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1. Forbidden hugs in pandemic times: II. The luminous red nova variety: AT 2020hat and AT 2020kog

   https://doi.org/10.1051/0004-6361/202039953  

   Pastorello, A.; Valerin, G.; Fraser, M.; Elias-Rosa, N.; Valenti, S.; Reguitti, A.; Mazzali, P. A.; Amaro, R. C.; Andrews, J. E.; Dong, Y.; et al (March 2021, Astronomy & Astrophysics)

   null (Ed.)

   We present the results of our monitoring campaigns of the luminous red novae (LRNe) AT 2020hat in NGC 5068 and AT 2020kog in NGC 6106. The two objects were imaged (and detected) before their discovery by routine survey operations. They show a general trend of slow luminosity rise, lasting at least a few months. The subsequent major LRN outbursts were extensively followed in photometry and spectroscopy. The light curves present an initial short-duration peak, followed by a redder plateau phase. AT 2020kog is a moderately luminous event peaking at ∼7 × 10 40 erg s −1 , while AT 2020hat is almost one order of magnitude fainter than AT 2020kog, although it is still more luminous than V838 Mon. In analogy with other LRNe, the spectra of AT 2020kog change significantly with time. They resemble those of type IIn supernovae at early phases, then they become similar to those of K-type stars during the plateau, and to M-type stars at very late phases. In contrast, AT 2020hat already shows a redder continuum at early epochs, and its spectrum shows the late appearance of molecular bands. A moderate-resolution spectrum of AT 2020hat taken at +37 d after maximum shows a forest of narrow P Cygni lines of metals with velocities of 180 km s −1 , along with an H α emission with a full-width at half-maximum velocity of 250 km s −1 . For AT 2020hat, a robust constraint on its quiescent progenitor is provided by archival images of the Hubble Space Telescope. The progenitor is clearly detected as a mid-K type star, with an absolute magnitude of M F 606 W  = −3.33 ± 0.09 mag and a colour of F 606 W  −  F 814 W  = 1.14 ± 0.05 mag, which are inconsistent with the expectations from a massive star that could later produce a core-collapse supernova. Although quite peculiar, the two objects nicely match the progenitor versus light curve absolute magnitude correlations discussed in the literature. 

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2. The Carnegie Supernova Project II: Observations of the intermediate-luminosity red transient SNhunt120

   https://doi.org/10.1051/0004-6361/202038018  

   Stritzinger, M. D.; Taddia, F.; Fraser, M.; Tauris, T. M.; Suntzeff, N. B.; Contreras, C.; Drybye, S.; Galbany, L.; Holmbo, S.; Morrell, N.; et al (July 2020, Astronomy & Astrophysics)

   null (Ed.)

   We present multiwavelength observations of two gap transients that were followed by the Carnegie Supernova Project-II. The observations are supplemented with data obtained by a number of different programs. Here in the first of two papers, we focus on the intermediate-luminosity red transient (ILRT) designated SNhunt120, while in a companion paper we examine the luminous red novae AT 2014ej. Our data set for SNhunt120 consists of an early optical discovery, estimated to be within three days after outburst, the subsequent optical and near-infrared broadband followup extending over a period of about two months, two visual and two near-infrared wavelength spectra, and Spitzer Space Telescope observations extending from early (+28 d) to late (+1155 d) phases. SNhunt120 resembles other ILRTs such as NGC 300-2008-OT and SN 2008S, and like these other ILRTs, SNhunt120 exhibits prevalent mid-infrared emission at both early and late phases. From the comparison of SNhunt120 and other ILRTs to electron-capture supernova simulations, we find that the current models underestimate the explosion kinetic energy and thereby produce synthetic light curves that overestimate the luminosity. Finally, examination of pre-outburst Hubble Space Telescope images yields no progenitor detection. 

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3. Light-curve Model for Luminous Red Novae and Inferences about the Ejecta of Stellar Mergers

   https://doi.org/10.3847/1538-4357/ac6269  

   Matsumoto, Tatsuya; Metzger, Brian D. (October 2022, The Astrophysical Journal)

   Abstract The process of unstable mass transfer in a stellar binary can result in either a complete merger of the stars or successful removal of the donor envelope leaving a surviving more compact binary. Luminous red novae (LRNe) are the class of optical transients believed to accompany such merger/common envelope events. Past works typically model LRNe using analytic formulae for supernova light curves that make assumptions (e.g., radiation-dominated ejecta, neglect of hydrogen recombination energy) not justified in stellar mergers due to the lower velocities and specific thermal energy of the ejecta. We present a one-dimensional model of LRN light curves that accounts for these effects. Consistent with observations, we find that LRNe typically possess two light-curve peaks, an early phase powered by initial thermal energy of the hot, fastest ejecta layers and a later peak powered by hydrogen recombination from the bulk of the ejecta. We apply our model to a sample of LRNe to infer their ejecta properties (mass, velocity, and launching radius) and compare them to the progenitor donor star properties from pretransient imaging. We define the maximum luminosity achievable for a given donor star in the limit that the entire envelope is ejected, finding that several LRNe violate this limit. Shock interaction between the ejecta and predynamical mass loss may provide an additional luminosity source to alleviate this tension. Our model can also be applied to the merger of planets with stars or stars with compact objects. 

