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We present H\alpha images of an ionized nebula surrounding the M2-5Ia redsupergiant (RSG) W26 in the massive star cluster Westerlund 1. The nebulaconsists of a circumstellar shell or ring ~0.1pc in diameter and a triangularnebula ~0.2pc from the star that in high-resolution Hubble Space Telescopeimages shows a complex filamentary structure. The excitation mechanism of bothregions is unclear since RSGs are too cool to produce ionizing photons and weconsider various possibilities. The presence of the nebula, high stellarluminosity and spectral variability suggest that W26 is a highly evolved RSGexperiencing extreme levels of mass-loss. As the only known example of anionized nebula surrounding a RSG W26 deserves further attention to improve ourunderstanding of the final evolutionary stages of massive stars.

The Ionized Nebula surrounding the Red Supergiant W26 in Westerlund 1