{
  "abstract": "Aims. We revisit the carbon abundance in the primitive iron-poor star J0815+4729 ([Fe/H] = -5.49). J0815+4729 is so carbon-enhanced and oxygen-rich that, in addition to CH and C2 molecular lines, weaker neutral atomic transitions are measurable in its spectrum, providing a unique opportunity to explore the line formation of C I and O I transitions in such an extremely iron-deficient star. Methods. Based on high-resolution spectroscopy and newly constructed atomic models for carbon and oxygen, we present the first investigation to derive carbon and oxygen abundances of this hyper iron-poor star from neutral atomic transitions under non-local thermodynamic equilibrium (NLTE) conditions. Results. The carbon abundance derived from C I lines is A(C)NLTE = 8.08+-0.07, consistent with that inferred from C2 lines (A(C)LTE =7.96 +- 0.08), but around 0.6 dex higher than the value from CH lines (A(C)LTE = 7.47 +- 0.17). Since CH lines are the most commonly-used carbon abundance indicator for extremely metal-poor stars, this result casts doubt on the quantitative interpretation of CH absorption in the most primitive stars. We confirm that J0815+4729 is one of the most carbon-enhanced stars with [Fe/H] <= -4.5 known to date. We have also determined A(O)NLTE as 7.25 +- 0.11, with NLTE corrections of around -0.11 dex. The carbon-to-oxygen ratio of the star is [C/O] = 1.06 +- 0.1.",
  "abstract_shown": true,
  "affiliations": [],
  "article_type": null,
  "arxiv": "2608.26731",
  "author_total": 10,
  "authors": [
    {
      "name": "Junbo Zhang"
    },
    {
      "name": "Carlos Allende Prieto"
    },
    {
      "name": "Yeisson Osorio"
    },
    {
      "name": "Jonay I. González Hernández"
    },
    {
      "name": "David S. Aguado"
    },
    {
      "name": "Ivan Hubeny"
    },
    {
      "name": "Shuai Liu"
    },
    {
      "name": "Jianrong Shi"
    },
    {
      "name": "Rafael Rebolo"
    },
    {
      "name": "Gang Zhao"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/atnd9kw15j2ws.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2608.26731",
  "fulltext_url": null,
  "id": "atnd9kw15j2ws",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2608.26731",
  "language": null,
  "licence": "CC-BY-NC-ND-4.0",
  "licence_label": "CC BY-NC-ND 4.0",
  "licence_source": "article",
  "licence_url": "http://creativecommons.org/licenses/by-nc-nd/4.0/",
  "metadata_licence": "CC0-1.0",
  "native_subjects": [
    [
      "arxiv",
      "astro-ph.GA"
    ],
    [
      "arxiv",
      "astro-ph.SR"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2608.26731",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-09",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/atnd9kw15j2ws.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:28:36Z",
      "native_id": "2608.26731",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2608.26731"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.GA",
      "subfield": "galaxies"
    },
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.SR",
      "subfield": "stellar"
    }
  ],
  "title": "Carbon in J0815+4729: Casting a shadow of doubt on the interpretation of CH absorption in the most primitive stars",
  "title_text": "Carbon in J0815+4729: Casting a shadow of doubt on the interpretation of CH absorption in the most primitive stars",
  "type": "preprint",
  "updated_at": "2026-10-30T12:28:36Z",
  "url": "https://research.draconitic.com/paper/atnd9kw15j2ws",
  "venue": "arXiv",
  "versions": []
}