Observational signature of circumstellar interaction and ${}^{56}$Ni-mixing in the Type II Supernova 2016gfy

Abstract

The optical and ultra-violet broadband photometric and spectroscopic observations of the Type II supernova (SN) 2016gfy are presented. The $V$-band light curve (LC) shows a distinct plateau phase with a slope, $s_2$ $\sim$ 0.12 mag / 100 d and a duration of 90 $\pm$ 5 d. Detailed analysis of SN 2016gfy provided a mean ${}^{56}\mathrm{Ni}$ mass of 0.033 $\pm$ 0.003 ${\mathrm{M}}_{\odot }$, a progenitor radius of $\sim$ 350-700 ${\mathrm{R}}_{\odot }$, a progenitor mass of $\sim$ 12-15 ${\mathrm{M}}_{\odot }$ and an explosion energy of 0.9-1.4$\times 10^{51}\mathrm{erg}{\mathrm{s}}^{-1}$. The P-Cygni profile of H$\hat{\mathrm{I}}\pm$ in the early phase spectra ($\sim$ 11-21 d) shows a boxy emission. Assuming that this profile arises from the interaction of the SN ejecta with the pre-existing circumstellar material (CSM), it is inferred that the progenitor underwent a recent episode (30-80 years prior to the explosion) of enhanced mass loss. Numerical modeling suggests that the early LC peak is reproduced better with an existing CSM of 0.15 ${\mathrm{M}}_{\odot }$ spread out to $\sim$ 70 AU. A late-plateau bump is seen in the $VRI$ LCs during $\sim$ 50-95 d. This bump is explained as a result of the CSM interaction and/or partial mixing of radioactive ${}^{56}\mathrm{Ni}$ in the SN ejecta. Using strong-line diagnostics, a sub-solar oxygen abundance is estimated for the supernova H II region (12 + log(O/H) = 8.50 $\pm$ 0.11), indicating an average metallicity for the host of a Type II SN. A star formation rate of $\sim$ 8.5 ${\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ is estimated for NGC 2276 using the archival $GALEX$ FUV data.

30 pages, 25 figures, Accepted for publication in ApJ