The Great Dimming of the Hypergiant Star RW Cephei: CHARA Array Images and Spectral Analysis
Gies, Douglas R.; Monnier, John D.; Schaefer, Gail H.; Lanthermann, Cyprien; Anugu, Narsireddy; Kraus, Stefan; Gardner, Tyler; Le Bouquin, Jean-Baptiste; Baron, Fabien; Roettenbacher, Rachael M.; Setterholm, Benjamin R.; Davies, Claire L.; Labdon, Aaron; Ennis, Jacob; Brummelaar, Theo ten; Shepard, Katherine A.; Vollmann, Wolfgang; Sigismondi, Costantino
United States, United Kingdom, France, Chile, Germany, Italy
Abstract
The cool hypergiant star RW Cephei is currently in a deep photometric minimum that began several years ago. This event bears a strong similarity to the Great Dimming of the red supergiant Betelgeuse that occurred in 2019-2020. We present the first resolved images of RW Cephei that we obtained with the CHARA Array interferometer. The angular diameter and Gaia distance estimates indicate a stellar radius of 900-1760 R ⊙, which makes RW Cephei one of the largest stars known in the Milky Way. The reconstructed, near-infrared images show a striking asymmetry in the disk illumination with a bright patch offset from the center and a darker zone to the west. The imaging results depend on assumptions made about the extended flux, and we present two cases with and without allowing extended emission. We also present a recent near-infrared spectrum of RW Cep that demonstrates that the fading is much larger at visual wavelengths compared to that at near-infrared wavelengths as expected for extinction by dust. We suggest that the star's dimming is the result of a recent surface mass ejection event that created a dust cloud that now partially blocks the stellar photosphere.