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4. SN 2024afav: A Superluminous Supernova with Multiple Light-curve Bumps and Spectroscopic Signatures of Circumstellar Interaction

   https://doi.org/10.3847/2041-8213/ae3749  

   Kumar, Harsh; Blanchard, Peter K; Berger, Edo; Athukoralalage, Wasundara; Hiramatsu, Daichi; Gomez, Sebastian; Andrews, Moira; Bostroem, K Azalee; Farah, Joseph R; Howell, D Andrew; et al (February 2026, The Astrophysical Journal Letters)

   Abstract We present a comprehensive optical and near-infrared (NIR) spectroscopic study of SN 2024afav, a hydrogen-poor superluminous supernova (SLSN-I) that peaks at ≈−20.7 mag and exhibits an unusual multibumped light curve. Our spectroscopic observations, spanning phases of −14 to +160 days, reveal several unusual features: (i) a narrow (1800 km s⁻¹) and blueshifted (11,000 km s⁻¹) absorption from Hαstarting at +20 days; (ii) persistent optical and NIR Heilines at all available phases, showing double absorption structure in NIR spectra at +23 days, with a high-velocity component at a similar velocity to Hα; (iii) early appearance of nebular [Oiii] emission starting at ≈+50 days; and (iv) a strong [Oii] + [Caii] 7300 Å emission complex starting at ≈+110 days. These unusual features, and their onset at the time of the light-curve bumps, provide compelling evidence of circumstellar interaction between the SN ejecta and a nearby hydrogen-rich shell, as well as the presence of helium in both the outer layers of the progenitor star and the circumstellar medium. A comparison of SN 2024afav to other SLSNe-I showing bumpy light curves and similar spectral properties (PTF 10hgi, SN 2017egm, SN 2019hge) points to a rare subgroup of SLSNe-I in which circumstellar medium interaction provides an important modulation to the energy input. 

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5. A Luminous Red Supergiant and Dusty Long-period Variable Progenitor for SN 2023ixf

   https://doi.org/10.3847/2041-8213/ace618  

   Jencson, Jacob E.; Pearson, Jeniveve; Beasor, Emma R.; Lau, Ryan M.; Andrews, Jennifer E.; Bostroem, K. Azalee; Dong, Yize; Engesser, Michael; Gomez, Sebastian; Guolo, Muryel; et al (July 2023, The Astrophysical Journal Letters)

   Abstract We analyze pre-explosion near- and mid-infrared (IR) imaging of the site of SN 2023ixf in the nearby spiral galaxy M101 and characterize the candidate progenitor star. The star displays compelling evidence of variability with a possible period of ≈1000 days and an amplitude of Δ m ≈ 0.6 mag in extensive monitoring with the Spitzer Space Telescope since 2004, likely indicative of radial pulsations. Variability consistent with this period is also seen in the near-IR J and K s bands between 2010 and 2023, up to just 10 days before the explosion. Beyond the periodic variability, we do not find evidence for any IR-bright pre-supernova outbursts in this time period. The IR brightness ( M K s = − 10.7 mag) and color ( J − K s = 1.6 mag) of the star suggest a luminous and dusty red supergiant. Modeling of the phase-averaged spectral energy distribution (SED) yields constraints on the stellar temperature ( T eff = 3500 − 1400 + 800 K) and luminosity ( log L / L ⊙ = 5.1 ± 0.2 ). This places the candidate among the most luminous Type II supernova progenitors with direct imaging constraints, with the caveat that many of these rely only on optical measurements. Comparison with stellar evolution models gives an initial mass of M init = 17 ± 4 M ⊙ . We estimate the pre-supernova mass-loss rate of the star between 3 and 19 yr before explosion from the SED modeling at M ̇ ≈ 3 × 10 − 5 to 3 × 10 −4 M ⊙ yr −1 for an assumed wind velocity of v w = 10 km s −1 , perhaps pointing to enhanced mass loss in a pulsation-driven wind. 

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