AT 2022aedm and a New Class of Luminous, Fast-cooling Transients in Elliptical Galaxies

The object AT 2022aedm was discovered in the ATLAS transient stream processed by the ATLAS Transient Science Server (Smith et al. 2020). Calibrated ATLAS data in the cyan (c) and orange (o) bands were obtained using the ATLAS forced photometry service (Tonry et al. 2018; Smith et al. 2020; Shingles et al. 2021). ATLAS typically obtains four exposures per night in a given band; we combined each quad into a single average flux measurement to improve the signal-to-noise ratio. After AT 2022aedm had faded below o ∼ 19 mag, observations were binned over neighboring nights to improve the signal-to-noise ratio.

Follow-up photometry was obtained from Pan-STARRS in the i, z, and y bands; the Las Cumbres Observatory (LCO) global telescope network (as part of the Global Supernova Project) in the B, g, V, r, and i bands; and the Liverpool Telescope (LT) in the u, g, r, i, and z bands. Data from the European Southern Observatory New Technology Telescope (NTT) were obtained using both EFOSC2 for the optical g, r, i, and z bands and SOFI for the near-infrared (NIR) J, H, and K bands as part of ePESSTO+. The data were reduced (debiased and flat-fielded) either automatically by facility pipelines or manually using the PESSTO pipeline (Smartt et al. 2015) in the case of the NTT data.

Photometry was performed using a custom pipeline, Photometry Sans Frustration (or psf). ³⁸ This is a fully python-based code employing aperture and point-spread function (PSF) fitting photometry routines from astropy (Astropy Collaboration et al. 2013, 2018) and photutils (Bradley et al. 2020). As well as deriving the image background, PSF, and zero-point, the code provides options to automatically download local star catalogs and reference images using astroquery (Ginsburg et al. 2019), solve the coordinate system using astrometry.net (Lang et al. 2010), clean cosmic rays using lacosmic (van Dokkum et al. 2012), align and stack images using astroalign (Beroiz et al. 2020), and subtract transient-free reference images of the field using pyzogy (Zackay et al. 2016; Guevel & Hosseinzadeh 2017).

All NTT, LCO, and LT images were first cleaned and stacked within each night. Calibration stars and template images were obtained in g, r, i, and z from Pan-STARRS (Flewelling et al. 2020); u from the SDSS; and J, H, and K from the VISTA Kilo-degree Galaxy Survey (Edge et al. 2013). The LCO B and V reference images were obtained after the transient faded. The PSF for each science and template image was determined using the local reference stars. All transient fluxes were measured after template subtraction to remove host galaxy light. The full photometric data set is shown in Figure 1.

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We obtained ultraviolet (UV) and X-ray data using target-of-opportunity observations with the UV-Optical Telescope (UVOT; Roming et al. 2005) and X-ray Telescope (XRT; Burrows et al. 2005) on board the Neil Gehrels Swift Observatory (Swift; Gehrels et al. 2004). The UVOT imaging was carried out in the uvw2, uvm2, and uvw1 filters. The light curves were measured using a 5'' aperture. Count rates were obtained using the Swift uvotsource tools and converted to magnitudes (in the AB system) using the UVOT photometric zero-points (Breeveld et al. 2011). No host subtraction was performed in the UV bands, as host contamination is negligible at these wavelengths (this is confirmed by the later UVOT visits that only result in nondetections).

We processed the XRT data using the online analysis tools provided by the UK Swift Science Data Centre (Evans et al. 2007, 2009). Object AT 2022aedm is not detected in the combined 15.3 ks exposure, with a limiting count rate <8.53 × 10⁻⁴ s⁻¹. Assuming a power-law spectrum with Γ = 2 (Coppejans et al. 2020) and a Galactic hydrogen column density toward AT 2022aedm of 4.2 × 10²⁰ cm⁻² (HI4PI Collaboration et al. 2016), this corresponds to an unabsorbed 0.3–10 keV luminosity L_X < 3.8 × 10⁴² erg s⁻¹. This upper limit is deeper than the observed X-ray luminosities of ∼10^43–44 erg s⁻¹ in AT 2018cow (Margutti et al. 2019), AT 2020xnd (Ho et al. 2022), and AT 2022tsd (Matthews et al. 2023).

