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

Jacob E. Jencson Steward Observatory, University of Arizona, 933 North Cherry
Avenue, Tucson, AZ 85721-0065, USA  David J. Sand Steward Observatory,
University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
Jennifer E. Andrews Gemini Observatory/NSF’s NOIRLab, 670 N. A’ohoku Place,
Hilo, Hawai’i, 96720, USA  Nathan Smith Steward Observatory, University of
Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA  Jeniveve Pearson
Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson,
AZ 85721-0065, USA  Jay Strader Center for Data Intensive and Time Domain
Astronomy, Department of Physics and Astronomy, Michigan State University,
East Lansing, MI 48824, USA  Stefano Valenti Department of Physics and
Astronomy, University of California, 1 Shields Avenue, Davis, CA 95616-5270,
USA  Emma R. Beasor Hubble Fellow  NSF’s NOIR Lab, 950 N. Cherry Avenue,
Tucson, AZ 85721, USA  Barry Rothberg LBT Observatory, University of Arizona,
933 N.Cherry Ave,Tucson AZ 85721, USA  George Mason University, Department of
Physics & Astronomy, MS 3F3, 4400 University Drive, Fairfax, VA 22030, USA

###### 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. 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). Modelling 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 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 a red supergiant, 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 supernovae.

Red supergiant stars(1375) — Peculiar variable stars(1202) — Stellar mass
loss(1613) — Evolved stars(481)

††facilities: HST (ACS, WFC3, WFPC2), Gemini:Gillett (NIRI), MMT (MMIRS),
AAVSO††software: AstroDrizzle, TweakReg (http://drizzlepac.stsci.edu/; Hack et
al., 2012), SExtractor, PSFEx (https://www.astromatic.net/software/; Bertin &
Arnouts, 1996; Bertin, 2011), DOLPHOT
(http://americano.dolphinsim.com/dolphot/; Dolphin, 2000, 2016, DRAGONS
(https://dragons.readthedocs.io/en/v2.1.1/index.html), Astropy
(https://www.astropy.org/; Astropy Collaboration et al., 2013; The Astropy
Collaboration et al., 2018), photutils, EPSFBuilder
(https://photutils.readthedocs.io/en/stable/; Bradley et al., 2020)

##  1 Introduction

Massive stars ($\gtrsim$8 M⊙) end their life in core collapse after nuclear
burning is no longer sustainable. Often the collapse of a massive star results
in the ejection of the stellar envelope and a luminous core-collapse supernova
(SN), but this may not always be the case. Indeed, one surprising claim is
that standard type IIP SNe, for which progenitor stars have been identified in
deep pre-explosion data, arise from red supergiant (RSG) stars with only
modest initial masses $\lesssim$18 M⊙ (e.g., Smartt et al., 2009; Smartt,
2015), even though RSGs as a population extend to $\gtrsim$25 M⊙ (Humphreys &
Davidson, 1979). The apparent lack of high-mass RSG progenitors to SN IIP
suggests that either they are systematically missed by transient surveys
(e.g., they explode preferentially in heavily obscured regions) or that the
highest mass RSGs do not end their life as SNe. However, the value of the mass
cutoff at $\sim$18 $M_{\odot}$, its statistical validity and its
interpretation are controversial due to the complexities of connecting the
limited observations to uncertain models (e.g., Davies et al., 2007; Smith et
al., 2011; Walmswell & Eldridge, 2012; Kochanek et al., 2012; Davies & Beasor,
2018, 2020a, 2020b; Kochanek, 2020).

The putative lack of SN type II progenitors with initial masses $\gtrsim$18 M⊙
has led to the hypothesis that more massive stars cannot yield successful SN
explosions, instead ending their lives as “failed” SNe that collapse directly
into black holes (e.g., Kochanek et al., 2008), possibly with an associated
fast, blue (Kashiyama & Quataert, 2015) or faint, $\approx$yr-long transient
(Piro, 2013; Lovegrove & Woosley, 2013). Producing simulations that yield
successful high-energy SN explosions has been a long-standing difficulty.
Modern simulations and theoretical work have found general agreement with this
picture of a large fraction of failed SNe from high-mass RSGs, in that a
majority of lower-mass RSG stars have compact cores that are more likely to
produce successful explosions, while more massive
($\gtrsim$16–$20\,M_{\odot}$) stars may be more likely to collapse into black
holes (e.g., O’Connor & Ott, 2011; Ugliano et al., 2012; Sukhbold & Woosley,
2014; Pejcha & Thompson, 2015; Ertl et al., 2016; Sukhbold et al., 2016).
Though not strictly monotonic in mass, the general trend may be related to a
transition in the star’s core between convective and radiative carbon burning
near the end of its life (Sukhbold & Adams, 2020).

Verification that massive RSG stars can directly collapse into black holes
without an accompanying SN would shed light on the observed compact remnant
population (Kochanek et al., 2014) and point to the stellar-mass progenitors
of the black hole mergers observed by gravitational wave experiments (The LIGO
Scientific Collaboration et al., 2018; Abbott et al., 2020). The ideal
observational signature of a failed SN would manifest as a bright RSG star
that winks out of existence (or nearly so) from one epoch to the next, and
stays in that state permanently. With no accompanying luminous SN, the only
explanation for the disappearance of such a massive star would be a direct
collapse into a black hole, although some fading, optical/near-IR emission may
remain from a weak envelope ejection or fallback accretion onto the newly
formed black hole (Lovegrove & Woosley, 2013; Perna et al., 2014).

Multiple projects have searched for disappearing massive stars. The first and
most comprehensive of these has utilized the Large Binocular Telescope (LBT)
to re-image nearby galaxy fields ($\lesssim$10 Mpc) with a cadence of
$\approx$months over the last decade (Gerke et al., 2015; Adams et al., 2017a;
Neustadt et al., 2021). This search has identified the best known candidate
for a failed SN, dubbed NG6946-BH1 (Adams et al., 2017b), which was an
apparent $\sim$25 M⊙ ($\sim$105.5 L⊙) RSG which brightened on a year timescale
before nearly vanishing in the optical and infrared (IR). Follow-up
observations show some fading remnant emission at the position, consistent
with expectations of fallback accretion (Adams et al., 2017b; Basinger et al.,
2020). The ongoing LBT survey has identified other, potential failed SN
candidates, including a blue supergiant star in M101 (Neustadt et al., 2021).
A separate, multi-epoch archival Hubble Space Telescope (HST) search also
identified a yellow supergiant (YSG) star that apparently vanished (Reynolds
et al., 2015).

A primary challenge in identifying true failed SNe is that massive, evolved
stars are subject to several forms of large-amplitude variability on
essentially all timescales, some which may mimic a disappearing star at
optical wavelengths. Eruptive variability, like that of the luminous blue
variables (LBVs; e.g., Humphreys et al., 1999; Smith et al., 2001b; Smith,
2017) or massive stellar mergers can form dust in their dense outflows,
obscuring the surviving star in the optical, though producing bright mid-IR
emission (e.g., Smith et al., 2016, 2018; Blagorodnova et al., 2017; Pejcha et
al., 2016a, b, see also, IR-dominated transients in Jencson et al., 2019).

Massive, cool supergiants, while subject to pulsational instabilities (Heger
et al., 1997; Yoon & Cantiello, 2010) manifesting as periodic or semiregular
variations on timescales of months to years (e.g., Kiss et al., 2006; Yang &
Jiang, 2011, 2012; Soraisam et al., 2018), can also undergo episodes of
enhanced mass loss accompanied by observed LBV-like color changes (Smith et
al., 2004). These episodes may produce complex circumstellar material (CSM)
like that observed for nearby, Galactic Y/RSGs that suggests prior, distinct
mass ejections (e.g., Humphreys et al., 1997; Smith et al., 2001a; Smith,
2004; Monnier et al., 2004). During such an event, increased opacity from a
dense molecular wind (as in Mira variables, Reid & Goldston, 2002; and for
recent work on RSGs see, e.g., Davies & Plez, 2021) or the formation of dust
can obscure star causing a dramatic visual dimming. Such a scenario has, for
example, been discussed at length to explain the recent historic minimum of
Betelgeuse that occurred in 2019–2020, known as the “Great Dimming” (e.g.,
Levesque & Massey, 2020; Montargès et al., 2021). Modern extragalactic
transient searches are designed to find bright, explosive outbursts that can
be seen at large distances. Dramatic fading events may also signify important
instabilities in massive evolved stars, however, as with failed SNe, they are
more challenging to find and our knowledge of their impact on stellar
evolution is limited.

In this paper, we describe the first results from a program to search for
failed SNe using a set of new HST data directed at nearby galaxies that have
both a long history of archival HST imaging, and have hosted multiple core-
collapse SNe. We have identified a massive, cool supergiant star in M51 which
underwent a significant dimming in 2019, preceded by a roughly year-long
brightening phase, both of which pointed to a potential failed SN. As the
first compelling candidate from our search, we named the object M51-DS1 ($=$
M51 “Dimming Star” 1) and began a dedicated follow-up campaign. Our subsequent
HST and ground-based near-IR imaging showed that the star had rebrightened in
2021 ruling out a failed SN explanation for the dimming event. Nevertheless,
we take a detailed look at the available data for the object as both a false-
positive in our failed SN search and as a prime example of exceptional
variability for a massive, evolved supergiant.

