{
  "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 post-heating 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 $1.8σ$, 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 $3σ$ limits are $ΔH=5.28$~mag and $ΔK=6.48$~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 $1.011\\pm0.004$, below the adopted $3σ$ 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": [],
  "article_type": null,
  "arxiv": "2608.28829",
  "author_total": 7,
  "authors": [
    {
      "name": "Narsireddy Anugu"
    },
    {
      "name": "Gail H. Schaefer"
    },
    {
      "name": "Douglas R. Gies"
    },
    {
      "name": "Anni Kasikov"
    },
    {
      "name": "John D. Monnier"
    },
    {
      "name": "Stefan Kraus"
    },
    {
      "name": "Karolina Kubiak"
    }
  ],
  "bibtex": "https://research.draconitic.com/paper/bd5esc7a97b2x.bib",
  "cited_by": 0,
  "doi": "10.48550/arxiv.2608.28829",
  "fulltext_url": null,
  "id": "bd5esc7a97b2x",
  "issns": [],
  "landing_url": "https://arxiv.org/abs/2608.28829",
  "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.SR"
    ]
  ],
  "openalex": null,
  "pdf_url": "https://arxiv.org/pdf/2608.28829",
  "pmcid": null,
  "pmid": null,
  "published": "2026-08-28",
  "publisher": null,
  "ris": "https://research.draconitic.com/paper/bd5esc7a97b2x.ris",
  "sources": [
    {
      "first_seen": "2026-10-30T12:17:41Z",
      "native_id": "2608.28829",
      "source": "arxiv",
      "url": "https://arxiv.org/abs/2608.28829"
    }
  ],
  "subjects": [
    {
      "field": "astronomy",
      "origin": "arxiv:astro-ph.SR",
      "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": "preprint",
  "updated_at": "2026-10-30T12:17:41Z",
  "url": "https://research.draconitic.com/paper/bd5esc7a97b2x",
  "venue": "arXiv",
  "versions": []
}