We observed AT 2022aedm with the Arcminute Microkelvin Imager–Large Array (AMI–LA; Zwart et al. 2008; Hickish et al. 2018) over 4 epochs beginning 20 days after discovery. The AMI–LA is an eight-dish interferometer based in Cambridge, UK. Each dish is 12.8 m in diameter, enabling an angular resolution of ∼30''. The facility observes at a central frequency of 15.5 GHz with a bandwidth of 5 GHz.

Data taken with AMI–LA were reduced using a custom pipeline, reduce_dc. We used 3C 286 and J1119+0410 as the primary and secondary calibrators, respectively, to perform amplitude and phase calibration. The pipeline also flags the data for radio-frequency interference, effects of poor weather, and antenna shadowing. The data were then exported in uvfits format ready for imaging.

Further flagging and imaging were conducted in the Common Astronomy Software Applications (casa, version 4.7.0; McMullin et al. 2007; CASA Team et al. 2022) using the tasks tfcrop, rflag, and clean. Object AT 2022aedm is not detected in any of the final images, with 3σ upper limits of F_ν < [41, 210, 156, 96] μJy at 20, 66, 107, and 110 days after the first optical detection. These correspond to limits on the spectral luminosity of L_ν < 2.3 × 10²⁸–1.2 × 10²⁹ erg s⁻¹ Hz⁻¹. For comparison, Cow-like RETs typically exhibit radio emission at the level of L_ν ≳ 10²⁹ erg s⁻¹ Hz⁻¹ (at ∼10 GHz) on timescales of months (Coppejans et al. 2020; Ho et al. 2020).

We obtained optical spectra of AT 2022aedm using EFOSC2 on the NTT (through ePESSTO+), the LCO 2 m telescopes, the SuperNova Integral Field Spectrograph (Lantz et al. 2004; Tucker et al. 2022) on the University of Hawaii 2.2 m telescope, and Binospec on the 6.5 m MMT (Fabricant et al. 2019). Spectroscopy from ePESSTO+ commenced on 2022 December 31 (within a day after the object was flagged by ATLAS) and continued until 2023 February 20, by which time the spectrum was indistinguishable from a preexplosion host galaxy spectrum from SDSS.

Standard reductions of these data, including debiasing, flat-fielding, spectral extraction, and flux and wavelength calibration, were performed using instrument-specific pipelines. The reduced spectra are plotted in Figure 1 and labeled with the instrument and phase with respect to our estimated explosion date. We assume that host galaxy extinction is negligible, supported by the lack of Na i D absorption in these spectra (Poznanski et al. 2012) and the early blue colors in our spectra and photometry. All data will be made publicly available via WISeREP (Yaron & Gal-Yam 2012).

The rising light curve of AT 2022aedm is well constrained by the early ATLAS o-band detections. The discovery point on MJD 59,941.1 at o = 19.51 mag is a factor of ≃5 below the peak o-band flux. Fitting a second-order polynomial to the early flux light curve (Figure 1) indicates the explosion occurred on MJD 59,940.0 ± 0.5 (≈1 day before the first detection), reaching the o-band peak on MJD 59,950.6 ± 0.5. We take these as the dates of explosion and peak throughout. We note, however, that the last nondetection is 15 days before detection, so a slightly earlier explosion date cannot be entirely ruled out if the early light-curve shape is more complex. The rest-frame rise time from half the peak flux is t_r,1/2 = 6.6 days, much shorter than the SLSNe that reach comparable peak luminosities but with t_r,1/2 = 10–40 days (Nicholl et al. 2015; De Cia et al. 2018; Lunnan et al. 2018).