##  2 A New HST Search for Failed SNe

As described above, the clear observational signature of a failed SN is the
permanent disappearance of a massive star. In order to observe more examples
of the failed SN phenomenon, we chose nearby galaxies that are both proven
core-collapse SN producers and have multiple epochs of HST imaging over the
last $\sim$30 years. To do this, we first identified galaxies that have hosted
two or more core-collapse SNe in the modern era using the Asiago Supernova
Catalogue (Barbon et al., 1999), supplemented by SNe found in the last several
years using a query of the Transient Name Server111https://www.wis-tns.org/.
We cross-matched this list of galaxies with those that had two or more
existing epochs of HST data in the $F814W$ filter, and we reobserved the
resulting 31 galaxies (all with $D\lesssim$45 Mpc) over 41 orbits with the
Advanced Camera for Surveys (ACS) in Cycle 26 (PI: D. Sand, PID: 15645),
matching the footprint of previous HST imaging as best as possible. The
$F814W$ filter was chosen as our discovery filter because it is among the most
sensitive to the supergiant progenitors to failed SNe, and has been commonly
used throughout HST’s lifetime with ACS, the Wide Field Planetary Camera 2
(WFPC2), and the Wide Field Camera 3 (WFC3); $F814W$ is also less sensitive to
extinction in regions of star formation than bluer bands. Each newly acquired
$F814W$ image is a full orbit with a standard sub-pixel dither sequence, and
has a sufficient depth ($F814W\approx 27.2$ mag with a signal-to-noise ratio
[S/N] of $\approx$5) to observe massive stars with initial stellar masses
$\gtrsim$10 M⊙ throughout the distance range we cover, although some of the
archival imaging we use is only sensitive to initial stellar masses of
$\gtrsim$15 M⊙.

We identify massive star candidates that have vanished using difference
imaging, subtracting older HST epochs with our newly acquired data. We outline
the general procedure in Section 3 as we describe the identification of
M51-DS1. Key to our strategy is the wide range of data available in both the
HST and ground-based data archives for each of our target galaxies, which
allows us to establish a baseline of existence for the progenitor star of any
given failed SN candidate stretching back years prior to disappearance. We
will provide further details about our strategy and expected number of failed
SNe in a future work, as analysis is ongoing.

##  3 Observations and Data Processing

Here we describe the observations and data processing associated with the
identification of M51-DS1 as a failed SN candidate, along with subsequent
follow-up observations.

###  3.1 HST Image Processing and Subtraction

To search for large amplitude variables and “disappearing” luminous stars, we
performed image subtraction with the available archival WFC3/UVIS and ACS/WFC
images in the $F814W$ filter that overlap with the footprint of our Cycle 26
program observations. For M51, this includes ACS/WFC frames taken from 2005
January 12–22 (PI: S. Beckwith, PID: 10452) and a large campaign consisting of
34 epochs taken between 2016 October 5 and 2017 February 17 to study massive
star variability (PI C. Conroy, PID: 14704; see Conroy et al., 2018; hereafter
C18).

We used the Cycle 26 ACS/WFC $F814W$ observations of M51 as template images
for subtraction with each of the archival frames. These include two spatially
overlapping visits taken on 2019 May 24 and 31, respectively, designed to
maximize coverage of M51 and overlap with the archival images. Each visit
consisted of four dithered 564 s exposures. We first downloaded the CALACS
calibrated and charge-transfer-efficiency (CTE)-corrected flc frames for both
visits from the Mikulski Archive for Space Telescopes. We processed the images
using the AstroDrizzle software package, including automated cosmic ray
rejection, sub-pixel alignments with TweakReg, and final combination into a
single, drizzled mosaic at the native WFC pixel scale of $0\farcs 05$ with an
effective PSF (ePSF) of $\approx 2$ pixels.

We then processed the available archival $F814W$ frames from ACS/WFC in a
similar manner. The 2005 observations consisted of a 6-pointing, mosaicked
mapping of M51, with each pointing composed of four dithered, individual 340 s
exposures. We aligned each of the 24 individual frames to the drizzled Cycle
26 template mosaic using TweakReg, typically using between $\approx$300 and
$4000$ common stars depending on the size of the overlap region with the
template. For the 2016–2017 data, each of the 34 epochs consists of 4
individual exposures totalling 2200 s. We again aligned each frame to the
template using TweakReg, where the number of common stars used for alignment
varied between $\approx$100-500 stars. We achieved ($\mathrm{rms}\lesssim 0.1$
WFC pixels in the $x$ and $y$ directions) for every frame. We then ran the
aligned images through AstroDrizzle. For the purposes of image subtraction, we
produced one large drizzled mosaic of the 2005 images and split the 2016–2017
into three batches consisting of observations taken between 2016 October –
2017 January, 2017 February–May, and 2017 June–September. We used the 2019
template image as a reference in Astrodrizzle so that all the output images
were drizzled onto the same pixel grid.

To prepare the drizzled images for subtraction, we ran each through the
Astromatic packages SExtractor and PSFEx (Bertin & Arnouts, 1996; Bertin,
2011) to generate source catalogs and models of the point-spread function
(PSF) for each image. The images and corresponding noise maps generated from
the inverse-variance-map output of AstroDrizzle were then converted to units
of electrons on the scale of the 2019 template using the zeropoint and
exposure time information in the image headers. The background level of the
images were also estimated and removed using the minimum value from a median
filter on a 15$\times$15 grid of the region of overlap with the template.

We then performed image subtraction using the ZOGY algorithm (Zackay et al.,
2016) on the background-subtracted images, with the noise maps and PSF model
postage stamps as additional inputs. The subtraction produces a difference
image, the difference image PSF, and a Scorr image (Equation 25 in Zackay et
al., 2016), which is a match-filtered S/N image optimized for point source
detection. In order to use the Scorr image for the detection of variable
objects, we normalize the Scorr values down to have a standard deviation of 1
over the image. This is necessary to account for the contribution of
correlated pixel noise inherent in our drizzled images, which is not included
in the calculation of the Scorr image with ZOGY. Finally, SExtractor is run on
both the “positive” (archival-minus-template) and “negative” (template-minus-
archival) difference images to build source catalogs of both fading and
brightening objects. These are cross-matched with the corresponding,
normalized Scorr image to select significant detections of $\mathrm{S/N}\geq
5$ (those objects with peak normalized Scorr values $\geq$5 in at least one
pixel).

###  3.2 Candidate Identification

To select initial candidates, we require that a source be detected at
$|\Delta\lambda L_{\lambda}|\gtrsim 10^{4}~{}L_{\odot}$ in the “positive”
difference images from at least two archival $F814W$ frames compared to the
2019 template, corresponding to an absolute magnitude in the difference image
photometry of $M_{F814W}\leq-6.38$ mag (Vega system) and indicating a luminous
object that has faded significantly. This cut-off in the change in luminosity
is the same as that used for the LBT search (Gerke et al., 2015; Adams et al.,
2017a; Neustadt et al., 2021), which is sufficient to exclude lower mass
large-amplitude variables including R CrB stars and Miras ($M_{I}>-6$ mag,
Tisserand et al., 2009; Soszyński et al., 2009). We also require that the two
detections be separated in time by at least 1 month. This establishes an
archival baseline for a given source as a persistent object, i.e., to reject
real astrophysical transient events such as novae as well as chance cosmic ray
hits and other image processing and subtraction artifacts.

We then visually inspect the position of the object in all available $F814W$
HST imaging to further reject any obvious image artifacts and examine the full
history of the source in that band. A strong failed SN candidate will appear
as a star-like source that is present in all archival frames of sufficient
depth prior to its first significant fading, after which the source will not
appear to rebrighten. We note that we do not require that a source actually
completely “disappear”, because a possible companion or unrelated, nearby star
in a crowded region may continue to be detectable at the location even after a
true failed SN. Any source that passes this visual inspection will be assigned
a “DS” name (for “Dimming Star”) and flagged for detailed analysis, including
PSF-fitting photometry of the available HST imaging in other filters (see
Section 3.3.1 for details).

A subset of the sequence of available HST $F814W$ imaging for M51-DS1, our
first compelling candidate (as defined by the criteria outlined above), is
shown in the bottom row of panels in Figure 1. The star was clearly detected
in the 2005 and 2016 ACS/WFC images (also in 1995 and 2008 WFPC2 images not
shown). In the 2019 image obtained as part of our search the star has
seemingly vanished and is clearly detected as a fading candidate in our
difference images.

Figure 1:  HST imaging of M51-DS1 and the surrounding region. From left to
right along the bottom row of panels, we show the sequence of ACS/WFC $F814W$
images from 2005, 2016, 2019, the 2016–2019 difference image, and the most
recent 2021 image. The source underwent a dramatic fading, nearly disappearing
in the 2019 frame (highlighted in the red-bordered panels), before
rebrightening in 2021. The large, upper-left panel shows the color-composite
ACS/WFC images from 2005 ($F435W$ in blue, $F555W$ in green, and $F814W$ in
red) of the location of M51-DS1 along an inner spiral arm and prominent dust
lane $\approx$36$\arcsec$ east of the galaxy nucleus. The positions of the
nearest two H II regions from Croxall et al. (2015) are indicated with yellow
circles of 1$\arcsec$ radius and labelled with the derived $E(B-V)$ values for
each one. A zoomed-in view of the immediate $1\farcs 5\times 1\farcs 5$ (62.4
pc on a side) region around M51-DS1 is shown in the upper-rightmost panel.
Below this, down the right side of the figure, we then show the same region in
the ACS/WFC $F606W$ images from 2016 and 2021, and the WFC3/IR $F110W$ images
from 2012 and 2021, demonstrating the dimming of the source across visible and
near-IR wavelengths.

