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

Abstract

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 s2 ∼ 0.12 mag (100 day) −1 and a duration of 90 ± 5 days. Detailed analysis of SN 2016gfy provided a mean 56Ni mass of 0.033 ± 0.003 Me, a progenitor radius of ∼350–700 Re, a progenitor mass of ∼12–15 Me, and an explosion energy of (0.9–1.4) × 1051 erg s−1 . The P-Cygni profile of Hα in the early-phase spectra (∼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 Me spread out to ∼70 au. A late-plateau bump is seen in the VRI LCs during ∼50–95 days. This bump is explained as a result of the CSM interaction and/or partial mixing of radioactive 56Ni 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 ∼8.5 Me yr−1 is estimated for NGC 2276 using the archival GALEX FUV data.