The fading timescale of AT 2022aedm is also much quicker than most other luminous transients. The g-band light curve fades by 2 mag in the 15 days after peak, and by 18 days, it has declined to 10% of the peak g-band flux. In the o band, where we also have the rise, the FWHM (i.e., the total time spent within 50% of peak flux) is t_1/2 = 19 ± 1 days. These timescales are well within the distributions measured for RETs discovered in the DES (Pursiainen et al. 2018). Although the measured t_1/2 for AT 2022aedm is longer than the t_1/2 < 12 days defining RETS in PS1 (Drout et al. 2014) and ZTF (Ho et al. 2023), this is attributable to using different photometric filters; we measured t_1/2 in o, where the rate of fading in AT 2022aedm is ≈55% slower than in the g band. Correcting by this factor gives an estimated g-band t_1/2 of ≈12 days.

The extinction-corrected g-band peak luminosity of AT 2022aedm, M_g = −22.04 ± 0.05, makes it one of the brightest RETs discovered to date. It outshines all but one event (DES 16E1bir) in the combined PS1+DES+ZTF sample. The closest spectroscopically classified RETs in terms of luminosity are the Cow-like RETs, typically reaching ≈−21 mag (Ho et al. 2023). To highlight the exceptional luminosity of AT 2022aedm, we show a combined g- and c-band rest-frame light curve in Figure 2, compared to representative examples of different types of fast-fading transients. Object AT 2022aedm is broader and brighter than AT 2018cow but fades faster than the fastest SLSN, SN 2018bgv (Chen et al. 2023). It is much more luminous than a typical RET (Drout et al. 2014); the fastest TDE, AT 2020neh (Angus et al. 2022); the fastest broad-lined SNe Ic, such as iPTF16asu (Whitesides et al. 2017; see also SN 2018gep and SN 2018fcg; Pritchard et al. 2021; Gomez et al. 2022); or any fast interacting transients of Types IIn (Ofek et al. 2010), Ibn (Hosseinzadeh et al. 2017), or Icn (Fraser et al. 2021; Perley et al. 2022).

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Figure 2 also shows the g − r (or B − V) color evolution of AT 2022aedm compared to the same sample of objects (where multiple bands are available). From an initial g − r = −0.37 at 10 days after explosion, AT 2022aedm dramatically reddens by 1.8 mag in color index over the next 35 rest-frame days. Cow-like RETs, TDEs, and interacting transients generally show a more gradual or flat color evolution. The color change in AT 2022aedm is more consistent with events with expanding, cooling photospheres; it lies intermediate between iPTF 16asu and SN 2018bgv. Object PS1-10bjp shows a similar color evolution over the first 10 days.

Two unclassified fast transients show a comparable color evolution in combination with a peak absolute magnitude brighter than −20 mag. One is AT 2020bot, the only RET in the ZTF sample that did not fit into any of the stripped-envelope, interacting, or Cow-like subpopulations (Ho et al. 2023). It is fainter than AT 2022aedm, with a faster rise and redder average color. A stronger similarity is exhibited by "Dougie" (Vinkó et al. 2015), a mysterious transient discovered by ROTSE in 2009. This event peaked at −22.5 mag after a fast rise of ≈10 days. The early light-curve shape is very similar to that of AT 2022aedm, though the decline may flatten after 30–40 days. However, this flattening could also be due to a host contribution in its UVOT photometry. While the light curve could be plausibly interpreted as a super-Eddington TDE (Vinkó et al. 2015), its position offset from the host nucleus and lack of distinct spectroscopic features make this classification far from certain.

To measure the overall energetics of AT 2022aedm, we integrate our multiband photometry using superbol (Nicholl 2018). We construct a pseudobolometric light curve using the excellent g-, r-, i-, z-, and o-band coverage and estimate the full bolometric light curve by fitting blackbody functions to these bands and our UV and NIR photometry where available. These light curves are shown in Figure 3.

