An Exceptional Dimming Event for a Massive, Cool Supergiant in M51

We present the discovery of an exceptional dimming event in a cool supergiant star in the Local Volume spiral M51. The star, dubbed M51-DS1, was found as part of a Hubble Space Telescope (HST) search for failed supernovae (SNe). The supergiant, which is plausibly associated with a very young (≲26 Myr) stellar population, showed clear variability (amplitude ΔF814W ≈ 0.7 mag) in numerous HST images obtained between 1995 and 2016, before suddenly dimming by >2 mag in F814W sometime between late 2017 and mid-2019. In follow-up data from 2021, the star rebrightened, ruling out a failed supernova. Prior to its near-disappearance, the star was luminous and red (M F814W ≲2 - 7.6 mag, F606W - F814W = 1.9-2.2 mag). Modeling of the pre-dimming spectral energy distribution of the star favors a highly reddened, very luminous ( log[L/L⊙]=5.4 -5.7) star with T eff ≈ 3700-4700 K, indicative of a cool yellow or post-red supergiant (RSG) with an initial mass of ≈26-40 M ⊙. However, the local interstellar extinction and circumstellar extinction are uncertain, and could be lower: the near-IR colors are consistent with an RSG, which would be cooler (T eff ≲2 3700 K) and slightly less luminous ( log[L/L⊙]=5.2 -5.3), giving an inferred initial mass of ≈19-22 M ⊙. In either case, the dimming may be explained by a rare episode of enhanced mass loss that temporarily obscures the star, potentially a more extreme counterpart to the 2019-2020 "Great Dimming"of Betelgeuse. Given the emerging evidence that massive evolved stars commonly exhibit variability that can mimic a disappearing star, our work highlights a substantial challenge in identifying true failed SNe.