{
  "abstract": "The carbon-to-oxygen (C/O) ratio of a giant planet's atmosphere has long been theorised to hold compositional information that can be traced back to its formation history. Typically, the C/O ratio of an exoplanetary atmosphere is inferred from abundance measurements of major C- and O-bearing molecules such as CO, CO$_2$, H$_2$O, and OH. However, some exoplanets have such elevated temperatures that molecules can be nearly completely thermally dissociated at the pressure levels that observations probe, making it difficult to measure their C/O ratio via these traditional tracers. Here, rather than using molecules, we aim to retrieve the C/O ratio of KELT-9b (T$_\\mathrm{eq} = 4000$K) directly from atomic C and O, which are detected in its atmosphere. We analysed two transits of KELT-9b observed with the MAROON-X high-resolution spectrograph, finding absorption cross-correlation signals from atomic C and O as well as refractory metals. From this, we inferred the vertical temperature structure, the relative proportions of volatile and refractory species, and the C/O ratio of KELT-9b using a 1D local thermodynamic equilibrium atmospheric retrieval framework applied only to spectral regions mostly unaffected by non-local thermodynamic equilibrium effects. We measure the atomic C/O ratio of the atmosphere of KELT-9b to be $0.20_{-0.07}^{+0.13}$, which is slightly lower than the stellar value of $0.38\\pm0.15$ and significantly below the solar value of $0.59\\pm0.07$. We otherwise confirm previous investigations of the terminator region, finding KELT-9b's atmosphere to be thermally inverted and slightly metal-rich. We measure the volatile-to-refractory ratio, a proxy for the ice-to-rock ratio, to be $1.24_{-0.78}^{+1.93}$ $\\times$ solar ($[$M$_{\\mathrm{vol}}$/M$_{\\mathrm{ref}}]$ = $0.09_{-0.20}^{+0.19}$), which is consistent with both KELT-9 and the Sun.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2609.13051",
  "author_total": 15,
  "authors": [
    {
      "name": "Stefan Pelletier"
    },
    {
      "name": "Nicholas W. Borsato"
    },
    {
      "name": "Jacob L. Bean"
    },
    {
      "name": "Andreas Seifahrt"
    },
    {
      "name": "David Kasper"
    },
    {
      "name": "Julian Stürmer"
    },
    {
      "name": "Ana Rita Costa Silva"
    },
    {
      "name": "David Ehrenreich"
    },
    {
      "name": "Luca Fossati"
    },
    {
      "name": "Jens Hoeijmakers"
    },
    {
      "name": "Bibiana Prinoth"
    },
    {
      "name": "Michal Steiner"
    },
    {
      "name": "Brian Thorsbro"
    },
    {
      "name": "Valentina Vaulato"
    },
    {
      "name": "Daniel B. Zucker"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/efw0v6h4gjpr8.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2609.13051",
  "fulltext_url": null,
  "id": "efw0v6h4gjpr8",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2609.13051",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.EP"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2609.13051",
  "pmcid": null,
  "pmid": null,
  "published": "2026-09-11",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/efw0v6h4gjpr8.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:21:07Z",
      "native_id": "2609.13051",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2609.13051"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.EP",
      "subfield": "planetary"
    }
  ],
  "title": "The atomic C/O ratio of KELT-9b",
  "title_text": "The atomic C/O ratio of KELT-9b",
  "type": "preprint",
  "updated_at": "2026-10-30T12:21:07Z",
  "url": "https://research.draconitic.com/paper/efw0v6h4gjpr8",
  "venue": "arXiv",
  "versions": []
}