The Hot, Continuum-compact State of the Yellow Hypergiant V509 Cassiopeiae: CHARA Diameters, CO-band Angular-extension Constraints, and Companion Limits
4 affiliations
- 1The CHARA Array of Georgia State University
- 2University of Tartu
- 3University of Michigan
- 4University of Exeter
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.
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BibTeX
@article{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},
journal = {The Astrophysical Journal},
year = {2026},
month = oct,
doi = {10.3847/1538-4357/aea08a},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea08a},
note = {CC BY 4.0},
}
RIS
TY - JOUR
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 - 2026/10/12
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aea08a
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aea08a
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 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.
SN - 0004-637X
SN - 1538-4357
ER -