SN 2016coi/ASASSN-16fp: An example of residual helium in a type Ic supernova?

Abstract

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 Lp~3 × 1042 erg s-1. Spectral models, including the nebular epoch, show that the SN ejected 2.5-4M⊙ of material, with ~1.5M⊙ below 5000 km s-1, and with a kinetic energy of (4.5-7) × 1051 erg. The explosion synthesized ~0.14M⊙ of 56Ni. There are significant uncertainties in E(B - V)host and the distance, however, which will affect Lp andMNi. 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 MZAMS of 23-28M⊙ and was stripped almost entirely down to its C/O core at explosion.