{
  "abstract": "CO isotopolog emission is widely used to infer gas masses and volatile carbon abundances in protoplanetary disks, but converting line emission into a CO column depends on optical depth, temperature structure, line width, and isotope ratios. Inferring CO/H ₂ additionally requires an independent constraint on the local hydrogen column. We use high-resolution ¹³ C ¹⁸ O 2–1 observations of HD 163296 to derive a spatially localized empirical constraint on the CO column at the resolved CO snowline edge. We focus on the 70–75 au annulus, on the inner, high-column side of the observed profile steepening near 75 au. Using RADEX slab calculations conditioned on a two-dimensional temperature structure, we infer an effective beam-averaged CO column from the absolute integrated intensity. Across representative temperatures, isotope-ratio pairs, and effective local line widths of 0.30 and 0.50 km s ⁻¹ , we find N CO beam = ( 1.6 –2.4) × 10 ²⁰ cm ⁻² and ¹³ C ¹⁸ O line-center optical depths τ = 0.39–0.97. Thus, even this rare isotopolog is not safely optically thin at the snowline edge. Adopting N H2 = 2.5 × 10 ²⁴ cm ⁻² from a published parametric gas surface density profile gives the conditional abundance CO/H ₂ = (6.4–9.5) × 10 ⁻⁵ . A beam-forward radial-profile analysis gives consistent columns, and a physical disk model with a near-canonical warm-layer CO abundance supplies a comparable CO column. The measurement is consistent with the higher C ¹⁷ O-based MAPS estimate. For the adopted hydrogen column, the inferred CO/H ₂ ratio is consistent with a near-canonical abundance on the warm side of the snowline.",
  "abstract_shown": true,
  "affiliations": [
    "Boston University",
    "National Astronomical Observatory of Japan",
    "Center for Astrophysics ∣ Harvard & Smithsonian"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 4,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Chunhua Qi",
      "orcid": "0000-0001-8642-1786"
    },
    {
      "affs": [
        1
      ],
      "name": "Takahiro Ueda",
      "orcid": "0000-0003-4902-222X"
    },
    {
      "affs": [
        2
      ],
      "name": "David J. Wilner",
      "orcid": "0000-0003-1526-7587"
    },
    {
      "affs": [
        0
      ],
      "name": "Catherine C. Espaillat",
      "orcid": "0000-0001-9227-5949"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/dnt8p7ece3ex1.bib",
  "cited_by": 0,
  "doi": "10.3847/1538-4357/aea097",
  "fulltext_url": null,
  "id": "dnt8p7ece3ex1",
  "issns": [
    "0004-637X",
    "1538-4357"
  ],
  "landing_url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea097",
  "language": null,
  "licence": "CC-BY-4.0",
  "licence_label": "CC BY 4.0",
  "licence_source": "article",
  "licence_url": "https://creativecommons.org/licenses/by/4.0/",
  "metadata_licence": null,
  "native_subjects": [
    [
      "openalex",
      "3103|Astrophysics and Star Formation Studies"
    ]
  ],
  "openalex": "W7215048189",
  "pdf_url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea097/pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-01",
  "publisher": "American Astronomical Society",
  "ris": "https://research.draconitic.com/paper/dnt8p7ece3ex1.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:11Z",
      "native_id": "10.3847/1538-4357/aea097",
      "source": "crossref",
      "url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea097"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "openalex:3103|Astrophysics and Star Formation Studies",
      "subfield": "stellar"
    }
  ],
  "title": "Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and ¹³ C ¹⁸ O Optical Depth",
  "title_text": "Empirical Constraints on the CO Snowline Transition in HD 163296: The Local CO Column and ¹³ C ¹⁸ O Optical Depth",
  "type": "peer-reviewed",
  "updated_at": "2026-10-30T12:18:11Z",
  "url": "https://research.draconitic.com/paper/dnt8p7ece3ex1",
  "venue": "The Astrophysical Journal",
  "versions": []
}