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The peak pseudobolometric luminosity is typical of SLSNe, reaching L_griz = 10^43.8 erg s⁻¹, but the light-curve rise and decline rates fall well outside the SLSN distribution. The decay rate is comparable to AT 2018cow, though the rise is longer than the <2 days exhibited by that event. The estimated full bolometric luminosity of AT 2022aedm at peak is exceptionally high, reaching ≈10⁴⁵ erg s⁻¹. This is due to a high temperature, T ≳ 30,000 K (shown in the top right panel), suggested by the very blue g − r and r − i colors in the first LCO images. The temperature exhibits a monotonic decline with a scaling of roughly T ∝ 1/t. The blackbody radius increases throughout our observations, suggestive of an expanding photosphere that remains optically thick. The estimated bolometric light curves of Dougie and AT 2020bot, also constructed using superbol, peak at similar luminosities to AT 2022aedm.

The spectroscopic evolution of AT 2022aedm is shown in Figure 1. The cooling observed in the photometry is also evident in its spectra. The first NTT spectrum 3.5 days after explosion shows a strong blue continuum, which weakens and disappears by day 27. By day 48, the spectrum is indistinguishable from an archival SDSS spectrum of the host galaxy.

The spectra mostly lack the obvious broad emission, absorption, or P Cygni lines typically seen in SNe. The spectra on days 3 and 14 (both from NTT) and 20 (from MMT) with the best signal-to-noise ratios are examined in more detail in Figure 4. Weak broad features may exist at around 4000–5000 Å after day 20, though these could also be caused by contamination from the host galaxy, which is around 2 mag brighter than AT 2022aedm at this phase. We show this explicitly by adding an arbitrary 20,000 K blackbody to the host, finding that this reasonably reproduces the overall shape of the day 20 spectrum.

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Figure 4 does show several narrow spectral lines of a clearly transient nature. The day 3 spectrum shows a sharply peaked emission line consistent with He ii λ4686, possibly with a broader base. This line is often observed in very young SNe (Gal-Yam et al. 2014; Khazov et al. 2016; Bruch et al. 2021) and in TDEs (Gezari et al. 2012; Arcavi et al. 2014) due to the high radiation temperatures capable of ionizing helium. We also detect a weak narrow Hα emission line. Other Balmer lines are not visible at this signal-to-noise ratio. The He ii is not detected in any later spectra, likely because the temperature has fallen too low to maintain helium ionization.

The spectra on days 14 and 20 show narrow absorption, rather than emission, from hydrogen and neutral helium. The first three transitions of the Balmer series are clearly visible but blueshifted from their rest wavelengths by ≈2700 km s⁻¹. The high-resolution MMT/Binospec data on day 20 also clearly show He i λ5875 absorption blueshifted by ≈2500 km s⁻¹. These lines are not visible in the host spectrum, confirming their association with the transient.

Figure 4 also includes a comparison between the early spectra of AT 2022aedm and other fast transients. The initial He ii and Hα emission is reminiscent of some SNe Ibn (Hosseinzadeh et al. 2017), as well as the fast-evolving TDE AT 2020neh (Angus et al. 2022). The earliest spectrum is also a reasonable match for young SNe IIn, including the shock breakout candidate PT 09uj (Ofek et al. 2010). However, AT 2022aedm never develops the strong emission lines typically seen in these classes at later times. The largely featureless spectrum even at 15 days is a poor match for any fast-evolving SNe Ic, SLSNe, or SNe Icn.

