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 similar to(7 +/- 2) M-circle dot with a final explosion energy of similar to(6 +/- 1) x 10(51) erg with M-Ni = 0.27 +/- 0.05 M-circle dot. The early spectra are hot and blue with weak He I lines, and a complicated H alpha region suggested to be a multicomponent feature. Modelling the spectra required similar to 0.08 M-circle dot of H at velocities >11 000 km s(-1) and a total He mass of similar to 1.0 M-circle dot 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-circle dot(-1). The high luminosity and explosion energy results in a broadened H alpha 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-circle dot, 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 Ni-56 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.

Automated summary

Using a combined spectral and light-curve modelling approach, a massive and energetic explosion model was fitted to the luminous Type Ib/IIb SN 2020cpg. The model suggested an ejected mass of around 7 solar masses, with a final explosion energy of approximately 6 x 10^51 erg and M-Ni of 0.27 solar masses. The early spectra showed hot and blue characteristics with weak He I lines, and a complex H alpha region. The modelling of the spectra required a significant amount of H and He masses at high velocities.