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

Abstract

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 ($\lesssim 6$ Myr) stellar population, showed clear variability (amplitude $\Delta F814W\approx 0.7$ 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 ($M_{F814W}\lesssim -7.6$ 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 ($\log [L/L_{\odot }]=5.4$ - $5.7$) star with $T_{\mathrm{eff}}\approx 3700$ - $4700$ K, indicative of a cool yellow or post-red supergiant (RSG) with an initial mass of $\approx 26$ - $40$ $M_{\odot }$. 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 ($T_{\mathrm{eff}}\lesssim 3700$ K) and slightly less luminous ($\log [L/L_{\odot }]=5.2$ - $5.3$), giving an inferred initial mass of $\approx 19$ - $22$ $M_{\odot }$. 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.

21 pages, 6 figures, accepted in ApJ

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