SN 2016coi/ASASSN-16fp: an example of residual helium in a typeIc supernova?

Abstract

The optical observations of Ic-4 supernova (SN) 2016coi/ASASSN-16fp, from\n$\\sim 2$ to $\\sim450$ days after explosion, are presented along with analysis\nof its physical properties. The SN shows the broad lines associated with SNe\nIc-3/4 but with a key difference. The early spectra display a strong absorption\nfeature at $\\sim 5400$ \\AA\\ which is not seen in other SNe~Ic-3/4 at this\nepoch. This feature has been attributed to He I in the literature. Spectral\nmodelling of the SN in the early photospheric phase suggests the presence of\nresidual He in a C/O dominated shell. However, the behaviour of the He I lines\nare unusual when compared with He-rich SNe, showing relatively low velocities\nand weakening rather than strengthening over time. The SN is found to rise to\npeak $\\sim 16$ d after core-collapse reaching a bolometric luminosity of Lp\n$\\sim 3\\times10^{42}$ \\ergs. Spectral models, including the nebular epoch, show\nthat the SN ejected $2.5-4$ \\msun\\ of material, with $\\sim 1.5$ \\msun\\ below\n5000 \\kms, and with a kinetic energy of $(4.5-7)\\times10^{51}$ erg. The\nexplosion synthesised $\\sim 0.14$ \\msun\\ of 56Ni. There are significant\nuncertainties in E(B-V)host and the distance however, which will affect Lp and\nMNi. SN 2016coi exploded in a host similar to the Large Magellanic Cloud (LMC)\nand away from star-forming regions. The properties of the SN and the\nhost-galaxy suggest that the progenitor had $M_\\mathrm{ZAMS}$ of $23-28$ \\msun\\\nand was stripped almost entirely down to its C/O core at explosion.\n