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

TL;DR

This paper reports the discovery of a rare, dramatic dimming event in a massive supergiant star in M51, challenging the identification of failed supernovae due to variability in evolved stars.

Contribution

It presents the first detailed observation of an extreme dimming in a supergiant star, suggesting variability can mimic failed supernova signatures, complicating their detection.

Findings

Star dimmed by over 2 magnitudes between 2017 and 2019

Rebrightening in 2021 rules out failed supernova

Dimming likely caused by enhanced mass loss or circumstellar obscuration

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 ($\lesssim6$ Myr) stellar population, showed clear variability (amplitude $\Delta F814W\approx0.7$ 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 $T_{\mathrm{eff}}\approx3700$ - $4700$ K, indicative of a cool yellow or post-red supergiant (RSG) with an initial mass of $\approx26$ - $40$ $M_{\odot}$. 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 ($T_{\mathrm{eff}}\lesssim3700$ K) and slightly less luminous ($\log[L/L_{\odot}] = 5.2$ - $5.3$), giving an inferred initial mass of $\approx19$ - $22$ $M_{\odot}$. 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.