Cow-like RETs exhibit quite featureless spectra at peak, but the prototype AT 2018cow showed increasingly clear H and He emission lines as it evolved, the opposite of the case in AT 2022aedm. Interestingly, the spectra of Dougie remained featureless for ≳30 days (Vinkó et al. 2015) and seemed to cool in a manner similar to AT 2022aedm. Object AT 2020bot lacks a full spectroscopic time series, making a detailed comparison difficult. It shows an unusual spectrum at maximum light. Ho et al. (2023) noted the presence of possible broad features, weak compared to typical SN lines and without an obvious identification. In Section 3.1, we observed that these two events also shared some key photometric properties with AT 2022aedm.

The host galaxy of AT 2022aedm is LEDA 1245338 (or SDSS J111927.73+030632.7). This is a bright, red galaxy with M_r = −22.8 mag; Figure 5 illustrates this with a color image obtained from Pan-STARRS. An SDSS spectrum is also available (shown in Figure 4). Both the SDSS spectral fitting and Galaxy Zoo morphological analysis classify LEDA 1245338 as an elliptical galaxy.

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The SDSS data release also includes an automated analysis of the SDSS spectrum with the Portsmouth pipeline (using the method of Maraston et al. 2009). The spectral fitting measures a total stellar mass of ≈10^11.5 M_⊙ and a star formation rate (SFR) consistent with zero. They find a mean age of the stellar population of 4.8 Gyr. Given that spectral energy distribution (SED) modeling is highly sensitive to the assumed functional form of the star formation history (Carnall et al. 2019; Leja et al. 2019), we run our own analysis of the host photometry over a wider wavelength range using prospector (Johnson et al. 2021). In particular, we use the prospector-α model employing a nonparametric star formation history, with six equal-mass star-forming bins of flexible width (see Leja et al. 2017, for details). We include archival host photometry from SDSS, the Two Micron All Sky Survey (Skrutskie et al. 2006), and the Wide-field Infrared Survey Explorer (Wright et al. 2010). The fit is shown in Figure 5. We measure M_* = 10^11.45 M_⊙, consistent with the SDSS results, and a specific SFR (sSFR) in the last 50 Myr of $\mathrm{log}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})=-11.69$.

The SDSS analyses and our prospector results confirm that the host of AT 2022aedm is a massive "red and dead" galaxy. This is surprising; recent work by Irani et al. (2022) shows that less than 1% of core-collapse explosions occur in such environments. Moreover, this galaxy is especially unlike the hosts of most transients with comparable luminosity. The SLSNe occur almost exclusively in low-mass galaxies with <10¹⁰ M_⊙ (Lunnan et al. 2014; Leloudas et al. 2015; Perley et al. 2016; Chen et al. 2017; Schulze et al. 2018), and their average sSFR is 3 orders of magnitude greater than our measurement for AT 2022aedm. Bright RETs are also found in relatively low-mass, star-forming host galaxies; the sSFRs of the RET hosts in PS1 and DES span $-10\lesssim \mathrm{log}(\mathrm{sSFR}/{\mathrm{yr}}^{-1})\lesssim -8$, with only three (out of 73) fitting best to a passive galaxy model and only two having a stellar mass greater than 10¹¹ M_⊙. Wiseman et al. (2020) conducted a systematic analysis of RET hosts in DES, finding evidence for star formation in all of the 49 galaxies for which redshifts were available, and the five RETs from HSC were also found in star-forming galaxies (Tampo et al. 2020).

Notably, two other fast transients from our photometric and spectroscopic comparisons also occurred in elliptical galaxies: Dougie and AT 2020bot (Vinkó et al. 2015; Ho et al. 2023). The Pan-STARRS images of their hosts are shown alongside AT 2022aedm in Figure 5. We also note that one SN Ibn, PS1-12sk, exploded in a bright elliptical, prompting Hosseinzadeh et al. (2019) to suggest that not all SNe Ibn result from massive stars.