###  3.3 Archival imaging, Follow-up Observations and Photometry

Following our identification of M51-DS1 as a disappearing star and promising
candidate for a failed SN, we began examining the wealth of multiband,
archival HST imaging, including the ACS/WFC $F814W$ images described above and
the complimentary $F435W$, $F555W$, and $F658N$ (2005) and F606W (2016–2017)
from the same programs. There is also coverage with WFC3/IR ($0\farcs 13$
pixels) in $F110W$ and $F128N$ (Pa$\beta$) taken on 2012 September 4 (PI: J.
Koda, PID: 12490). As shown in Figure 1, M51-DS1 is notably red in appearance
in the available archival imaging, and the optical source appears to be
associated with a bright, relatively isolated star in the near-IR frames. Of
the available WFC3/UVIS observations, we use only the 2012 observations in the
redder $F673N$ and $F689M$ filters (PI: K. Kuntz, PID: 12762) where the star
is clearly identifiable in the drizzled images. Similarly for WFPC2, while
there are numerous prior observations in many filters that cover the location
on the WF cameras, we consider only the $F814W$ images from 1995 (PI: R.
Kirshner; PID: 5777) and 2008 (PI: M. Meixner, PID: 11229) given the red color
of the star and the increasing likelihood of contamination in bluer filters
for the larger pixels of the WF chips ($0\farcs 0996$).

We were also awarded an HST Cycle 28 mid-cycle proposal for additional deep,
optical and near-IR imaging of the field (PI: J. Jencson, PID: 16508) in order
to verify the extreme fading of this source. The images for this program were
obtained with ACS/WFC in $F606W$ and $F814W$ (total exposure time of 2208 s
each filter) on 2021 April 28–29 and with WFC3/IR in $F110W$ and $F160W$ on
2021 June 6 (1200 s exposure per filter). These new images revealed a
surviving star at the location of M51-DS1 that had clearly rebrightened at
$F814W$ from the 2019 minimum (see the bottom right corner of Figure 1). While
this rules out a failed SN scenario for this object, the star is notably
fainter at each of $F606W$, $F814W$, and $F110W$ compared to the corresponding
pre-disappearance imaging.

####  3.3.1 DOLPHOT Photometry

We processed the available HST imaging mentioned above with DOLPHOT (Dolphin,
2000, 2016) to obtain PSF-fitting photometry of M51-DS1 and the surrounding
stars in its vicinity. As inputs to DOLPHOT, we use the CTE-corrected flc
frames for ACS/WFC and WFC3/UVIS observations, flt frames for WFC3/IR
observations, and the standard exposure level c0f and data quality c1f frames
for WFPC2. Each input image was also run first through Astrodrizzle to flag
cosmic ray hits. We employ the parameter settings used for the HST PHAT survey
(Dalcanton et al., 2012; Williams et al., 2014). These settings were also used
by C18 to build their photometry catalogs of M51, to which we can directly
compare our results.

For most of the data, we ran DOLPHOT separately for each instrument and filter
combination over the available epochs, using the corresponding drizzled images
as references for alignment. Generally, DOLPHOT produced excellent alignments
better than $\approx$0.2 pixels rms for ACS/WFC and WFC3/UVIS, and
$\approx$0.3 pixels rms for WFC3/IR and WFPC2. Owing to the large quantity of
ACS/WFC $F814W$ and $F606W$ data (primarily from the C18 program), we had to
run them in batches. In order to obtain consistent results across all runs for
a given filter, we use the “warmstart” option in DOLPHOT, in which we use the
output star catalog from an initial batch where M51-DS1 is well detected to
fix the positions of stars in the subsequent runs for a given filter. DOLPHOT
computes and applies aperture corrections to a radius of $0\farcs 5$ for the
reported photometry. We then applied the appropriate corrections to infinite
apertures for each instrument and filter combination.222See (Bohlin, 2016) for
ACS; for WFC3: https://www.stsci.edu/hst/instrumentation/wfc3/data-
analysis/photometric-calibration; for WFPC2:
https://www.stsci.edu/itt/review/dhb_2010/WFPC2/wfpc2_dhb.pdf To estimate the
photometric uncertainties, we compute the rms deviations of the measurments
for the individual frames that comprise a given HST observing visit of all
stars within a 100 pixel radius of M51-DS1 as a function of magnitude, added
in quadrature with the nominal statistical uncertainty reported by DOLPHOT.

For all the data that we ran, a “good” star (“object type”$=$1) was detected
by DOLPHOT at the location of M51-DS1 as measured in the 2016 $F814W$ ACS
frames within $\approx$0.5 ACS pixels, with the exception of the 2005 $F435W$
and $F555W$ ACS images. In these bluer filters, photometry of M51-DS1 is
limited by crowding of nearby sources. We therefore adopt the magnitudes of
the nearest detect star (1.3 and 0.8 pixels away for $F435W$ and $F555W$,
respectively) as limits. Our final DOLPHOT photometry is presented in Table 1.
Throughout this work, all photometry is reported on the Vega magnitude system.

Our measurements of M51-DS1 at $F814W$ from the ACS/WFC 2016–2017 data are
typically about 0.05–0.1 mag fainter than those reported for the star in the
C18 catalog, though the offset is comparable with our measurement
uncertainties. The offset is somewhat larger at $F606W$, with our measurements
systematically at $\approx$0.3 mag fainter that those of C18. In experimenting
with DOLPHOT runs, we found that the choice of the alignment reference image
as well as the specific images included in a given batch affected the final
star list. In our final catalogs, there were stars immediately adjacent to
M51-DS1 that were not in the C18 catalog. The simultaneous fitting of these
additional stars by DOLPHOT may explain our lower flux measurements.

Table 1: HST DOLPHOT Photometry

UT Date | MJD | Inst. | Band | App. MagnitudeaaVega magnitudes. 1$\sigma$ uncertainties are given in parentheses.
---|---|---|---|---
|  |  |  | (mag)
1995 Jan 15.76 | 49732.76 | WFPC2 | $F814W$ | 22.03 (0.20)
2005 Jan 21.36 | 53391.36 | ACS/WFC | $F435W$ | $>24.8$
2005 Jan 21.37 | 53391.37 | ACS/WFC | $F555W$ | $>25.0$
2005 Jan 21.38 | 53391.38 | ACS/WFC | $F814W$ | 22.25 (0.11)
2008 Mar 31.14 | 54556.14 | WFPC2 | $F814W$ | 22.79 (0.20)
2012 Sep 04.82 | 56174.82 | WFC3/IR | $F110W$ | 20.09 (0.17)
2012 Sep 04.83 | 56174.83 | WFC3/IR | $F128N$ | 19.54 (0.09)
2016 Oct 05.25 | 57666.25 | ACS/WFC | $F606W$ | 24.51 (0.10)
2016 Oct 05.31 | 57666.31 | ACS/WFC | $F814W$ | 22.44 (0.05)
2016 Oct 14.25 | 57675.25 | ACS/WFC | $F606W$ | 24.54 (0.09)

Note. — Table 1 will be published in its entirety in the machine-readable
format. A portion is shown here for guidance regarding its form and content.

####  3.3.2 Ground-based Imaging and Photometry

We obtained follow-up imaging in the near-IR with the MMT and Magellan
Infrared Spectrograph (MMIRS; $0\farcs 2$ pixels; McLeod et al., 2012) on the
6.5-m MMT Observatory telescope on Mt. Hopkins at the Smithsonian’s Fred
Lawrence Whipple Observatory. We obtained $JHK_{s}$ imaging over a two-night
run on UT 2021 February 23 and 24, consisting of dithered sequences
alternating between the target position in the central regions of M51 and an
offset blank-sky field every few minutes to allow for accurate subtraction of
the bright near-IR sky background. We reduced the images using a custom
pipeline333Adapted from the MMIRS imaging pipeline developed by K. Paterson
available here: https://github.com/CIERA-Transients/Imaging_pipelines that
performs standard dark-current subtraction, flat-fielding, sky background
estimation and subtraction, astrometric alignments, and final stacking of the
individual exposures.

We also obtained $J$ and $K_{s}$-band imaging with the Near-Infrared
Imager444http://www.gemini.edu/instrumentation/niri (NIRI) on the 8-m Gemini N
Telescope on Maunakea through a Directors Discretionary Time program (PI: J.
Jencson; PID: GN-2021A-DD-101). The images were obtained withe the f/6 camera
($0\farcs 117$ pixels) over multiple nights from 2021 April 1–6, again using
sequences of dithered images alternating between the target position and an
offset sky position. We also downloaded NIRI $JHK$ images from the Gemini
Observatory archive covering the location of M51-DS1 and taken with in a
similar manner the same camera setup on 2005 June 27 (PI: S. Smartt; PID:
GN-2005A-Q-49). We reduced all the images using
DRAGONS555https://dragons.readthedocs.io/en/v2.1.1/index.html, a Python-based
platform for reducing Gemini data, and following the procedures for extended
sources outlined in the NIRI imaging reduction
tutorial666https://dragons.readthedocs.io/projects/niriimg-
drtutorial/en/stable/.

With the large field-of-view of the MMIRS imager ($6.9\times 6.9$ arcmin), we
were able to calibrate the photometric zeropoints using aperture photometry of
relatively isolated stars in the MMIRS images with cataloged $JHK_{s}$-band
magnitudes in the Two Micron All Sky Survey (2MASS; Skrutskie et al., 2006).
We then derived a model of the ePSF for each image by fitting bright, isolated
stars using the EPSFBuilder tool of the photutils package in Astropy. We
performed PSF-fitting photometry at the location of M51-DS1 as well as for a
set of approximately 60 stars spread across the images with varying degrees of
crowding and background galaxy emission. We include a low-order, two-
dimensional polynomial in the fit to account for the spatially varying
background for each star, taking care to avoid over-fitting the data. We adopt
the rms error of the fit residuals, scaled by a factor of the square root of
the reduced $\chi^{2}$ (typically $\gtrsim 1$) for the fit, as the nominal
statistical uncertainty per pixel, and multiply by the effective footprint, or
number of “noise pixels”, of the ePSF777A derivation of this quantity is
provided by F. Masci here:
http://web.ipac.caltech.edu/staff/fmasci/home/mystats/noisepix_specs.pdf to
obtain an estimate of the statistical uncertainty for each flux measurement.
We used the set of 2MASS calibration stars to derive aperture corrections
($\lesssim 0.1$ mag in all three filters) to place the PSF-fitting magnitudes
on the scale of the image photometric zeropoints. We adopt the statistical
flux uncertainty, summed in quadrature with the rms error of the stars used in
estimations of the zeropoint and ePSF aperture correction, as the total
uncertainty in our final magnitudes. Owing to the limited number of isolated
2MASS stars, even with the large field-of-view of MMIRS, the zeropoint rms
(typically $\approx$0.1 mag) dominates the error budget.

