The Hot, Continuum-compact State of the Yellow Hypergiant V509 Cassiopeiae: CHARA Diameters, CO-band Angular-extension Constraints, and Companion Limits
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±0.004, 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.
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BibTeX
@misc{anugu2026continuumcompact,
title = {{The Hot, Continuum-compact State of the Yellow Hypergiant V509 Cassiopeiae: CHARA Diameters, CO-band Angular-extension Constraints, and Companion Limits}},
author = {Narsireddy Anugu and Gail H. Schaefer and Douglas R. Gies and Anni Kasikov and John D. Monnier and Stefan Kraus and Karolina Kubiak},
howpublished = {arXiv},
year = {2026},
month = aug,
doi = {10.48550/arxiv.2608.28829},
eprint = {2608.28829},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2608.28829},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The Hot, Continuum-compact State of the Yellow Hypergiant V509 Cassiopeiae: CHARA Diameters, CO-band Angular-extension Constraints, and Companion Limits
AU - Narsireddy Anugu
AU - Gail H. Schaefer
AU - Douglas R. Gies
AU - Anni Kasikov
AU - John D. Monnier
AU - Stefan Kraus
AU - Karolina Kubiak
PY - 2026
DA - 2026/08/28
PB - arXiv
DO - 10.48550/arxiv.2608.28829
UR - https://arxiv.org/abs/2608.28829
AB - 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±0.004, 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.
ER -