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1. Low luminosity Type II supernovae – IV. SN 2020cxd and SN 2021aai, at the edges of the sub-luminous supernovae class

   https://doi.org/10.1093/mnras/stac1182  

   Valerin, G; Pumo, M L; Pastorello, A; Reguitti, A; Elias–Rosa, N; Gútierrez, C P; Kankare, E; Fraser, M; Mazzali, P A; Howell, D A; et al (May 2022, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Photometric and spectroscopic data for two Low Luminosity Type IIP Supernovae (LL SNe IIP) 2020cxd and 2021aai are presented. SN 2020cxd was discovered 2 d after explosion at an absolute magnitude of Mr  = −14.02 ± 0.21 mag, subsequently settling on a plateau which lasts for ∼120 d. Through the luminosity of the late light curve tail, we infer a synthesized 56Ni mass of (1.8 ± 0.5) × 10−3 M⊙. During the early evolutionary phases, optical spectra show a blue continuum ($$T\, \gt $$8000 K) with broad Balmer lines displaying a P Cygni profile, while at later phases, Ca ii, Fe ii, Sc ii, and Ba ii lines dominate the spectra. Hydrodynamical modelling of the observables yields $$R\, \simeq$$ 575 R⊙ for the progenitor star, with Mej  = 7.5 M⊙ and $$E\, \simeq$$ 0.097 foe emitted during the explosion. This low-energy event originating from a low-mass progenitor star is compatible with both the explosion of a red supergiant (RSG) star and with an Electron Capture Supernova arising from a super asymptotic giant branch star. SN 2021aai reaches a maximum luminosity of Mr  = −16.57 ± 0.23 mag (correcting for AV = 1.92 mag), at the end of its remarkably long plateau (∼140 d). The estimated 56Ni mass is (1.4 ± 0.5) × 10−2 M⊙. The expansion velocities are compatible with those of other LL SNe IIP (few 103 km s−1). The physical parameters obtained through hydrodynamical modelling are $$R\, \simeq$$ 575 R⊙, Mej = 15.5 M⊙, and E = 0.4 foe. SN 2021aai is therefore interpreted as the explosion of an RSG, with properties that bridge the class of LL SNe IIP with standard SN IIP events. 

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2. SN 2018is: A low-luminosity Type IIP supernova with narrow hydrogen emission lines at early phases

   https://doi.org/10.1051/0004-6361/202452507  

   Dastidar, R; Misra, K; Valenti, S; Sand, D J; Pastorello, A; Reguitti, A; Pignata, G; Benetti, S; Bose, S; Gangopadhyay, A; et al (February 2025, Astronomy & Astrophysics)

   We present a comprehensive photometric and spectroscopic study of the Type IIP supernova (SN) 2018is. TheVband luminosity and the expansion velocity at 50 days post-explosion are −15.1 ± 0.2 mag (corrected for A_V= 1.34 mag) and 1400 km s⁻¹, classifying it as a low-luminosity SN II. The recombination phase in theVband is shorter, lasting around 110 days, and exhibits a steeper decline (1.0 mag per 100 days) compared to most other low-luminosity SNe II. Additionally, the optical and near-infrared spectra display hydrogen emission lines that are strikingly narrow, even for this class. The Fe IIand Sc IIline velocities are at the lower end of the typical range for low-luminosity SNe II. Semi-analytical modelling of the bolometric light curve suggests an ejecta mass of ∼8 M_⊙, corresponding to a pre-supernova mass of ∼9.5 M_⊙, and an explosion energy of ∼0.40 × 10⁵¹erg. Hydrodynamical modelling further indicates that the progenitor had a zero-age main sequence mass of 9 M_⊙, coupled with a low explosion energy of 0.19 × 10⁵¹erg. The nebular spectrum reveals weak [O I]λλ6300,6364 lines, consistent with a moderate-mass progenitor, while features typical of Fe core-collapse events, such as He I, [C I], and Fe I, are indiscernible. However, the redder colours and low ratio of Ni to Fe abundance do not support an electron-capture scenario either. As a low-luminosity SN II with an atypically steep decline during the photospheric phase and remarkably narrow emission lines, SN 2018is contributes to the diversity observed within this population. 

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3. Stable nickel production in type Ia supernovae: A smoking gun for the progenitor mass?

   https://doi.org/10.1051/0004-6361/202142323  

   Blondin, S.; Bravo, E.; Timmes, F. X.; Dessart, L.; Hillier, D. J. (April 2022, Astronomy & Astrophysics)