The field-of-view of the NIRI images is smaller ($\approx 2\arcmin\times
2\arcmin$), and there were not enough isolated 2MASS stars in the central
regions of M51 to do a direct calibration. We instead cross-calibrated our
PSF-photometry of stars in the NIRI image, performed in the same manner as
described above, to the set of $\approx$15–20 common stars with the
corresponding MMIRS image in the same filter. For our measurements of M51-DS1,
we adopt the statistical uncertainty from the PSF-fitting (as above), summed
in quadrature with the zeropoint uncertainty (from the standard deviation of
the individual stars used in the cross-calibration) as our measurement
uncertainty.

The image quality of all the ground-based near-IR data was between
$\approx$0$\farcs 5$ and $0\farcs 8$ FWHM—a factor of $\approx$2–3 times that
of WFC3/IR—generally with better seeing in the redder $H$ and $K$ bands. As
noted in Section 3.3, M51-DS1 is relatively isolated in the near-IR, with the
nearest comparably bright object in the WFC3/IR images at a separation of
$\approx$1$\arcsec$. Still, our ground-based photometry may suffer some
contamination from nearby unrelated sources, particularly in the $J$ band. The
2005 and 2021 $J$-band fluxes are a bit higher than those from the comparable
$F110W$ images, though the discrepancies are $\lesssim 1.7\sigma$. The 2021
$H$-band flux is fully consistent with the comparable $F160W$ measurement, and
we also expect any contamination to be small or negligble in the $K$-band. Our
final photometry of M51-DS1 from the ground-based near-IR images is provided
in Table 2.

Table 2: Ground-based Near-IR Photometry  UT Date | MJD | Tel./Inst. | Band | App. MagnitudeaaVega magnitudes on the 2MASS system. 1$\sigma$ uncertainties are given in parentheses.
---|---|---|---|---
|  |  |  | (mag)
2005 Jun 27.34 | 53548.34 | Gemini/NIRI | $J$ | 19.61 (0.14)
2005 Jun 27.36 | 53548.36 | Gemini/NIRI | $H$ | 19.14 (0.16)
2005 Jun 27.38 | 53548.38 | Gemini/NIRI | $K$ | 18.54 (0.21)
2021 Feb 23.36 | 59268.36 | MMT/MMIRS | $J$ | 20.04 (0.11)
2021 Feb 23.49 | 59268.49 | MMT/MMIRS | $H$ | 19.26 (0.09)
2021 Feb 24.40 | 59269.40 | MMT/MMIRS | $K_{s}$ | 18.73 (0.13)
2021 Apr 01.45 | 59305.45 | Gemini/NIRI | $J$ | 20.04 (0.08)
2021 Apr 02.41 | 59306.41 | Gemini/NIRI | $K_{s}$ | 18.83 (0.12)

##  4 Analysis of M51-DS1

###  4.1 Extinction, Distance, Host Galaxy Environment, and Metallicity

M51-DS1 is located at an R.A. and decl. of
$13^{\mathrm{h}}29^{\mathrm{m}}56\fs 16,+47\arcdeg 11\arcmin 47\farcs 8$
(J2000.0), near the inner regions of the nearby, star-forming galaxy M51.
Throughout this work, we assume a distance modulus for M51 from McQuinn et al.
(2016) of $m-M=29.67\pm 0.02$ (statistical) $\pm 0.07$ (systematic; Rizzi et
al., 2007) mag ($D=8.58$ Mpc) based on the luminosity of the tip of the red
giant branch (TRGB) method, and that the systematic uncertainties associated
with calibrating this method dominate over the statistical measurement
uncertainties. We adopt the value from the NASA/IPAC Infrared Science Archive
(IRSA) for the Galactic extinction toward the position of M51-DS1 of
$E(B-V)=0.03$ mag, based on the Schlafly & Finkbeiner (2011) recalibration of
the Schlegel et al. (1998) dust maps, and assuming a standard (Fitzpatrick,
1999) reddening law with $R_{V}=3.1$.

The star is $\approx$36″ from the galaxy nucleus along the outer edge of a
densely populated spiral arm and prominent dust lane (see Figure 1). We thus
expect the region may also be subject to substantial and spatially-variable
extinction from the host. Croxall et al. (2015) presented spectroscopic, gas-
phase chemical abundances for 29 H II regions in M51 including extinction
estimates derived from Balmer-line ratios as part of the CHemical Abundances
of Spirals (CHAOS) program. The two nearest H II regions to M51-DS1 from this
study are labelled in Figure 1. One is centered on the dark dust lane with
$E(B-V)=1.0$ mag, while the other is along the inside edge of the spiral arm
in a region that appears visually similar to that of M51-DS1 with $E(B-V)=0.4$
mag. Wei et al. (2021) recently derived extinction maps of the M51 system from
UV/optical and IR photometry, from which we again find M51-DS1 to be
associated with a region of higher extinction with $A_{V}\approx 1.0$–$1.2$
mag, or $E(B-V)\approx 0.3$–0.4 mag for $R_{V}=3.1$. These estimates are based
on spatial averages in seeing-limited spectra ($\approx$1$\arcsec$; for CHAOS)
or lower resolution space-based imaging (e.g., $\approx$3$\arcsec$ with
Spitzer in Wei et al., 2021) and while they are useful guideposts, the
appearance of the dust lanes in the higher resolution HST images indicates
that there is likely substantial variation in the foreground host extinction
on smaller spatial scales. In our analysis below, we therefore consider the
implications of various amounts of additional reddening due to dust in the
interstellar medium of M51, ranging from $E(B-V)$ = 0 mag to 1.6 mag.

From their set of H II regions, Croxall et al. (2015) found O/H abundances in
the central regions of M51 to be approximately solar or somewhat supersolar
(assuming solar abundances of $12+\log(\mathrm{O/H})_{\odot}=8.69$–8.78,
Asplund et al., 2009; Ayres et al., 2013). Similarly, the metallicity map
derived by Wei et al. (2021) suggests somewhat supersolar metallicities of
$\log(Z/Z_{\odot})\approx 0.0$–0.3 dex in the vicinity of M51-DS1 along the
inner spiral arm.

###  4.2 Optical/near-IR Light Curves and Colors

The light curves of M51-DS1, dating back to 1995, are shown in Figure 2. The
magnitudes displayed and discussed here have been corrected only for Galactic
extinction. The star was detected dozens of times at $F814W$ between 1995 and
2017, exhibiting some variability at the level of $\approx$0.7 mag between
$F814W=22.74\pm 0.20$ and $22.08\pm 0.06$ mag ($M_{F814W}=-6.9$ to $-7.6$
mag). During 2016–2017, the evolution of the source was tracked at high
cadence in both $F814W$ and $F606W$ with ACS/WFC (see inset in Figure 2). The
source displays a smooth, slow rise in both bands at a relatively constant,
red color between $F606W-F814W=1.6$ and 1.9 mag, where the spread in the color
measurements is comparable to their uncertainties.

Figure 2:  The light curves of M51-DS1 in $F606W$, $F814W$, and $F110W$ from
archival and new HST images (filled symbols) going back more than 26 yr to
1995. The region enclosed in the dash-lined box of high-cadence measurements
from $F814W$ and $F606W$ imaging taken between 2016–2017 is shown in more
detail in the inset zoom-in panel. Ground-based near-IR measurements from 2005
and new 2021 images are shown as open symbols. All photometry has been
corrected for Galactic extinction to M51.

Subsequently, the star underwent a dramatic fading in the 2019 ACS/WFC $F814W$
observations, dropping to at least $F814W>24.2$ ($>-5.5$ mag) in our DOLPHOT
measurement, $>2.2$ mag below the brightest prior detection. Though DOLPHOT
nominally obtained a detection (Section 3.3.1), the source looks to have
nearly completely disappeared (Fig 2; bottom, center panel) at $F814W$, and
given the high degree of crowding at this location, we treat this measurement
as an upper limit. Then, in our 2021 follow-up observations approximately two
years later, the source has rebrightened partially to $F814W=22.97\pm 0.09$
mag ($-6.6$ mag; similar to the prior $F814W$ observed in 2008), and displays
a slightly redder color at $F606W-F814W=1.97\pm 0.16$ mag compared to the
2016–2019 measurements.

As expected given its red optical color, the source was brighter in the near-
IR at $F110W=20.05\pm 0.17$ mag ($M_{F110W}=-9.6$ mag) in the 2012 WFC3/IR
image. In our recent 2021 HST follow-up imaging, around the time of the
partial recovery seen in the optical, the star is also fainter at
$F110W=20.44\pm 0.18$ ($M_{F110W}=-9.2$ mag) with a red near-IR color of
$F110W-F160W=1.0\pm 0.2$ mag. The ground-based imaging with NIRI and MMIRS
tells a similar story. In the 2005 NIRI imaging, the star is detected at
$J=19.58\pm 0.14$ ($M_{J}=-10.1$ mag), with $J-K=1.05\pm 0.25$ mag.
Subsequently, in our 2021 ground-based follow-up imaging, the star is again
fainter in the near-IR filters with NIRI (MMIRS) compared to the pre-2019
levels at $J=20.01\pm 0.08$ mag ($20.01\pm 0.11$ mag) and $J-K_{s}=1.20\pm
0.14$ mag ($1.29\pm 0.17$ mag).

