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1. SN 2021foa: the bridge between SN IIn and Ibn

   https://doi.org/10.1093/mnras/staf187  

   Gangopadhyay, Anjasha; Dukiya, Naveen; Moriya, Takashi_J; Tanaka, Masaomi; Maeda, Keiichi; Howell, D_Andrew; Singh, Mridweeka; Singh, Avinash; Sollerman, Jesper; Kawabata, Koji_S; et al (January 2025, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT We present the long-term photometric and spectroscopic analysis of a transitioning SN IIn/Ibn from –10.8 d to 150.7 d post V-band maximum. SN 2021foa shows prominent He i lines comparable in strength to the H $$\alpha$$ line around peak, placing SN 2021foa between the SN IIn and SN Ibn populations. The spectral comparison shows that it resembles the SN IIn population at pre-maximum, becomes intermediate between SNe IIn/Ibn, and at post-maximum matches with SN IIn 1996al. The photometric evolution shows a precursor at –50 d and a light curve shoulder around 17 d. The peak luminosity and colour evolution of SN 2021foa are consistent with most SNe IIn and Ibn in our comparison sample. SN 2021foa shows the unique case of an SN IIn where the narrow P-Cygni in H $$\alpha$$ becomes prominent at 7.2 d. The H $$\alpha$$ profile consists of a narrow (500–1200 km s$$^{-1}$$) component, intermediate width (3000–8000 km s$$^{-1}$$) and broad component in absorption. Temporal evolution of the H $$\alpha$$ profile favours a disc-like CSM geometry. Hydrodynamical modelling of the light curve well reproduces a two-component CSM structure with different densities ($$\rho \propto$$ r$$^{-2}$$–$$\rho \propto$$ r$$^{-5}$$), mass-loss rates (10$$^{-3}$$–10$$^{-1}$$ M$$_{\odot }$$ yr$$^{-1}$$) assuming a wind velocity of 1000 km s$$^{-1}$$ and having a CSM mass of 0.18 M$$_{\odot }$$. The overall evolution indicates that SN 2021foa most likely originated from an LBV star transitioning to a WR star with the mass-loss rate increasing in the period from 5 to 0.5 yr before the explosion or it could be due to a binary interaction. 

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2. SN 2019zrk, a bright SN 2009ip analog with a precursor

   https://doi.org/10.1051/0004-6361/202243452  

   Fransson, Claes; Sollerman, Jesper; Strotjohann, Nora L.; Yang, Sheng; Schulze, Steve; Barbarino, Cristina; Kool, Erik C.; Ofek, Eran O.; Crellin-Quick, Arien; De, Kishalay; et al (October 2022, Astronomy & Astrophysics)

   We present photometric and spectroscopic observations of the Type IIn supernova SN 2019zrk (also known as ZTF 20aacbyec). The SN shows a > 100 day precursor, with a slow rise, followed by a rapid rise to M  ≈ −19.2 in the r and g bands. The post-peak light-curve decline is well fit with an exponential decay with a timescale of ∼39 days, but it shows prominent undulations, with an amplitude of ∼1 mag. Both the light curve and spectra are dominated by an interaction with a dense circumstellar medium (CSM), probably from previous mass ejections. The spectra evolve from a scattering-dominated Type IIn spectrum to a spectrum with strong P-Cygni absorptions. The expansion velocity is high, ∼16 000 km s −1 , even in the last spectra. The last spectrum ∼110 days after the main eruption reveals no evidence for advanced nucleosynthesis. From analysis of the spectra and light curves, we estimate the mass-loss rate to be ∼4 × 10 −2   M ⊙ yr −1 for a CSM velocity of 100 km s −1 , and a CSM mass of 1  M ⊙ . We find strong similarities for both the precursor, general light curve, and spectral evolution with SN 2009ip and similar SNe, although SN 2019zrk displays a brighter peak magnitude. Different scenarios for the nature of the 09ip-class of SNe, based on pulsational pair instability eruptions, wave heating, and mergers, are discussed. 

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3. Isoscaling in central Sn+Sn collisions at 270 MeV/u

   https://doi.org/10.1140/epja/s10050-022-00851-2  

   Lee, J. W.; Tsang, M. B.; Tsang, C. Y.; Wang, R.; Barney, J.; Estee, J.; Isobe, T.; Kaneko, M.; Kurata-Nishimura, M.; Lynch, W. G.; et al (October 2022, The European Physical Journal A)
4. SN 2019yvq Does Not Conform to SN Ia Explosion Models

   https://doi.org/10.3847/1538-4357/abf93b  

   Tucker, M. A.; Ashall, C.; Shappee, B. J.; Vallely, P. J.; Kochanek, C. S.; Huber, M. E.; Anand, G. S.; Keane, J. V.; Hsiao, E. Y.; Holoien, T. W.-S. (June 2021, The Astrophysical Journal)

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5. SN 2022jox: An Extraordinarily Ordinary Type II SN with Flash Spectroscopy

   https://doi.org/10.3847/1538-4357/ad2a49  

   Andrews, Jennifer E.; Pearson, Jeniveve; Hosseinzadeh, Griffin; Bostroem, K. Azalee; Dong 董, Yize 一泽; Shrestha, Manisha; Jencson, Jacob E.; Sand, David J.; Valenti, S.; Hoang, Emily; et al (April 2024, The Astrophysical Journal)

   Abstract We present high-cadence optical and ultraviolet (UV) observations of the Type II supernova (SN), SN 2022jox which exhibits early spectroscopic high-ionization flash features of Hi, Heii, Civ, and Nivthat disappear within the first few days after explosion. SN 2022jox was discovered by the Distance Less Than 40 Mpc survey ∼0.75 day after explosion with follow-up spectra and UV photometry obtained within minutes of discovery. The SN reached a peak brightness ofM_V∼ −17.3 mag, and has an estimated⁵⁶Ni mass of 0.04M_⊙, typical values for normal Type II SNe. The modeling of the early light curve and the strong flash signatures present in the optical spectra indicate interaction with circumstellar material (CSM) created from a progenitor with a mass-loss rate of $\dot{M}\sim {10}^{-3}\text{–}{10}^{-2}{M}_{\odot }{\mathrm{y}\mathrm{r}}^{-1}$. There may also be some indication of late-time CSM interaction in the form of an emission line blueward of Hαseen in spectra around 200 days. The mass-loss rate of SN 2022jox is much higher than the values typically associated with quiescent mass loss from red supergiants, the known progenitors of Type II SNe, but is comparable to inferred values from similar core-collapse SNe with flash features, suggesting an eruptive event or a superwind in the progenitor in the months or years before explosion. 

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