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 (≲6 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 ≲ − 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 ≲ 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.

Weak Mass Loss from the Red Supergiant Progenitor of the Type II SN 2021yja

“…A −0.6 to −1.4 mag discrepancy between this expectation and our derived limits could be explained by variability in the progenitor star V-band magnitude confined to a single epoch 24.5 yr prior to core collapse, similar to the variability observed in RSGs such as Betelgeuse and M51-DS1 (Levesque & Massey 2020;Jencson et al 2022). The peak-to-peak variability in Betelgeuse was ≈1.2 mag such that even a fraction of this change in the SN 2021yja progenitor star would completely obscure a <15 M e star in our WFPC2/F606W imaging.…”

“…Thus while we rely on the WFPC2 image from 26 March 1997, 24.5 yr before explosion, for our best constraints on the nature of any progenitor star of SN 2021yja, we can place meaningful constraints on that star at later times when the DECam and Spitzer imaging were obtained. These limits are informative in the scenario where that star was highly variable and had a lower flux in F606W compared to its average flux ( i.e., similar to variability in α Ori and the recent detection of a dimming RSG in M51 described in Levesque & Massey 2020;Jencson et al 2022). We further explore the implications of this scenario below.…”

A Rapidly Fading Star as a Type II Obscuring Intermediate Luminosity Optical Transient (ILOT) in a Triple-star System

“…The attenuation in wavelengths of λ < 5 μm can be more than 3 orders of magnitude (Soker 2021). Another object that might be explained by this scenario is the massive star M51-DS1 that reappeared after its near disappearance Jencson et al (2022). This behavior can be explained by a dust cloud that temporarily obscured the star, supporting the notion that dusty circumstellar matter can almost completely obscure massive stars.…”

Hubble Space Telescope Imaging Reveals that SN 2015bh is Much Fainter than its Progenitor

“…Filippenko, PIDs: 14668, 15166). Additional deep imaging with the Advanced Camera for Surveys (ACS) Wide Field Channel (WFC) in the F 814W filter was obtained on 2019 March 28.7 as part of an HST search for disappearing massive stars as failed SNe that form black holes (PI D. Sand, PID 15645;Jencson et al 2022).…”