2022MNRAS.517.5678T

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 M_Ni = 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$_{-0.3}^{+0.5}$ 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.