The Great Dimming of the hypergiant star RW Cephei: CHARA Array images and spectral analysis

TL;DR

This study presents the first resolved images and spectral analysis of RW Cephei during its deep dimming event, revealing asymmetrical surface features and suggesting dust cloud formation from a recent mass ejection as the cause.

Contribution

First resolved interferometric images of RW Cephei, providing insights into its asymmetrical surface features and the dimming mechanism involving dust ejection.

Findings

RW Cephei has a radius of 900-1760 solar radii, making it one of the largest stars in the Milky Way.

The star's dimming is more pronounced at visual wavelengths due to dust extinction.

Images show asymmetry with bright and dark regions on the stellar disk.

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_sun which makes RW Cep 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 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.