The Hot, Continuum-compact State of the Yellow Hypergiant V509 Cassiopeiae: CHARA Diameters, CO-band Angular-extension Constraints, and Companion Limits
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.