{
  "abstract": "Yellow hypergiants are rare, evolved massive stars generally interpreted as post-red-supergiant objects evolving blueward through or near the dynamically unstable yellow evolutionary void. V509 Cassiopeiae is a remarkable example: its spectroscopically inferred effective temperature increased from approximately 5000 K in the mid-1980s to about 7900 K by the late 1990s and has since remained near this value during its postheating state. We use simultaneous six-telescope CHARA observations obtained in 2023 with MIRC-X in the H band and MYSTIC in the K band to measure its photospheric diameter, search for a luminous companion, and constrain any spatially extended CO atmosphere. Interferometric data fitting with fixed atmosphere-model center-to-limb variation profiles yield limb-darkened angular diameters of 1.1840–1.1950 mas. Although nominally about 5% smaller than the previous Palomar Testbed Interferometer measurement, the difference is only 1.8 σ , indicating no significant secular change over 17–21 yr. The closure phases are consistent with a centrosymmetric brightness distribution, and no near-infrared-bright companion is detected over the sampled separations. The median 3 σ limits are Δ H = 5.28 mag and Δ K = 6.48 mag. The B1 V companion proposed from ultraviolet spectroscopy is expected to lie below these limits and is not excluded. The CO first-overtone channels give a weighted mean CO-to-continuum diameter ratio of 1.011 ± 0.004, below the adopted 3 σ detection criterion. We find no significant CO-band angular extension. V509 Cas is thus hot and molecularly compact despite retaining spectroscopically visible atomic circumstellar gas.",
  "abstract_shown": true,
  "affiliations": [
    "The CHARA Array of Georgia State University",
    "University of Tartu",
    "University of Michigan",
    "University of Exeter"
  ],
  "article_type": null,
  "arxiv": null,
  "author_total": 7,
  "authors": [
    {
      "affs": [
        0
      ],
      "name": "Narsireddy Anugu",
      "orcid": "0000-0002-2208-6541"
    },
    {
      "affs": [
        0
      ],
      "name": "Gail H. Schaefer",
      "orcid": "0000-0001-5415-9189"
    },
    {
      "affs": [
        0
      ],
      "name": "Douglas R. Gies",
      "orcid": "0000-0001-8537-3583"
    },
    {
      "affs": [
        1
      ],
      "name": "Anni Kasikov",
      "orcid": "0000-0002-1823-3975"
    },
    {
      "affs": [
        2
      ],
      "name": "John D. Monnier",
      "orcid": "0000-0002-3380-3307"
    },
    {
      "affs": [
        3
      ],
      "name": "Stefan Kraus",
      "orcid": "0000-0001-6017-8773"
    },
    {
      "affs": [
        0
      ],
      "name": "Karolina Kubiak",
      "orcid": "0000-0001-9253-7785"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/4n4j251ntrvv1.bib",
  "cited_by": 0,
  "doi": "10.3847/1538-4357/aea08a",
  "fulltext_url": null,
  "id": "4n4j251ntrvv1",
  "issns": [
    "0004-637X",
    "1538-4357"
  ],
  "landing_url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea08a",
  "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|Stellar, planetary, and galactic studies"
    ]
  ],
  "openalex": "W7204964100",
  "pdf_url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea08a/pdf",
  "pmcid": null,
  "pmid": null,
  "published": "2026-10-12",
  "publisher": "American Astronomical Society",
  "ris": "https://research.draconitic.com/paper/4n4j251ntrvv1.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:18:11Z",
      "native_id": "10.3847/1538-4357/aea08a",
      "source": "crossref",
      "url": "https://iopscience.iop.org/article/10.3847/1538-4357/aea08a"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "openalex:3103|Stellar, planetary, and galactic studies",
      "subfield": "stellar"
    }
  ],
  "title": "The Hot, Continuum-compact State of the Yellow Hypergiant V509 Cassiopeiae: CHARA Diameters, CO-band Angular-extension Constraints, and Companion Limits",
  "title_text": "The Hot, Continuum-compact State of the Yellow Hypergiant V509 Cassiopeiae: CHARA Diameters, CO-band Angular-extension Constraints, and Companion Limits",
  "type": "peer-reviewed",
  "updated_at": "2026-10-30T12:18:11Z",
  "url": "https://research.draconitic.com/paper/4n4j251ntrvv1",
  "venue": "The Astrophysical Journal",
  "versions": []
}