Type Ic supernova of a 22 M⊙ progenitor

© 2020 The Author(s). Type Ic supernovae (SNe Ic) are a sub-class of core-collapse SNe that exhibit no helium or hydrogen lines in their spectra. Their progenitors are thought to be bare carbon-oxygen cores formed during the evolution of massive stars that are stripped of their hydrogen and helium envelopes sometime before collapse. SNe Ic present a range of luminosities and spectral properties, from luminous GRB-SNe with broad-lined spectra to less luminous events with narrow-line spectra. Modelling SNe Ic reveals a wide range of both kinetic energies, ejecta masses, and 56Ni masses. To explore this diversity and how it comes about, light curves and spectra are computed from the ejecta following the explosion of an initially 22 M⊙ progenitor that was artificially stripped of its hydrogen and helium shells, producing a bare CO core of ∼5 M⊙, resulting in an ejected mass of ∼4 M⊙, which is an average value for SNe Ic. Four different explosion energies are used that cover a range of observed SNe. Finally, 56Ni and other elements are artificially mixed in the ejecta using two approximations to determine how element distribution affects light curves and spectra. The combination of different explosion energy and degree of mixing produces spectra that roughly replicate the distribution of nearpeak spectroscopic features of SNe Ic. High explosion energies combined with extensive mixing can produce red, broad-lined spectra, while minimal mixing and a lower explosion energy produce bluer, narrow-lined spectra

A massive, energetic model for the luminous transitional Type Ib/IIb SN 2020cpg

Using a combined spectral and light-curve modelling approach, we fit a massive and energetic explosion model to the luminous Type Ib/IIb SN 2020cpg. This model has an ejected mass of ∼(7 ± 2) M⊙ with a final explosion energy of ∼(6 ± 1) × 1051 erg with MNi = 0.27 ± 0.05 M⊙. The early spectra are hot and blue with weak He I lines, and a complicated Hα region suggested to be a multicomponent feature. Modelling the spectra required ∼0.08 M⊙ of H at velocities >11 000 km s−1 and a total He mass of ∼1.0 M⊙ at velocities >9500 km s−1 above CO-rich ejecta. This model has a ratio of kinetic energy and ejected mass of 0.85+0.5−0.3 foe M⊙−1. The high luminosity and explosion energy results in a broadened Hα line that is blended with Si II, C II, and He I, which led to the initial classification of SN 2020cpg as a Type Ib. We instead classify SN 2020cpg as a bright transitional event between the Type Ib and Type IIb classes. Comparing our model parameters to stellar evolution models, a progenitor mass of 25–30 M⊙, i.e. stripped of most of the hydrogen shell and of some of the helium shell prior to collapse produces a He core of comparable mass. The excess 56Ni production in SN 2020cpg as compared to objects of similar ejected mass may suggest evidence of additional energy sources such as a failed GRB or weak magnetar energy injection, or a smaller remnant mass

Flat-topped NIR profiles originating from an unmixed helium shell in the Type IIb SN 2020acat

The Near Infrared (NIR) spectra of the Type IIb supernova (SN IIb) SN 2020acat, obtained at various times throughout the optical follow-up campaign, are presented here. The dominant He i 1.0830 μm and 2.0581 μm features are seen to develop flat-topped P-Cygni profiles as the NIR spectra evolve towards the nebular phase. The nature of the NIR helium peaks imply that there was a lack of mixing between the helium shell and the heavier inner ejecta in SN 2020acat. Analysis of the flat-top features showed that the boundary of the lower velocity of the helium shell was ∼3 − 4 × 103 km s−1. The NIR spectra of SN 2020acat were compared to both SN 2008ax and SN 2011dh to determine the uniqueness of the flat-topped helium features. While SN 2011dh lacked a flat-topped NIR helium profile, SN 2008ax displayed NIR helium features that were very similar to those seen in SN 2020acat, suggesting that the flat-topped feature is not unique to SN 2020acat and may be the product of the progenitors structure

How much H and He is 'hidden' in SNe Ib/c? - II. Intermediate-mass objects: A 22M<inf>⊙</inf>progenitor case study

Stripped envelope supernovae are a sub-class of core-collapse supernovae showing several stages of H/He shell stripping that determines the type: H-free/He-poor SNe are classified as Type Ic, H-poor/He-rich are Type Ib, and H/He-rich are Type IIb. Stripping H/He with only stellar wind requires significantly higher mass-loss rates than observed while binary-involved mass transfer may usually not strip enough to produce H/He free SNe. Type Ib/c SNe are sometimes found to include weak H/He transient lines as a product of a trace amount of H/He left over from stripping processes. The extent and mass of the H/He required to produce these lines is not well known. In this work, a 22 M⊙progenitor model is stripped of the H/He shells in five steps prior to collapse and then exploded at four explosion energies. Requiring both optical and near-infrared He I lines for helium identification does not allow much He mass to be hidden in SE-SNE. Increasing the mass of He above the CO core delays the visibility of OI 7774 in early spectra. Our SN Ib-like models are capable of reproducing the spectral evolution of a set of observed SNe with reasonable estimated Ek accuracy. Our SNIIb-like models can partially reproduce low energy observed SN IIb, but we find no observed comparison for the SN IIb-like models with high Ek

Abundance stratification in Type Ia supernovae – VI. The peculiar slow decliner SN 1999aa