####  4.2.1 Near-IR Photometric Classification and Bolometric Luminosity

At $M_{K}=-11.1$ mag (Galactic extinction correction only), the star is well
above the TRGB, and moreover, is brighter than any asymptotic giant branch
(AGB) stars identified in nearby galaxies including the Large and Small
Magellanic Clouds (L/SMC), M31, and M33 (see, e.g., Cioni et al., 2006; Boyer
et al., 2011; Massey et al., 2021, and references therein). Its near-IR color
of $J-K_{s}\approx 1.0$–1.2 mag is also consistent with the range often used
to identify RSGs, though it may be somewhat bluer given the likelihood of
significant foreground host extinction (see Section 4.1). The $K$ band is
particularly useful as a luminosity indicator for RSGs, both because the
effects of extinction are reduced compared to the optical and bluer near-IR
bands, and because the bolometric correction, ${\rm BC}_{K}$, is found
empirically by Davies & Beasor (2018) to be constant across early- to late-M
spectral types for cool supergiants in Milky Way and LMC star clusters. Thus,
assuming an M-type spectrum ($T_{\rm eff}\lesssim 3700$ K) for M51-DS1 and
adopting their value of ${\rm BC}_{K}=3.0$ mag, we obtain bolometric
luminosities in the range of $\log(L/L_{\odot})\approx 5.15$–5.37 depending on
the amount of extinction assumed in excess of the Milky Way foreground from
$E(B-V)=0.0$ up to 1.6 mag (the highest value found to produce good fits in
our modelling of the SED in Section 4.4.1). The value for zero extinction,
$\log(L/L_{\odot})=5.15$, can likely be viewed as a robust lower limit, as any
foreground extinction or an earlier intrinsic spectral type ($<$M0 with ${\rm
BC}_{K}<3.0$), will both increase the inferred bolometric luminosity of the
star. We discuss the possible location of the star in the Hertzsprung–Russell
Diagram (HRD) and inferred evolutionary state in comparison with the results
of our SED fitting in Section 4.5.

###  4.3 Color-magnitude diagrams

In Figure 3, we present the F606W–F814W color-magnitude diagram (CMDs) derived
from our DOLPHOT photometry on the 2021 ACS/WFC images to place M51-DS1 in the
context of the nearby stellar population. Correcting only for Milky Way
extinction, M51-DS1 appears as the most luminous red star ($F606W-F814W\gtrsim
2$ mag) at $F814W$ within a projected distance of 50 ($1\farcs 2$) or 100 pc
($2\farcs 4$) at its 2016–2017 level. Notably as well, it is one of only two
stars within 50 pc that appears so red—the other is indicated by the magenta
circle in both of the figure panels. Compared to the single-star, non-
rotating, solar-metallcity stellar tracks from the Mesa Isochrones and Stellar
Tracks models (MIST; Choi et al., 2016, 2017), its location in the CMD would
correspond to an RSG with an initial mass of $\approx 12$–15 $M_{\odot}$
(2016–2017), or $\approx 8$–10 $M_{\odot}$ (2021), and an age below
$\approx$15 or 30 Myr, respectively. As noted in Section 4.1, however, there
is likely to be significant, and spatially variable foreground extinction in
the region. For values of $E(B-V)$ between $0.4$ and $1.0$ mag—in the range
inferred from previous analysis of nearby H II regions by (Croxall et al.,
2015)—the position of the star would correspond to higher masses up to
$\approx$20–25 $M_{\odot}$, and younger ages $\lesssim$7–10 Myr. Our analysis
of the full SED of M51-DS1 in Section 4.4.1 suggests even higher extinction
values of $E(B-V)=0.8$–1.6 mag, possibly pointing to significant circumstellar
extinction, corresponding to intial masses as high as $\approx$25–40
$M_{\odot}$ and implying an age as young as $\approx$5 Myr in the CMD.

Figure 3:  Left: Immediate environment of M51-DS1 (red “x”-symbol) in the 2005
ACS/WFC images (color-composite of $F435W$, $F555W$, and $F814W$, as in Figure
1) showing a mix of red and blue stars and dark regions that are likely
obscured by dust. Radii corresponding to 20 ($0\farcs 48$) and 50 pc ($1\farcs
2$) projected distances from M51-DS1 are indicated. Sources indicated by
multicolor symbols correspond to those objects marked in the CMD in the right
panel. Right: The CMD of the region around M51-DS1 from the 2021 ACS/WFC
$F606W$ and $F814W$ images. Stars within 20, 50, and 100 pc (projected;
$0\farcs 48$, $1\farcs 2$, and $2\farcs 4$, respectively) from the location of
M51-DS1 are shown as orange, blue, and gray points, respectively. Additional
multicolor markers correspond to those objects indicated in the left panel. We
also show single-star, solar metallicity, non-rotating stellar evolutionary
tracks from MIST as the black and gray dashed curves for a range of masses
between 8 and 40 $M_{\odot}$. Measurements for M51-DS1 are shown as the large
stars with varying degrees of foreground host extinction, $E(B-V)$, indicated
by different colors as labeled. We include several M51-DS1 epochs of $F606W$
and $F814W$ imaging including 2016–2017 and 2021. The nominal 2019 $F814W$
DOLPHOT measurement (uncertainty) is indicated as the red horizontal line
(shaded bar), though we consider this as an upper limit (see main text) as
indicated by the downward arrow.

As is evident from the tricolor image shown in Figure 3, the immediate region
surrounding M51-DS1 is populated by a range of blue-, yellow- and red-
appearing stars. This is reflected as a notable spread in the
$\mathrm{F606W}-\mathrm{F814W}$ color between $\approx-0.3$ and $1.6$ mag for
the bulk of the stars in the CMD within the 20- and 50-pc radii. If one
assumes a coeval population of stars undergoing isolated evolution, this could
be attributed to effects of foreground dust, possibly implying variable
extinction in the range $E(B-V)\approx 0.0$–$2.0$ mag. Interpretation of the
CMD as a whole, however, is complicated by additional factors on top of the
uncertain extinction, including the possibility of multiple, overlapping
stellar populations of different ages, and the effects of binary
interaction—known to be increasingly common among more massive stars (e.g.,
Sana et al., 2012; Moe & Di Stefano, 2017)—on the location of a given source
in the diagram.

We look briefly at a few individual sources of particular note. Within 50 pc,
the apparently brightest object at $F814W$ in the CMD, along with two somewhat
fainter objects, are associated with what appears to be a partially resolved
star cluster (indicated with cyan squares) and may not represent secure
detections of individual stars. After this, there are two bright, yellowish
stars (indicated with yellow diamonds), at $M_{\mathrm{F814W}}\approx-7.4$ mag
and $\mathrm{F606W}-\mathrm{F814W}=0.7$ mag, which, in the absence of
additional foreground extinction, would correspond to stars of at least
$\approx$12–15 $M_{\odot}$ evolving across the Hertzsprung gap. Within a
smaller radius of 20 pc ($0\farcs 48$), the most luminous objects after
M51-DS1 top out at $M_{\mathrm{F814W}}\approx-6.5$ mag, including a relatively
blue (presumably unreddened) source at $\mathrm{F606W}-\mathrm{F814W}=-0.3$
mag (indicated in both the image and CMD by a blue circle). This would
correspond to a $\approx$30 $M_{\odot}$ star at the end of its main-sequence
lifetime and implying an age $\lesssim$6 Myr. This is consistent with the
higher mass and younger age inferred for M51-DS1 above when one allows for
significant foreground and/or circumstellar extinction ($E[B-V]\gtrsim 0.8$
mag) and assuming it is part of the same, coeval population.

####  4.3.1 Comparison to Luminous, Red, Large-amplitude Variable Stars

Regardless of the magnitude of the foreground extinction to M51-DS1, the track
of possible locations it occupies in the CMD in Figure 3 essentially aligns
with the red-end points of the MIST evolutionary tracks for stars of initial
masses $\gtrsim 15~{}M_{\odot}$. RSGs in this region are expected to be
susceptible to pulsational instabilities driven by partial ionization of
hydrogen in their envelopes (e.g., Li & Gong, 1994; Yoon & Cantiello, 2010,
and see region of “supergiant instabilities” in Figures 13 and 16 of C18).

To compare M51-DS1 to other such luminous, red variable stars, we selected
stars from the catalog of M51 luminous variable star light curves presented in
C18. Of the more than $\approx$72000 objects in their catalog (including only
the Milky Way extinction corrections already applied), we pulled out the 90
objects having $M_{\rm F814W}\leq-7.0$ mag at any epoch (1995, 2005, and
2016–2017 data), average colors (including epochs within 0.5 mag of the
$F814W$ peak) of $F606W-F814W\geq 1.5$ mag, and maximum $F814W$ amplitudes
$\Delta F814W\geq 1.0$ mag. We examined both the images and light curves of
these objects by eye to remove false positives such as foreground proper
motion stars and stars with large amplitude variability in the light curves
that is not apparent in visual inspection of the images, likely indicating
erroneous photometry. We also cull the sample by rejecting photometric points
with large uncertainties ($\sigma_{m}>0.2$ mag) and stars where the color
evolution is dominated by light curve scatter, usually in the fainter $F606W$
measurements. We thus select a clean sample of 8 luminous, red, large-
amplitude variables, and show their evolution in the CMD in Figure 4.

