. Azimuthal C/O variations in a planet-forming disk. Nat Astron (2023). https://doi.org/10.1038/s41550-023-01951-9

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