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

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 M_B = -17.75 ± 0.39 mag and a maximum pseudo-bolometric luminosity of L_max = 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 M_Ni ~ 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 M_ejc ~ 5.5 ± 2.0 M_⊙ and a kinetic energy of E_K ~ 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....