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1. SN 2020bio: A Double-peaked, H-poor Type IIb Supernova with Evidence of Circumstellar Interaction

   https://doi.org/10.3847/1538-4357/ace595  

   Pellegrino, C. ; Hiramatsu, D. ; Arcavi, I. ; Howell, D. A. ; Bostroem, K. A. ; Brown, P. J. ; Burke, J. ; Elias-Rosa, N. ; Itagaki, K. ; Kaneda, H. ; et al ( August 2023 , The Astrophysical Journal)

   We present photometric and spectroscopic observations of SN 2020bio, a double-peaked Type IIb supernova (SN) discovered within a day of explosion, primarily obtained by Las Cumbres Observatory and Swift. SN 2020bio displays a rapid and long-lasting initial decline throughout the first week of its light curve, similarly to other well-studied Type IIb SNe. This early-time emission is thought to originate from the cooling of the extended outer hydrogen-rich (H-rich) envelope of the progenitor star that is shock heated by the SN explosion. We compare SN 2020bio to a sample of other double-peaked Type IIb SNe in order to investigate its progenitor properties. Analytical model fits to the early-time emission give progenitor radius (≈100–1500R_⊙) and H-rich envelope mass (≈0.01–0.5M_⊙) estimates that are consistent with other Type IIb SNe. However, SN 2020bio displays several peculiarities, including (1) weak H spectral features indicating a greater amount of mass loss than other Type IIb progenitors; (2) an underluminous secondary light-curve peak that implies a small amount of synthesized⁵⁶Ni (M_Ni≈0.02M_⊙); and (3) low-luminosity nebular [Oi] and interaction-powered nebular features. These observations are more consistent with a lower-mass progenitor (M_ZAMS≈ 12M_⊙) that was stripped of most of its H-rich envelope before exploding. This study adds to the growing diversity in the observed properties of Type IIb SNe and their progenitors.

    

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2. 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)

   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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3. SN 2020cpg: an energetic link between Type IIb and Ib supernovae

   https://doi.org/10.1093/mnras/stab1761  

   Medler, K ; Mazzali, P A ; Teffs, J ; Prentice, S J ; Ashall, C ; Amenouche, M ; Anderson, J P ; Burke, J ; Chen, T W ; Galbany, L ; et al ( July 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT Stripped-envelope supernovae (SE-SNe) show a wide variety of photometric and spectroscopic properties. This is due to the different potential formation channels and the stripping mechanism that allows for a large diversity within the progenitors outer envelope compositions. Here, the photometric and spectroscopic observations of SN 2020cpg covering ∼130 d from the explosion date are presented. SN 2020cpg (z = 0.037) is a bright SE-SNe with the B-band peaking at MB = −17.75 ± 0.39 mag and a maximum pseudo-bolometric luminosity of Lmax = 6.03 ± 0.01 × 1042 erg s−1. Spectroscopically, SN 2020cpg displays a weak high- and low-velocity H α feature during the photospheric phase of its evolution, suggesting that it contained a detached hydrogen envelope prior to explosion. From comparisons with spectral models, the mass of hydrogen within the outer envelope was constrained to be ∼0.1 M⊙. From the pseudo-bolometric light curve of SN 2020cpg a 56Ni mass of MNi ∼ 0.27 ± 0.08 M⊙ was determined using an Arnett-like model. The ejecta mass and kinetic energy of SN 2020cpg were determined using an alternative method that compares the light curve of SN 2020cpg and several modelled SE-SNe, resulting in an ejecta mass of Mejc ∼ 5.5 ± 2.0 M⊙ and a kinetic energy of EK ∼ 9.0 ± 3.0 × 1051 erg. The ejected mass indicates a progenitor mass of 18−25 M⊙. The use of the comparative light curve method provides an alternative process to the commonly used Arnett-like model to determine the physical properties of SE-SNe. 

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4. Supernova 2020wnt: An Atypical Superluminous Supernova with a Hidden Central Engine

   https://doi.org/10.3847/1538-4357/acc6c3  

   Tinyanont, Samaporn ; Woosley, Stan E. ; Taggart, Kirsty ; Foley, Ryan J. ; Yan, Lin ; Lunnan, Ragnhild ; Davis, Kyle W. ; Kilpatrick, Charles D. ; Siebert, Matthew R. ; Schulze, Steve ; et al ( June 2023 , The Astrophysical Journal)

   We present observations of a peculiar hydrogen- and helium-poor stripped-envelope (SE) supernova (SN) 2020wnt, primarily in the optical and near-infrared (near-IR). Its peak absolute bolometric magnitude of −20.9 mag (L_{bol, peak}= (6.8 ± 0.3) × 10⁴³erg s⁻¹) and a rise time of 69 days are reminiscent of hydrogen-poor superluminous SNe (SLSNe I), luminous transients potentially powered by spinning-down magnetars. Before the main peak, there is a brief peak lasting <10 days post explosion, likely caused by interaction with circumstellar medium (CSM) ejected ∼years before the SN explosion. The optical spectra near peak lack a hot continuum and Oiiabsorptions, which are signs of heating from a central engine; they quantitatively resemble those of radioactivity-powered hydrogen/helium-poor Type Ic SESNe. At ∼1 yr after peak, nebular spectra reveal a blue pseudo-continuum and narrow Oirecombination lines associated with magnetar heating. Radio observations rule out strong CSM interactions as the dominant energy source at +266 days post peak. Near-IR observations at +200–300 days reveal carbon monoxide and dust formation, which causes a dramatic optical light-curve dip. Pair-instability explosion models predict slow light curve and spectral features incompatible with observations. SN 2020wnt is best explained as a magnetar-powered core-collapse explosion of a 28M_⊙pre-SN star. The explosion kinetic energy is significantly larger than the magnetar energy at peak, effectively concealing the magnetar-heated inner ejecta until well after peak. SN 2020wnt falls into a continuum between normal SNe Ic and SLSNe I, and demonstrates that optical spectra at peak alone cannot rule out the presence of a central engine.

    

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5. Extreme Mass Loss in Low-mass Type Ib/c Supernova Progenitors

   https://doi.org/10.3847/2041-8213/ac9b3d  

   Wu, Samantha C. ; Fuller, Jim ( November 2022 , The Astrophysical Journal Letters)

   Many core-collapse supernovae (SNe) with hydrogen-poor and low-mass ejecta, such as ultra-stripped SNe and type Ibn SNe, are observed to interact with dense circumstellar material (CSM). These events likely arise from the core collapse of helium stars that have been heavily stripped by a binary companion and have ejected significant mass during the last weeks to years of their lives. In helium star models run to days before core collapse we identify a range of helium core masses ≈2.5–3M_⊙whose envelopes expand substantially due to the helium shell burning while the core undergoes neon and oxygen burning. When modeled in binary systems, the rapid expansion of these helium stars induces extremely high rates of late-stage mass transfer ($\dot{M}\gtrsim {10}^{-2}{M}_{\odot }{\mathrm{yr}}^{-1}$) beginning weeks to decades before core collapse. We consider two scenarios for producing CSM in these systems: either mass transfer remains stable and mass loss is driven from the system in the vicinity of the accreting companion, or mass transfer becomes unstable and causes a common envelope event (CEE) through which the helium envelope is unbound. The ensuing CSM properties are consistent with the CSM masses (∼10⁻²–1M_⊙) and radii (∼10¹³–10¹⁶cm) inferred for ultra-stripped SNe and several type Ibn SNe. Furthermore, systems that undergo a CEE could produce short-period neutron star binaries that merge in less than 100 Myr.

    

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