Object AT 2022aedm is a puzzling event with a very unusual set of properties:

• 1.  
  a high peak luminosity in the optical, with M_g ≈ −22 mag;
• 2.  
  no luminous radio or X-ray emission;
• 3.  
  fast rise and decline rates, fading ∼1 mag week^–1 in the g band;
• 4.  
  rapid cooling from ∼30,000 to ∼4000 K in a few weeks following peak;
• 5.  
  a spectrum dominated by a smooth continuum with no high equivalent width absorption or emission lines at any phase; and
• 6.  
  a massive host galaxy comprised of an old stellar population, with no evidence for current star formation.

This combination is not consistent with any known class of transients. An extensive search of the literature reveals two other objects that share the key properties: M < −20 mag, rise time <10 days, fast decline and color evolution, and weak spectroscopic features at all times. Together, these events indicate a new class of fast transients with high optical luminosities and fast cooling after peak. Dougie (Vinkó et al. 2015) in particular shows a strong photometric and spectroscopic similarity, while AT 2022bot (Ho et al. 2023) may represent an even faster-evolving member of this class. All three occurred off-center within passive host galaxies, indicating that—uniquely among RETs—these new "luminous fast coolers" (LFCs) do not require young stellar populations.

We estimate the volumetric rate of these events using the ATLAS Survey Simulator (McBrien 2021; Srivastav et al. 2022). Interpolated absolute light curves of AT 2022aedm in the c and o bands are inserted into the simulation at 10,000 random times, positions, and redshifts (up to some ${z}_{\max }$) and compared to the cadence, footprint, and depth of the true ATLAS survey to determine the number of 5σ detections of each injected event.

We define a discovery as those objects detected in more than ${n}_{\det }$ images (or $\approx {n}_{\det }/4$ nights, since ATLAS typically obtains a quad of exposures per night at a given pointing). We then take all real transients brighter in apparent magnitude than the injected transient at $z={z}_{\max }$ and having at least the same number of ATLAS detections and take as our spectroscopic completeness the fraction of these transients that were classified. The rate is then

$$\begin{eqnarray}&&R=\displaystyle \frac{{N}_{\mathrm{events}}}{{f}_{\mathrm{disc}}{f}_{\mathrm{spec}}{\int }_{0}^{{z}_{\max }}T/(1+z)\tfrac{{dV}}{{dz}}{dz}},\end{eqnarray} \tag{ 1 }$$

where T = 2.5 yr is the duration of the mock survey (with the factor 1/(1 + z) accounting for time dilation over the observed redshift range), dV/dz is the differential comoving volume between redshift z and z + dz, f_disc and f_spec are the fractions discovered and classified, and N_events = 1 is the number of observed AT 2022aedm–like transients in ATLAS.

We set ${z}_{\max }=0.2$, covering the redshift range within which LFCs have been discovered, and set ${n}_{\det }=20$. The latter is motivated by the modal number of detections for real ATLAS transients and, with observations on 5 nights, should also enable identification of the fast light-curve shape. Varying ${n}_{\det }$ leads to less than a factor of 2 variation in our derived rate due to a trade-off between f_disc and f_spec; stricter requirements lead to a smaller discovered sample but with a higher spectroscopic completeness. For these parameters, our survey simulation returns f_disc = 0.35 and f_spec = 0.58, giving R ≈ 1 Gpc⁻³ yr⁻¹.

We caution that this estimate applies to the brightest LFCs such as AT 2022aedm and Dougie, and that fainter and faster events such as AT 2020bot are likely more common volumetrically but harder to detect. Nevertheless, our derived rate indicates that these events are very rare, ∼10⁻⁵ of the core-collapse SN rate. This rate is lower than the SLSN rate of a few × 10 Gpc⁻³ yr⁻¹ (Frohmaier et al. 2021) but may be consistent with the rate of Cow-like events, estimated as 0.3–420 Gpc⁻³ yr⁻¹ (Ho et al. 2023).

