Observational Signature of Circumstellar Interaction and 56Ni-mixing in the Type II Supernova 2016gfy

The optical and ultraviolet 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 of s(2) similar to 0.12 mag (100 day)(-1) and a duration of 90 +/- 5 days. Detailed analysis of SN 2016gfy provided a mean Ni-56 mass of 0.033 +/- 0.003 M-circle dot, a progenitor radius of similar to 350-700 R-circle dot, a progenitor mass of similar to 12-15 M-circle dot, and an explosion energy of (0.9-1.4) x 10(51) erg s(-1). The P-Cygni profile of H alpha in the early-phase spectra (similar to 11-21 days) 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 yr 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 M-circle dot spread out to similar to 70 au. A late-plateau bump is seen in the VRI LCs during similar to 50-95 days. This bump is explained as a result of the CSM interaction and/or partial mixing of radioactive Ni-56 in the SN ejecta. Using strong-line diagnostics, a subsolar oxygen abundance is estimated for the supernova H II region (12 + log(O/H) = 8.50 +/- 0.11), indicating an average metallicity for the host of an SN II. A star formation rate of similar to 8.5 M-circle dot yr(-1) is estimated for NGC 2276 using the archival GALEX FUV data.