The abundance distribution in the ejecta of the peculiar slowly declining Type Ia supernova (SN Ia) SN 1999aa is obtained by modelling a time series of optical spectra. Similar to SN 1991T, SN 1999aa was characterized by early-time spectra dominated by Fe iii features and a weak Si ii 6355 Å line, but it exhibited a high-velocity Ca ii H&amp;K line and morphed into a spectroscopically normal SN Ia earlier. Three explosion models are investigated, yielding comparable fits. The innermost layers are dominated by ∼0.3 M⊙ of neutron-rich stable iron-group elements, mostly stable iron. Above that central region lies a 56Ni-dominated shell, extending to $v \approx 11\, 000$–$12\, 000$ km s−1, with mass ∼0.65 M⊙. These inner layers are therefore similar to those of normal SNe Ia. However, the outer layers exhibit composition peculiarities similar to those of SN 1991T: The intermediate-mass elements shell is very thin, containing only ∼0.2 M⊙, and is sharply separated from an outer oxygen-dominated shell, which includes ∼0.22 M⊙. These results imply that burning suddenly stopped in SN 1999aa. This is a feature SN 1999aa shares with SN 1991T, and explains the peculiarities of both SNe, which are quite similar in nature apart from the different luminosities. The spectroscopic path from normal to SN 1991T-like SNe Ia cannot be explained solely by a temperature sequence. It also involves composition layering differences, suggesting variations in the progenitor density structure or in the explosion parameters

Transitional events in the spectrophotometric regime between stripped envelope and superluminous supernovae

The division between stripped-envelope supernovae (SE-SNe) and superluminous supernovae (SLSNe) is not well-defined in either photometric or spectroscopic space. While a sharp luminosity threshold has been suggested, there remains an increasing number of transitional objects that reach this threshold without the spectroscopic signatures common to SLSNe. In this work, we present data and analysis on four SNe transitional between SE-SNe and SLSNe; the He-poor SNe 2019dwa and 2019cri, and the He-rich SNe 2019hge and 2019unb. Each object displays long-lived and variable photometric evolution with luminosities around the SLSN threshold of Mr < -19.8 mag. Spectroscopically however, these objects are similar to SE-SNe, with line velocities lower than either SE-SNe and SLSNe, and thus represent an interesting case of rare transitional events.KM, MRM, and SJP are supported by H2020 ERC grant no. 758638. LG acknowledges financial support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 839090, and from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2019 Ramón y Cajal programme RYC2019-027683. TMB was funded by the CONICYT PFCHA / DOCTORADOBECAS CHILE/2017-72180113. MG is supported by the EU Horizon 2020 research and innovation programme under grant agreement no. 101004719. SGG acknowledges support by FCT under Project CRISP PTDC/FIS-AST-31546/2017. MN is supported by a Royal Astronomical Society Research Fellowship and H2020 ERC grant no. 948381. T-WC acknowledges the EU Funding under Marie Skłodowska-Curie grant H2020-MSCA-IF-2018-842471. The LT is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council

Transitional events in the spectrophotometric regime between stripped envelope and superluminous supernovae

KM, MRM, and SJP are supported by H2020 ERC grant no. 758638. LG acknowledges financial support from the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 839090, and from the Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2019 Ramon y Cajal programme RYC2019-027683. TMB ´ was funded by the CONICYT PFCHA / DOCTORADOBECAS CHILE/2017-72180113. MG is supported by the EU Horizon 2020 research and innovation programme under grant agreement no. 101004719. SGG acknowledges support by FCT under Project CRISP PTDC/FIS-AST-31546/2017. MN is supported by a Royal Astronomical Society Research Fellowship and H2020 ERC grant no. 948381. T-WC acknowledges the EU Funding under Marie Skłodowska-Curie grant H2020-MSCA-IF-2018-842471. The LT is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, as part of ePESSTO+ (the advanced Public ESO Spectroscopic Survey for Transient Objects Survey). ePESSTO+ observations were obtained under ESO programme ID 1103.D-0328 (PI: Inserra). The WHT is operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrof´ısica de Canarias. SJP thanks GPL for many insightful discussions at the bar over the last few years.The division between stripped-envelope supernovae (SE-SNe) and superluminous supernovae (SLSNe) is not well-defined in either photometric or spectroscopic space. While a sharp luminosity threshold has been suggested, there remains an increasing number of transitional objects that reach this threshold without the spectroscopic signatures common to SLSNe. In this work, we present data and analysis on four SNe transitional between SE-SNe and SLSNe; the He-poor SNe 2019dwa and 2019cri, and the He-rich SNe 2019hge and 2019unb. Each object displays long-lived and variable photometric evolution with luminosities around the SLSN threshold of Mr < -19.8 mag. Spectroscopically however, these objects are similar to SE-SNe, with line velocities lower than either SE-SNe and SLSNe, and thus represent an interesting case of rare transitional events.H2020 ERC grant no. 758638European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement no. 839090Spanish Ministry of Science, Innovation and Universities (MICIU) under the 2019 Ramon y Cajal programme RYC2019-027683CONICYT PFCHA / DOCTORADOBECAS CHILE/2017-72180113EU Horizon 2020 research and innovation programme under grant agreement no. 101004719FCT under Project CRISP PTDC/FIS-AST-31546/2017Royal Astronomical Society Research FellowshipH2020 ERC grant no. 948381UK Science and Technology Facilities CouncilESO programme ID 1103.D-0328 (PI: Inserra