Figure 4:  CMD of luminous ($M_{F814W}\leq-7.0$ mag), red ($F606W-F814W\geq
1.5$), high-amplitude ($\Delta F814W\geq 1.0$ mag) variable stars in M51 from
the catalog of C18 (small circles connected by dashed lines) compared to the
2016–2017, 2019, and 2021 data of M51-DS1 (red stars). The plotting colors of
the catalog variable points correspond to their photometric color evolution
during fading fading events: to the blue (blue), approximately constant
(yellow), and to the red (dark orange). We also show the $M_{I},V-I$ evolution
of Betelgeuse during its 2019 dimmming event from AAVSO, and the location of
the progenitor of NGC6946-BH1 from (Adams et al., 2017b). As in Figure 3, we
show single star tracks of various masses from MIST.

These 8 stars can then be separated according to their color evolution: Those
that evolve to the red as they fade, those that remain at a relatively
constant color, and those that evolve to the blue (shown as orange, yellow,
and blue circles in Figure 4, respectively). The distinctions between these
groups are seen more clearly in the differential CMD shown in the right panel
of the figure, in which the average $F814W$ magnitudes and $F606W-F814W$
colors (using points within 0.5 mag of the $F814W$ peak) have been subtracted
out for each source. Interestingly, the “yellow”-class objects appear near the
end of the RSG evolutionary tracks at the Hayashi limit (though we note that
only foreground Milky Way extinction corrections have been applied), possibly
indicating that their near-vertical evolution in the CMD may indeed be
attributed to large-amplitude, radial pulsations that are theoretically
expected for such stars. Between the 2016–2017 and 2021 data, the evolution of
M51-DS1 appears most similar to this group of stars. Unfortunately, during the
$F814W$ minimum in 2019, we lack any information on its color evolution.

The red-evolving objects notably become much redder than the end of the MIST
tracks at $F606W-F814W>2.5$ mag. While some of these stars may suffer
additional host reddening not accounted for here, their color evolution is
nonetheless striking. A natural explanation of this would be the formation of
dust, possibly during an episode of enhanced mass loss from the star, that
temporarily obscures and reddens the light from the photosphere.
Alternatively, an increase in the opacity of the strong molecular absorption
bands, in particular of TiO, that dominate RSG spectra can produce substantial
swings in the optical colors without a correspondingly large decrease in
temperature. The $F814W$ and $F606W$ filters of interest here notably both
contain strong TiO bands. Davies & Plez (2021) recently explored the effect of
winds on RSG spectra, finding that the addition of winds increases the
molecular absorption, effectively shifting the stars to later spectral types
at a fixed photospheric temperature and producing excess mid-IR emission
without needing to invoke dust.

These effects have been discussed extensively regarding the recent, historic
photometric minimum of the RSG Betelgeuse, dubbed the “Great Dimming”, in
which the star faded by more than a magnitude in the $V$ band and then
recovered over the course of $\approx$5 months between the end of 2019 and
early 2020 (Guinan et al., 2019, 2020; Dupree et al., 2020). Multi-epoch,
high-resolution imaging revealed the appearance of an optically-dark patch
over part of the stellar surface during the dimming and exclude a transiting
dust cloud (Montargès et al., 2021). UV observations of chromospheric
variability also indicate the formation of a dense, outflowing
structure—possibly enhanced by a timing coincidence with outward motion of the
pulsational cycle—just before the event (Dupree et al., 2020). The most likely
explanation for the dimming, then, was an increase in the opacity of this
ouflowing material related to the formation of molecules (see, e.g., evidence
from TiO-band monitoring and tomography in Harper et al., 2020; Kravchenko et
al., 2021, and discussion in Davies & Plez, 2021) and possibly also large-
grain dust (e.g., Levesque & Massey, 2020). As shown in Figure 4, the
evolution of Betelgeuse in $V$ and $I$ from the American Association of
Variable Star Observers888https://www.aavso.org/ (AAVSO) as it recovers from
near the photometric minimum of the 2019–2020 dimming is remarkably similar to
the “red”-class variables, though smaller in amplitude. We suggest that these
objects may represent similar or even more extreme instances of episodic,
enhanced mass-loss in RSGs, though we reserve a full analysis for future
investigations. This may signal that many RSGs experience Betelgeuse-like
dimming events during this phase of their evolution.

We briefly comment on the two blue-evolving sources shown in Figure 4 as well.
Given their red colors, we caution that the apparent evolution of these stars
may be affected by contamination from nearby objects during their minima at
$F606W$. Still, their evolution is reminiscent of that typical of bluer
massive stars, namely the characteristic S Doradus variations between a hot
(visibly fainter), quiescent state and a cool (visibly brighter) outburst
state (e.g., Humphreys & Davidson, 1994). Theoretically, massive stars in the
bluer portion of the CMD are expected to be susceptible to instabilities
arising in radiation-dominated envelopes that approach the local Eddington
limit (Paxton et al., 2013; Jiang et al., 2015; Owocki, 2015, e.g., and see
the “radiation-dominated instabilities” region in Figures 13 and 16 of C18).
We suggest that the “blue”-evolving variables could be intrinsically bluer
stars, possibly LBVs or yellow supergiants experiencing LBV-like episodes (as
in Smith et al., 2004), that are now reddened by the dust in the foreground
ISM or formed in their own circumstellar environments during prior eruptive
mass-loss events. A detailed examination of these objects is outside the scope
of the present work.

Finally, having laid out the modes of variability observed (and expected) for
luminous, red-appearing stars, we return to the variability of M51-DS1. As
noted above, where color information is available for the 2016–2017 and 2021
epochs, M51-DS1 tracks with the mostly vertical evolution of the yellow-
colored variables in Figure 4, consistent with expectations for pulsational
instabilities occuring in massive ($\gtrsim 15M_{\odot}$) RSGs, though large-
grain circumstellar dust may also produce extinction that is relatively gray
(see, e.g., Scicluna et al., 2015; Massey et al., 2006; Haubois et al., 2019).
The long period of brightening seen between 2016–2017 may correspond at most
to one half of a full pulsation cycle, requiring that any periodicity be
$\gtrsim$2 yr in duration. Given the star’s high luminosity ($M_{K}=-11.1$
mag), this is consistent with expectations from the RSG period–luminosity
relation observed in the Milky Way, L/SMC, M33 and M31 (Kiss et al., 2006;
Yang & Jiang, 2012; Soraisam et al., 2018). The maximum amplitude of this
variability (excluding the 2019 minimum), is $\approx$0.9 mag, only slightly
larger than pre-2019 variability observed at the level of $\Delta F814W\approx
0.7$ mag. At an exceptional $\Delta F814W\gtrsim 2.1$ mag, it is unlikely,
though not strongly excluded, that the 2019 minimum is part of the inferred
(semi)regular pulsation cycle ($\lesssim 10$% chance of finding the source in
the bottom portion of a pulsation only once in the 6 light curve samples over
26 years). We therefore suggest that the 2019 dimming event of M51-DS1
represents a relatively rare occurrence, possibly an exceptional mass-loss
event and more extreme cousin of “Great Dimming” of Betelgeuse. Though we lack
the necessary color information during the dimming event to confirm this for
M51-DS1, a statistical analysis of the ensemble of massive variables in M51
with the wealth of archival and newly obtained HST imaging in a future study
will establish the frequency and range of properties (i.e., mass-loss rates,
durations, dust masses) of episodic mass loss in RSGs.

###  4.4 SED Analysis

Figure 5 shows the multi-epoch SEDs of M51-DS1, constructed from the available
HST and ground-based photometry. The photometric magnitudes were converted to
band-luminosities, $\lambda L_{\lambda}$, using the zero point fluxes and
effective wavelengths available with the pysynphot package (STScI Development
Team, 2013) for HST instrument and filter setups, and those compiled by the
Spanish Virtual Observatory (SVO) Filter Profile Service999Documentation for
the SVO Filter Profile Service is available at
http://ivoa.net/documents/Notes/SVOFPSDAL/index.html and
http://ivoa.net/documents/Notes/SVOFPSDAL/index.html for the ground-based
observations (Rodrigo et al., 2012; Rodrigo & Solano, 2020).

Figure 5:  Left: Multi-epoch SEDs of M51-DS1 constructed from visible and
near-IR photometry. Space-based measurements from HST are shown as filled
symbols, while ground-based near-IR measurements are open symbols. Upper
limits are indicated with downward arrows. Center: The pre-2019 SED
constructed from the 2017 ACS/WFC, 2012 WFC3/UVIS and IR, and 2005 NIRI
photometry (black diamonds) is shown along with GRAMS RSG models that provide
good fits ($\chi^{2}\leq\chi^{2}_{\mathrm{min}}+\Delta\chi^{2}_{90}$; see
text) to the data. The best fitting model, described by the parameters listed
in the upper-left corner of the panel, is shown as the orange, thick-dashed
curve, with the corresponding synthetic photometry points in the available
bands shown as red squares. The input (unreddened) stellar photosphere to the
model is shown as the blue dashed curve. Right: Same as the center panel, but
for the 2021 SED constructed from our ACS/WFC, WFC3/IR and MMIRS photometry.

Prior to its dramatic dimming in 2019, the source was characterized by a very
red SED peaking in the near-IR between 1–2 $\mu$m at $\lambda
L_{\lambda}\approx 10^{5}~{}L_{\odot}$. The variability seen in the light
curves (Section 4.2) is also reflected in the SEDs constructed from the 2005,
2012, and 2016–2017 data, though the overall color and shape of the SED
remains largely constant. In 2019, the source faded by $\Delta\lambda
L_{\lambda}\gtrsim 2.6\times 10^{4}~{}L_{\odot}$ at $F814W$, a factor of
$\gtrsim 7$ or $\gtrsim 85$% of the flux in that band. Without multi-band
observations in 2019, we are unable to constrain the shape of the SED or total
drop in bolometric luminosity at that epoch. During our follow-up 2021
observations, the source has partially recovered in flux at $F814W$. The 2021
SED also peaks in the near-IR at $\lambda L_{\lambda}\approx 8.2\times
10^{4}~{}L_{\odot}$ in $F160W$. While overall similar in color and shape, the
source is dimmer across all the available optical and near-IR bands compared
to the pre-2019 SEDs.

