2018MNRAS.478.4162P

The optical observations of Ic-4 supernova (SN) 2016coi/ASASSN-16fp, from ∼2 to ∼450 d after explosion, are presented along with analysis of its physical properties. The SN shows the broad lines associated with SNe Ic-3/4 but with a key difference. The early spectra display a strong absorption feature at ∼5400 Å which is not seen in other SNe Ic-3/4 at this epoch. This feature has been attributed to He I in the literature. Spectral modelling of the SN in the early photospheric phase suggests the presence of residual He in a C/O dominated shell. However, the behaviour of the He I lines is unusual when compared with He-rich SNe, showing relatively low velocities and weakening rather than strengthening over time. The SN is found to rise to peak ∼16 d after core-collapse reaching a bolometric luminosity of L_p∼3 x 10⁴² erg s^–1. Spectral models, including the nebular epoch, show that the SN ejected 2.5-4 M_☉ of material, with ∼1.5 M_☉ below 5000 km s^–1, and with a kinetic energy of (4.5-7) x 10⁵¹ erg. The explosion synthesized ∼0.14 M_☉ of ⁵⁶Ni. There are significant uncertainties in E(B - V)_host and the distance, however, which will affect L_p and M_Ni. SN 2016coi exploded in a host similar to the Large Magellanic Cloud (LMC) and away from star-forming regions. The properties of the SN and the host-galaxy suggest that the progenitor had M_ZAMS of 23-28 M_☉ and was stripped almost entirely down to its C/O core at explosion.