4.3.1. Tidal Disruption Events

4.3.2. Nickel Powering and White Dwarf Explosions

4.3.3. Magnetar Birth

4.3.4. Shock Breakout

The old stellar populations hosting AT 2022aedm and other LFCs are more compatible with a compact object origin, rather than massive stars. However, we encountered inconsistencies in interpreting these objects as white dwarf explosions or TDEs from massive BHs. Fortunately, recent years have seen rapid progress in understanding the diversity of transients resulting from mergers involving neutron stars (NSs) and stellar-mass BHs, and we compare to such models here.

Kilonovae, transients powered by the decay of heavy elements ejected from NS mergers, have been discovered in targeted follow-up of gravitational waves (Abbott et al. 2017; Margutti & Chornock 2021) and gamma-ray bursts (Berger et al. 2013; Tanvir et al. 2013; Rastinejad et al. 2022). However, LFCs are much brighter and bluer than any plausible kilonovae, in which low ejecta masses of <0.1 M_⊙ and large opacities conspire to produce faint red transients visible for only a few days in the optical. While no definitive detections exist, kilonovae from NS–BH mergers are expected to be even fainter and redder than those from binary NSs (Gompertz et al. 2023). Object AT 2018kzr was a so-far unique event suggested to be the merger of an NS with a white dwarf (McBrien et al. 2019; Gillanders et al. 2020). It was somewhat brighter and longer-lived than the known kilonovae, peaking at M ≈ −18 mag. However, it still faded much faster than the LFCs (Figure 2) and showed broad metal absorption lines resembling SNe Ic, inconsistent with our events (Figure 4).

Lyutikov & Toonen (2019) proposed that Cow-like RETs can arise from accretion-induced collapse following the merger of a carbon–oxygen white dwarf with an oxygen–neon–magnesium white dwarf. This model produces a magnetar in combination with a low ejecta mass. If one of the white dwarfs retained a surface hydrogen layer (type DA), a residual precollapse wind can help to explain the observed lines in the spectrum of AT 2022aedm. This model may be a plausible progenitor channel for LFCs. However, it also predicts Cow-like nonthermal emission, which is ruled out in the case of AT 2022aedm. This channel also has a short delay time, such that the host galaxies are likely to still be star-forming, in tension with the elliptical hosts of our objects. Brooks et al. (2017) suggested another channel for RETs from the collapse of a long-lived remnant of a helium white dwarf merging with an oxygen–neon white dwarf, though these models were fainter than our LFCs.

Finally, mergers involving a main-sequence or evolved star with a stellar-mass BH have also been suggested as progenitors of Cow-like events. Given the older host environments in LFCs, mergers with Wolf–Rayet stars (Metzger 2022) are probably disfavored in this case. A merger of a BH with a lower-mass He core is more consistent with a long delay time. In this case, the accretion rate is expected to be much lower, though additional luminosity could be produced by interaction of disk winds with stellar material lost earlier in the merger (Metzger & Pejcha 2017).

Kremer et al. (2021, 2023) presented models for tidal disruptions of main-sequence stars by stellar-mass BHs in dense clusters. Some of their wind-reprocessed models exhibit optical rise times and postpeak temperatures similar to LFCs. However, while the bolometric luminosities can reach ∼10⁴⁴ erg s⁻¹, most of this energy is emitted in the UV and X-ray regime, and none of the models reach the peak optical magnitudes of our objects.

Noting the early similarity of AT 2022aedm to some TDE spectra, we encourage a broader exploration of the parameter space for stellar-mass TDEs. Another important test of TDE models (stellar or intermediate mass) will be identifying globular clusters at the positions of LFCs. The James Webb Space Telescope (JWST) can reach the peak of the globular cluster luminosity function (≈−7.5 mag; Harris & Racine 1979) in 10 ks with NIRCam for an event within 200 Mpc. At the distance of AT 2022aedm, it is possible to achieve the same constraint in ≈100 ks. We note, however, that the angular extent of a typical globular cluster at this distance is comparable to the NIRCam pixel scale, making identification challenging.