####  4.4.1 SED Modelling

To estimate physical parameters of the star, we attempted to fit the SEDs with
the Grid of Red supergiant and Asymptotic Giant Branch ModelS (GRAMS; Sargent
et al., 2011; Srinivasan et al., 2011). This suite of radiative transfer
models consists of a base grid of 1225 spectra from spherically symmetric
shells of varying amounts of silicate dust (Ossenkopf et al., 1992,
appropriate for RSGs) around stars of constant mass-loss rates computed using
the dust radiative transfer code 2-Dust (Ueta & Meixner, 2003). The published
grid uses input PHOENIX model photospheres (Kučinskas et al., 2005, 2006) for
1 $M_{\odot}$ stars (model spectra can be scaled for more luminous and
massive, i.e., supergiant, stars) with effective temperatures,
$T_{\mathrm{eff}}$, between 2100 and 4700 K, and at a fixed sub-solar
metallicity of $\log(Z/Z_{\odot})=-0.5$ and a fixed surface gravity of $\log
g=-0.5$. The amount of circumstellar dust is characterized in terms of the
optical depth at 1 $\mu$m, $\tau_{1}$, from which a dust mass-loss rate,
$\dot{M}_{\mathrm{d}}$ can be inferred assuming a wind speed of $v_{w}=10$ km
s-1. An additional parameter in the GRAMS grid is the inner radius of the dust
shell, $R_{\mathrm{in}}$. We found that our results were largely insensitive
to this parameter and we chose to fix it at $R_{\mathrm{in}}=11.0R_{*}$, where
$R_{*}$ is the stellar radius, to reduce the number of free parameters.

To model the source prior to its near-disappearance in 2019, we combined HST
measurements from the 2017 ACS/WFC observations in $F606W$ and $F814W$ (MJD
58011) and the 2012 WFC3/UVIS observations in $F673N$ and $F689M$ (MJD
56020–56027) and IR observations in $F110W$ and $F128N$ (MJD 56174), together
with the 2005 ground-based NIRI measurements in $J$, $H$ and $K$ (MJD 53548).
Given the observed variability of the source during this time period, we
experimented with fitting different combinations of the available pre-2019
data, but found that these epochs together gave the best results. We also
included additional reddening (Fitzpatrick, 1999 law with $R_{V}=3.1$) for a
range of $E(B-V)$ values between 0.0 and 3.0 mag to account for the likely
significant and uncertain host extinction (see Section 4.1). We then fit each
(reddened) model spectrum in the expanded grid (99225 models in total) by
computing $\chi^{2}$ values between the data points and synthetic photometry
on the spectra, weighted by data point uncertainties and allowing for an
overall scaling factor of the flux as a free parameter. In our analysis, we
will consider “good” models as those with
$\chi^{2}\leq\chi^{2}_{\mathrm{min}}+\Delta\chi^{2}_{90}$, where
$\chi^{2}_{\mathrm{min}}$ is minimum value of $\chi^{2}$ found for all the
models, and $\Delta\chi^{2}_{90}$ is the 90%-tile of the $\chi^{2}$
distribution with degrees of freedom $\nu$.

The results of this fitting procedure for the pre-2019 SED are shown in the
center panel of Figure 5. The best-fitting model ($\chi^{2}/\nu=3.12$) has
$T_{\mathrm{eff}}=4300$, $E(B-V)=1.28$ and $\log(L/L_{\odot})=5.60$. With
$\nu=5$ degrees of freedom, $\Delta\chi^{2}=9.24$, and we find the set of 257
good models with $T_{\mathrm{eff}}=3700$–4700 K, $E(B-V)=0.56$–1.56 mag and
$\log(L/L_{\odot})=5.35$–5.75. Unsurprisingly, $T_{\mathrm{eff}}$ and $E(B-V)$
are strongly positively correlated for the set of good models, and we note
that the range of $T_{\mathrm{eff}}$ they span extends to the maximum value in
the grid. The $\chi^{2}$ distribution is well behaved around the minimum at
$T_{\mathrm{eff}}=4300$ K in the range we tested, although highly reddened,
warmer models could, in principle, also provide acceptable fits to the SED.
These results point to a very luminous, cool ($\approx$K or early-M spectral
type) supergiant star, near the empirical Humphreys-Davidson limit
($\log[L/L_{\odot}]=5.5$–5.8; Humphreys & Davidson, 1979; Davies et al.,
2018). In comparison to single-star evolutionary tracks from the Mesa
Isochrones and Stellar Tracks models (MIST; Choi et al., 2016, 2017), this
would correspond to a star with an initial mass of $\approx$24–40 $M_{\odot}$
with terminal age $\lesssim 8$ Myr (see Figure 6 and further discussion in
Section 4.5).

We fit the post-fading SED, constructed from the 2021 ACS/WFC ($F606W$,
$F814W$) WFC3/IR ($F110W$, $F160W$), and MMIRS ($J$, $H$, $K_{s}$) imaging,
using the same procedure and show the results in the rightmost panel of Figure
5. In this case, the best-fitting model ($\chi^{2}/\nu=4.78$) has
$T_{\mathrm{eff}}=4300$, $E(B-V)=1.64$ and $\log(L/L_{\odot})=5.52$. The set
of 125 good models ($\nu=3$; $\Delta\chi^{2}_{90}=6.25$) now has
$T_{\mathrm{eff}}=3900$ – 4700 K, $E(B-V)=1.0$ – 1.9 mag and
$\log(L/L_{\odot})=5.31$–5.65. Overall, results for the 2021 SED suggest a
slightly dimmer star than the pre-fading data, though there is substantial
overlap in the allowed parameter ranges.

While the GRAMS grid includes models with high values of circumstellar
extinction up to $\tau_{1}\approx 60$, our fitting results for both the pre-
and post-fading data favor $\tau_{1}\lesssim 0.9$ and corresponding to dust
mass-loss rates below $\dot{M}_{\mathrm{d}}\lesssim 10^{-7}~{}M_{\odot}$ yr-1.
This is in accord with modern estimates of mass-loss rates for normal RSG
winds in this luminosity range (Beasor et al., 2020). At the same time, the
models favor relatively high values of foreground host (i.e., ISM-like)
extinction. Importantly, longer wavelength data ($\gtrsim 3$–5 $\mu$m and
especially at the 10 $\mu$m silicate feature) that are sensitive to emission
from warm dust would be required to directly constrain the amount of
circumstellar material (CSM), so our ability to distinguish circum- and
interstellar extinction should be viewed cautiously. Notably too, the
modelling results for the 2021 data favor higher values of the foreground
extinction to account for the redder colors of the source compared to the pre-
fading source. An increase in the amount of intervening ISM material is
presumably unphysical, indicating that other effects not captured by the
models may be at play in the observed SED evolution. These may include the
effects of increased molecular opacity during episodes of enhanced mass loss
(see, e.g., Davies & Plez, 2021), changes in composition or the grain-size
distribution of circumstellar dust, and/or the effects of non-spherical
geometry in stellar surface (e.g., large convective bubbles and cold spots) or
the CSM.

Lastly, the input PHOENIX model photospheres of the published GRAMS grid are
at a metallicity of $\log(Z/Z_{\odot})=-0.5$, originally chosen to be similar
to that of the LMC, while the local environment of M51-DS1 indicates that
models at higher, somewhat super-solar, metallicities
($\log[Z/Z_{\odot}]=0.0$–0.3; see Section 4.1) would be more appropriate.
Though metallicity will affect the photospheric spectrum in the optitcal/near-
IR, we do not expect this to be significant compared to the effects of of an
external dusty wind or the possible effects of a complex geometry mentioned
above. We examined this by also fitting the pre-fading SED with bare PHOENIX
photospheres at low, solar, and high metallcities
($\log[Z/Z_{\odot}]=-0.5,0.0,+0.5$), again including the amount foreground
extinction $E(B-V)$ as a free parameter. In general we find similar results
regardless of metallicity. The best fitting models have
$T_{\mathrm{eff}}=4300$ at low and solar metallicities and
$T_{\mathrm{eff}}=4500$ at high metallicity, all well within the preferred
parameter ranges found for the GRAMS models.

###  4.5 Location in the H–R Diagram and Evolutionary State

We examine possible locations M51-DS1 in the Hertzsprung–Russell diagram (HRD;
Figure 6) and discuss implications for the inferred initial mass and
evolutionary state of the star. Specifically, we directly compare the results
of applying a simple $K$-band bolometric correction (Davies & Beasor, 2018,
assuming an M-type RSG; see Section 4.2.1) to those of our SED model fitting
(Section 4.4.1). Overall, the luminosity of the star inferred from applying
$BC_{K}$ is lower than that from the SED fitting, falling in the range
$\log(L/L_{\odot})\approx 5.15$–5.37—notably above the range inferred for the
growing collection of SN progenitors and comparable to that inferred for the
failed SN candidate NGC 6946-BH1—depending on the value assumed for the
foreground extinction between $E(B-V)=0.0$ and 1.6 mag. The SED modelling
prefers somewhat warmer ($T_{\mathrm{eff}}=3700$–4700 K) photospheric models
at relatively high extinction ($E(B-V)=0.56$–1.56 mag) and luminosities in the
range $\log(L/L_{\odot})=5.35$–5.75. Given the constraints on ISM environment
discussed in Section 4.1, the high $E(B-V)$ values $\gtrsim 1$ mag would
likely point to significant circumstellar extinction.

