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1. 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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2. Volumetric Rates of Luminous Red Novae and Intermediate-luminosity Red Transients with the Zwicky Transient Facility

   https://doi.org/10.3847/1538-4357/acc2b9  

   Karambelkar, Viraj_R; Kasliwal, Mansi_M; Blagorodnova, Nadejda; Sollerman, Jesper; Aloisi, Robert; Anand, Shreya_G; Andreoni, Igor; Brink, Thomas_G; Bruch, Rachel; Cook, David; et al (May 2023, The Astrophysical Journal)

   Abstract Luminous red novae (LRNe) are transients characterized by low luminosities and expansion velocities, and they are associated with mergers or common-envelope ejections in stellar binaries. Intermediate-luminosity red transients (ILRTs) are an observationally similar class with unknown origins, but they are generally believed to be either electron-capture supernovae in super-asymptotic giant branch stars or outbursts in dusty luminous blue variables (LBVs). In this paper, we present a systematic sample of eight LRNe and eight ILRTs detected as part of the Census of the Local Universe (CLU) experiment on the Zwicky Transient Facility (ZTF). The CLU experiment spectroscopically classifies ZTF transients associated with nearby (<150 Mpc) galaxies, achieving 80% completeness form_r< 20 mag. Using the ZTF-CLU sample, we derive the first systematic LRNe volumetric rate of ${7.8}_{-3.7}^{+6.5}\times {10}^{-5}$Mpc⁻³yr⁻¹in the luminosity range −16 ≤M_r≤ −11 mag. We find that, in this luminosity range, the LRN rate scales as $\mathit{dN}/\mathit{dL}\propto L^{-2.5\pm 0.3}$—significantly steeper than the previously derived scaling ofL^−1.4±0.3for lower-luminosity LRNe (M_V≥ −10 mag). The steeper power law for LRNe at high luminosities is consistent with the massive merger rates predicted by binary population synthesis models. We find that the rates of the brightest LRNe (M_r≤ −13 mag) are consistent with a significant fraction of them being progenitors of double compact objects that merge within a Hubble time. For ILRTs, we derive a volumetric rate of ${2.6}_{-1.4}^{+1.8}\times {10}^{-6}$Mpc⁻³yr⁻¹forM_r≤ −13.5 mag, which scales as $\mathit{dN}/\mathit{dL}\propto L^{-2.5\pm 0.5}$. This rate is ∼1%–5% of the local core-collapse supernova rate and is consistent with theoretical ECSN rate estimates. 

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3. On the progenitor of the Crab Pulsar

   https://doi.org/10.1093/mnras/staf333  

   Cruz-Cruz, Elvira; Kochanek, C_S (February 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We investigate the progenitor of the Crab supernova by examining the remnant’s surrounding stellar population. The Crab is interesting because of the apparently low energy and mass of the supernova remnant. We also know it was not a binary at death and that the explosion formed a neutron star. Using Gaia EDR3 parallaxes and photometry, we analyse stars inside a cylinder with a projected radius of 100 pc and spanning distances from $$\sim 1600$$ to 2300 pc set by the $$2\sigma$$ uncertainties in the Crab’s parallax. We also individually model the most luminous stars local to the Crab. The two most luminous stars are blue, roughly main sequence stars with masses of $$\sim 11\, {\rm M}_{\odot }$$. We estimate the stellar population’s age distribution using solar metallicity PARSEC isochrones. The estimated age distribution of the 205 $$M_{\mathrm{ G}} < 0$$ stars modestly favour lower mass stars, consistent with an AGB star or a lower mass binary merger as the progenitor, but statistically we cannot rule out higher masses. This may be driven by contamination due to the $$\sim 700$$ pc span of the cylinder in distance. 

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4. Progenitor, precursor, and evolution of the dusty remnant of the stellar merger M31-LRN-2015

   https://doi.org/10.1093/mnras/staa1872  

   Blagorodnova, N; Karambelkar, V; Adams, S M; Kasliwal, M M; Kochanek, C S; Dong, S; Campbell, H; Hodgkin, S; Jencson, J E; Johansson, J; et al (August 2020, Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT M31-LRN-2015 is a likely stellar merger discovered in the Andromeda Galaxy in 2015. We present new optical to mid-infrared photometry and optical spectroscopy for this event. Archival data show that the source started to brighten ∼2 yr before the nova event. During this precursor phase, the source brightened by ∼3 mag. The light curve at 6 and 1.5 months before the main outburst may show periodicity, with periods of 16 ± 0.3 and 28.1 ± 1.4 d, respectively. This complex emission may be explained by runaway mass-loss from the system after the binary undergoes Roche lobe overflow, leading the system to coalesce in tens of orbital periods. While the progenitor spectral energy distribution shows no evidence of pre-existing warm dust in the system, the remnant forms an optically thick dust shell at approximately four months after the outburst peak. The optical depth of the shell increases dramatically after 1.5 yr, suggesting the existence of shocks that enhance the dust formation process. We propose that the merger remnant is likely an inflated giant obscured by a cooling shell of gas with mass ∼0.2 M⊙ ejected at the onset of the common envelope phase. 

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5. Progenitor, environment, and modelling of the interacting transient AT 2016jbu (Gaia16cfr)

   https://doi.org/10.1093/mnras/stac1228  

   Brennan, S. J.; Fraser, M.; Johansson, J.; Pastorello, A.; Kotak, R.; Stevance, H. F.; Chen, T. -W; Eldridge, J. J.; Bose, S.; Brown, P. J.; et al (May 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present the bolometric light curve, identification and analysis of the progenitor candidate, and preliminary modelling of AT 2016jbu (Gaia16cfr). We find a progenitor consistent with a ∼ 22–25 M⊙ yellow hypergiant surrounded by a dusty circumstellar shell, in agreement with what has been previously reported. We see evidence for significant photometric variability in the progenitor, as well as strong Hα emission consistent with pre-existing circumstellar material. The age of the environment, as well as the resolved stellar population surrounding AT 2016jbu, supports a progenitor age of >10 Myr, consistent with a progenitor mass of ∼22 M⊙. A joint analysis of the velocity evolution of AT 2016jbu and the photospheric radius inferred from the bolometric light curve shows the transient is consistent with two successive outbursts/explosions. The first outburst ejected material with velocity ∼650 km s−1, while the second, more energetic event ejected material at ∼4500 km s−1. Whether the latter is the core collapse of the progenitor remains uncertain. We place a limit on the ejected 56Ni mass of <0.016 M⊙. Using the Binary Population And Spectral Synthesis (BPASS) code, we explore a wide range of possible progenitor systems and find that the majority of these are in binaries, some of which are undergoing mass transfer or common-envelope evolution immediately prior to explosion. Finally, we use the SuperNova Explosion Code (SNEC) to demonstrate that the low-energy explosions within some of these binary systems, together with sufficient circumstellar material, can reproduce the overall morphology of the light curve of AT 2016jbu. 

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