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

Prentice, S.J. and Ashall, C. and Mazzali, P.A. and Zhang, J-J. and James, P.A. and Vinkó, József and Cseh, Borbála and Kriskovics, Levente and Pál, András and Sárneczky, Krisztián and Sódor, Ádám and Szakáts, Róbert and Székely, Péter and Verebélyi, Erika and Vida, Krisztián (2018) SN 2016coi/ASASSN-16fp: An example of residual helium in a type Ic supernova? MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, 478. pp. 4162-4192. ISSN 0035-8711

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Abstract

The optical observations of Ic-4 supernova (SN) 2016coi/ASASSN-16fp, from ˜2 to ˜450 days 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 Åwhich is not seen in other SNe Ic-3/4 at this epoch. This feature has been attributed to HeIin 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 HeIlines 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<SUB>p</SUB> ˜ 3 × 10<SUP>42</SUP> erg s<SUP>-1</SUP>. Spectral models, including the nebular epoch, show that the SN ejected 2.5 - 4 M<SUB>⊙</SUB>of material, with ˜1.5 M<SUB>⊙</SUB>below 5000 km s<SUP>-1</SUP>, and with a kinetic energy of (4.5 - 7) × 10<SUP>51</SUP> erg. The explosion synthesised ˜0.14 M<SUB>⊙</SUB>of 56<SUB>Ni</SUB>. There are significant uncertainties in E(B - V)<SUB>host</SUB> and the distance however, which will affect L<SUB>p</SUB> and M<SUB>Ni</SUB>. 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<SUB>ZAMS</SUB> of 23 - 28 M<SUB>⊙</SUB>and was stripped almost entirely down to its C/O core at explosion.

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