Figure 6:  HRD showing possible locations of M51-DS1. The result for best
fitting model for the pre-2019 SED is shown as the star symbol, while the
range of good models are represented by the small circles. The color of each
point corresponds to the value of the extinction (in excess of the Galactic
foreground), $E(B-V)$, for that model as indicated by the color bar. The
“x”-symbols show the locations of M51-DS1 assuming early, mid, and late M-type
spectra and applying the $K$-band bolometric corrections of Davies & Beasor
(2018) to the 2005 NIRI measurement, and with the same mapping of color to
$E(B-V)$. We show stellar evolutionary tracks from MIST (non-rotating, solar
metallicity) for a set of massive stars in the range $M=8$–40 $M_{\odot}$ as
black curves for comparison. We also indicate the locations of Betelgeuse
(purple diamond; Dolan et al., 2016), the RSG progenitor of NGC6949-BH1 (thin
black diamond; model P5 from Adams et al., 2017a), and the collection of
directly detected SN progenitors of Types II (light gray squares) and IIb
(dark gray circles) from Smartt (2015).

Even for an early M-type star ($T_{\mathrm{eff}}\approx 3700$), where there is
overlap between the two methods, the inferred luminosity from SED fitting is
about a factor of 1.3 higher than from applying $BC_{K}$ for the same
extinction of $E(B-V)=0.8$ mag. This modest discrepancy could be resolved if
the effective extinction law were grayer—either from a shallower ISM law
(i.e., $R_{V}>3.1$) for the central regions of M51 or perhaps related to
properties of the stellar wind or circumstellar dust not captured by the
models (e.g., nonspherical geometry or nonisotropic scattering by dust
grains)—resulting in a smaller correction at bluer wavelengths and a lower
bolometric luminosity for a given model. Alternatively, using a smaller
$BC_{K}$ at earlier spectral types (as found for cool supergiants in the
Magellanic Clouds; Elias et al., 1985; Levesque et al., 2006; Davies et al.,
2018, e.g.,) would also bring the luminosity estimates closer to agreement.

Altogether, we infer that M51-DS1 is likely a luminous RSG at
$\log(L/L_{\odot})\approx 5.2$–$5.3$ (for $T_{\mathrm{eff}}\lesssim 3700$ K
and reasonable values of $E(B-V)=0.4$–1.3) corresponding to a highly evolved
star of initial mass $M\approx 19$–22 $M_{\odot}$, or somewhat warmer
($T_{\mathrm{eff}}=3700$–4700 K)—possibly a YSG on a post-RSG track back to
higher temperatures —at higher luminosities up to $\log(L/L_{\odot})\approx
5.4$–$5.7$. This more extreme case would suggest an initial mass as high as
$M\approx 26$–40 $M_{\odot}$ for a star near the empirical Humphreys-Davidson
limit in an exceptionally short-lived and rare state of stellar evolution.

##  5 Summary and Conclusions

We have presented a detailed analysis and characterization of a remarkable
dimming event of a luminous, cool supergiant star in the nearby, star-forming
galaxy M51, found as part of a new search for failed SNe as “disappearing”
massive stars with HST. The object, which we call M51-DS1, was detected dozens
of times in the wealth of archival HST imaging for more than two decades since
1995 before undergoing a dramatic, near-total disappearance in the 2019
ACS/WFC $F814W$ images obtained as part of our search. Thus meeting our
criteria for a strong failed SN candidate (outlined in Section 3.2), we
conducted follow-up HST imaging observations with a Cycle 28 mid-cycle program
using ACS/WFC and WFC3/IR, supplemented with ground-based near-IR imaging from
Gemini-N/NIRI and MMT/MMIRS, to attempt to confirm the disappearance of the
star. The star was found to have partially rebrightened to near its pre-
disappearance levels across optical and near-IR, ruling out the terminal
collapse of the star in a failed SN. As summarized below, our analysis
indicates instead that an isolated episode of enhance mass loss on a very
massive, cool Y/RSG star—potentially a rare, more extreme cousin of the recent
“Great Dimming” of Betelgeuse—can explain its observed properties.

M51-DS1 is located in the inner regions of M51 along the edge of a spiral arm.
Previous studies of the host galaxy suggest an environment of approximately
solar or somewhat supersolar metallicity ($\log[Z/Z_{\odot}]\approx 0.0$–0.3)
and significant foreground extinction ($E[B-V]\approx 0.4$–1.0 mag).
Photometry of stars in the immediate vicinity indicate the presence of a young
population ($\lesssim 6$ Myr old) within a 20 pc radius. M51-DS1 itself is the
brightest red source in this region, implying an initial mass of at least
$\approx$30 $M_{\odot}$ if it is part of the same population. It is difficult,
however, to disentangle possible confounding effects, including a mix of
stellar populations of different ages and high spatial variation in the
foreground extinction toward individual sources up to $E(B-V)\approx 2.0$ mag.
At $M_{K}=-11.1$ mag in 2005 and with $J-K\approx 1.0$–1.2 mag (Galactic
extinction correction only), the star is consistent with a luminous RSG, and
applying the $K$-band bolometric correction of Davies & Beasor (2018) yields
$\log(L/L_{\odot})=5.2$–5.3, implying an initial mass $M=19$–22 $M_{\odot}$
(assuming an M-type supergiant with $T_{\mathrm{eff}}\lesssim 3700$ K and
foreground extinction $E[B-V]=0.4$–1.3 mag). Modelling of the SED suggests
higher temperatures ($T_{\mathrm{eff}}\approx 3700$–$4700$) and luminosities
($\log[L/L_{\odot}]=5.4$–5.7), pointing to a more massive (initial mass
$M=26$–40 $M_{\odot}$) YSG or post-RSG star that would be consistent with the
inferred age of the very young stellar population in the immediate vicinity
and possibly requiring significant circumstellar extinction.

The extended, 26-yr light curve of M51-DS1 indicates a history of variability
with $\Delta F814W\approx 0.7$–0.9 mag, consistent with semiregular variations
typical of Y/RSGs and expected theoretically for massive cool stars above
$\gtrsim$15$\,M_{\odot}$. At $\Delta F814W\gtrsim 2.1$ mag, we find that the
2019 minimum is unlikely to be part of the typical variation cycle for this
star, but rather an uncommon and exceptionally deep dimming event—notably,
more than $\gtrsim$1.5 mag deeper than the 2019-2020 historic minimum of
Betelgeuse in the comparable $I$-band. In the context of other large-amplitude
modes of variation seen in luminous red stars in M51, we suggest that the 2019
event of M51-DS1 could be associated with an enhanced episode of mass loss
from the star, in which increased opacity from the dense, molecular wind or
the formation of new dust grains temporarily obscured the star.

This discovery highlights a central challenge for the definitive
identification of failed SNe, namely, that the massive stars in question
exhibit optical variability that can mimic a disappearing star. In particular,
episodic mass loss in massive, cool supergiants remains poorly understood
(e.g., Smith, 2014), both in terms of the physical mechanisms that drive it
and critical properties such as the frequency, possible duration, and mass-
loss rates of the most extreme events—an extended mass loss event could
plausibly obscure a massive star in the optical for years. While Galactic
examples of episodic mass loss from cool supergiants, like Betelgeuse’s Great
Dimming, can be studied in real time and in exquisite detail, such events will
be few and far between. Still, significant progress can be made in mining the
ever-growing trove of archival data from both space-based (e.g., HST, Spitzer)
and numerous ground-based time-domain surveys to uncover and characterize the
population of such sources in nearby galaxies, both as possible contaminants
in ongoing failed SN searches and as direct probes of episodic mass-loss in
massive, evolved stars. At the same time, a continued, concerted effort to
monitor existing and newly uncovered failed SN candidates is vital, especially
observations in the near-IR and mid-IR, e.g., soon with the James Webb Space
Telescope and later with the Nancy Grace Roman Space Telescope, to test for
surviving obscured stars and direct signatures of a potential obscuring wind
or dust.

Time domain research by D.J.S. is also supported by NSF grants AST-1821987,
1813466, 1908972, & 2108032, and by the Heising-Simons Foundation under grant
#2020-1864. J.S. acknowledges support from NASA grant HST-GO-15645.003-A and
the Packard Foundation. Research by S.V. is supported by NSF grants
AST-1813176 and AST-2008108. E.R.B. is supported by NASA through Hubble
Fellowship grant HST-HF2-51428 awarded by the Space Telescope Science
Institute, which is operated by the Association of Universities for Research
in Astronomy, Inc., for NASA, under contract NAS5-26555. Based on observations
made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope
Science Institute, which is operated by the Association of Universities for
Research in Astronomy, Inc., under NASA contract NAS5-26555. These
observations are associated with programs # HST-GO-15645, 16508, 10452, 14704,
12490, 12762.Support for program # HST-GO-15645 and # HST-GO-16508 was
provided by NASA through a grant from the Space Telescope Science Institute,
which is operated by the Association of Universities for Research in
Astronomy, Inc., under NASA contract NAS5-26555. Observations reported here
were obtained at the MMT Observatory, a joint facility of the University of
Arizona and the Smithsonian Institution. Based on observations obtained at the
Gemini Observatory (Programs GN-2005A-Q-49,GN-2021A-DD-101), which is operated
by the Association of Universities for Research in Astronomy, Inc., under a
cooperative agreement with the NSF on behalf of the Gemini partnership: the
National Science Foundation (United States), National Research Council
(Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación
Productiva (Argentina), Ministério da Ciência, Tecnologia e Inovação (Brazil),
and Korea Astronomy and Space Science Institute (Republic of Korea). We
acknowledge with thanks the variable star observations from the AAVSO
International Database contributed by observers worldwide and used in this
research.

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