Atmospheric dynamics of the hypergiant RW Cep during the Great Dimming

Context. The hypergiant RW Cep is one of the largest stars in the Galaxy. The evolution and mass loss of such stars have profound effects on their surrounding regions and their galaxy as a whole. Between 2020 and 2024, RW Cep experienced a historic mass-loss event known as the Great Dimming.Aims. This study provides a spectroscopic analysis of RW Cep during the Great Dimming. We examine its atmospheric dynamics and place it in the context of the star’s variability behaviour since the early 2000s.Methods. We conducted high-cadence spectroscopic observations of RW Cep during the dimming event using the Tartu Observatory 1.5-meter telescope and the Nordic Optical Telescope. We analysed the atmospheric dynamics by measuring the radial velocities and line depths of Fe I and other spectral lines.Results. The radial velocities of the Fe I lines reveal a vertical velocity gradient of 10–20 km s^{−1 in the atmosphere, correlating with the strength of the spectral lines. Stronger lines, formed in higher atmospheric layers, have higher radial velocities. We measured the systemic velocity at −50.3 km s−1. During the dimming, radial velocities were affected by additional emission from the ejected gas, which was blue-shifted relative to the absorption lines. Post-dimming, we observed large-scale atmospheric motions with an amplitude of ~25 km s−1. Strong resonance lines of Ba II, K I, Na I, and Ca I showed stable central emission components at −56 km s−1, which are likely of circumstellar origin.}