   Context. At present, there are strong indications that white dwarf (WD) stars with masses well below the Chandrasekhar limit ( M Ch ≈ 1.4 M ⊙ ) contribute a significant fraction of SN Ia progenitors. The relative fraction of stable iron-group elements synthesized in the explosion has been suggested as a possible discriminant between M Ch and sub- M Ch events. In particular, it is thought that the higher-density ejecta of M Ch WDs, which favours the synthesis of stable isotopes of nickel, results in prominent [Ni  II ] lines in late-time spectra (≳150 d past explosion). Aims. We study the explosive nucleosynthesis of stable nickel in SNe Ia resulting from M Ch and sub- M Ch progenitors. We explore the potential for lines of [Ni  II ] in the optical an near-infrared (at 7378 Å and 1.94 μm) in late-time spectra to serve as a diagnostic of the exploding WD mass. Methods. We reviewed stable Ni yields across a large variety of published SN Ia models. Using 1D M Ch delayed-detonation and sub- M Ch detonation models, we studied the synthesis of stable Ni isotopes (in particular, 58 Ni) and investigated the formation of [Ni  II ] lines using non-local thermodynamic equilibrium radiative-transfer simulations with the CMFGEN code. Results. We confirm that stable Ni production is generally more efficient in M Ch explosions at solar metallicity (typically 0.02–0.08 M ⊙ for the 58 Ni isotope), but we note that the 58 Ni yield in sub- M Ch events systematically exceeds 0.01 M ⊙ for WDs that are more massive than one solar mass. We find that the radiative proton-capture reaction 57 Co( p ,  γ ) 58 Ni is the dominant production mode for 58 Ni in both M Ch and sub- M Ch models, while the α -capture reaction on 54 Fe has a negligible impact on the final 58 Ni yield. More importantly, we demonstrate that the lack of [Ni  II ] lines in late-time spectra of sub- M Ch events is not always due to an under-abundance of stable Ni; rather, it results from the higher ionization of Ni in the inner ejecta. Conversely, the strong [Ni  II ] lines predicted in our 1D M Ch models are completely suppressed when 56 Ni is sufficiently mixed with the innermost layers, which are rich in stable iron-group elements. Conclusions. [Ni  II ] lines in late-time SN Ia spectra have a complex dependency on the abundance of stable Ni, which limits their use in distinguishing among M Ch and sub- M Ch progenitors. However, we argue that a low-luminosity SN Ia displaying strong [Ni  II ] lines would most likely result from a Chandrasekhar-mass progenitor. 

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4. Circumstellar Interaction Signatures in the Low-luminosity Type II SN 2021gmj

   https://doi.org/10.3847/1538-4357/ad4d55  

   Meza-Retamal, Nicolás; Dong_董, Yize_一泽; Bostroem, K_Azalee; Valenti, Stefano; Galbany, Lluís; Pearson, Jeniveve; Hosseinzadeh, Griffin; Andrews, Jennifer_E; Sand, David_J; Jencson, Jacob_E; et al (August 2024, The Astrophysical Journal)

   Abstract We present comprehensive optical observations of SN 2021gmj, a Type II supernova (SN II) discovered within a day of explosion by the Distance Less Than 40 Mpc survey. Follow-up observations show that SN 2021gmj is a low-luminosity SN II (LL SN II), with a peak magnitudeM_V= −15.45 and an Feiivelocity of ∼1800 km s⁻¹at 50 days past explosion. Using the expanding photosphere method, we derive a distance of ${17.8}_{-0.4}^{+0.6}$Mpc. From the tail of the light curve we obtain a radioactive nickel mass of $M_{{}^{56}\mathrm{Ni}}$= 0.014 ± 0.001M_⊙. The presence of circumstellar material (CSM) is suggested by the early-time light curve, early spectra, and high-velocity Hαin absorption. Analytical shock-cooling models of the light curve cannot reproduce the fast rise, supporting the idea that the early-time emission is partially powered by the interaction of the SN ejecta and CSM. The inferred low CSM mass of 0.025M_⊙in our hydrodynamic-modeling light-curve analysis is also consistent with our spectroscopy. We observe a broad feature near 4600 Å, which may be high-ionization lines of C, N, or/and Heii. This feature is reproduced by radiation-hydrodynamic simulations of red supergiants with extended atmospheres. Several LL SNe II show similar spectral features, implying that high-density material around the progenitor may be common among them. 

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5. The Diverse Properties of Type Icn Supernovae Point to Multiple Progenitor Channels

   https://doi.org/10.3847/1538-4357/ac8ff6  

   Pellegrino, C.; Howell, D. A.; Terreran, G.; Arcavi, I.; Bostroem, K. A.; Brown, P. J.; Burke, J.; Dong, Y.; Gilkis, A.; Hiramatsu, D.; et al (October 2022, The Astrophysical Journal)

   Abstract We present a sample of Type Icn supernovae (SNe Icn), a newly discovered class of transients characterized by their interaction with H- and He-poor circumstellar material (CSM). This sample is the largest collection of SNe Icn to date and includes observations of two published objects (SN 2019hgp and SN 2021csp) and two objects not yet published in the literature (SN 2019jc and SN 2021ckj). The SNe Icn display a range of peak luminosities, rise times, and decline rates, as well as diverse late-time spectral features. To investigate their explosion and progenitor properties, we fit their bolometric light curves to a semianalytical model consisting of luminosity inputs from circumstellar interaction and radioactive decay of⁵⁶Ni. We infer low ejecta masses (≲2M_⊙) and⁵⁶Ni masses (≲0.04M_⊙) from the light curves, suggesting that normal stripped-envelope supernova (SESN) explosions within a dense CSM cannot be the underlying mechanism powering SNe Icn. Additionally, we find that an estimate of the star formation rate density at the location of SN 2019jc lies at the lower end of a distribution of SESNe, in conflict with a massive star progenitor of this object. Based on its estimated ejecta mass,⁵⁶Ni mass, and explosion site properties, we suggest a low-mass, ultra-stripped star as the progenitor of SN 2019jc. For other SNe Icn, we suggest that a Wolf–Rayet star progenitor may better explain their observed properties. This study demonstrates that multiple progenitor channels may produce SNe Icn and other interaction-powered transients. 

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