Time-Evolution Images of the Hypergiant RW Cephei During the Re-brightening Phase Following the Great Dimming

TL;DR

This study uses multi-epoch high-resolution imaging to analyze the time-evolution of dust and surface changes in the hypergiant RW Cephei during its re-brightening after a significant dimming event, revealing dynamic mass loss processes.

Contribution

First multi-epoch imaging analysis of RW Cep during re-brightening, showing dust evolution and surface changes linked to mass ejection events.

Findings

The star's apparent size increased by 8% during re-brightening.

The west side of RW Cep reappeared after being obscured during dimming.

Dust in front of the star evolved over time, affecting brightness.

Abstract

Stars with initial masses larger than 8 solar masses undergo substantial mass loss through mechanisms that remain elusive. Unraveling the origins of this mass loss is important for comprehending the evolutionary path of these stars, the type of supernova explosion and whether they become neutron stars or black hole remnants. In 2022 December, RW Cep experienced the Great Dimming in its visible brightness, presenting a unique opportunity to understand mass loss mechanisms. Our previous observations of RW Cep from the CHARA Array, taken during the dimming phase, show a compelling asymmetry in the star images, with a darker zone on the west side of the star indicating presence of dust in front of the star in our line of sight. Here, we present multi-epoch observations from CHARA while the star re-brightened in 2023. We created images using three image reconstruction methods and an analytical model fit. Comparisons of images acquired during the dimming and re-brightening phases reveal remarkable differences. Specifically, the west side of RW Cep, initially obscured during the dimming phase, reappeared during the subsequent re-brightening phase and the measured angular diameter became larger by 8%. We also observed image changes from epoch to epoch while the star is brightening indicating the time evolution of dust in front of the star. We suggest that the dimming of RW Cep was a result from a recent surface mass ejection event, generating a dust cloud that partially obstructed the stellar photosphere.