Supersolar Ni/Fe production in the Type IIP SN 2012ec

Abstract

SN 2012ec is a Type IIP supernova (SN) with a progenitor detection and comprehensive photospheric phase observational coverage. Here, we present Very Large Telescope and Public ESO Spectroscopic Survey of Transient Objects observations of this SN in the nebular phase. We model the nebular [O i] λλ6300, 6364 lines and find their strength to suggest a progenitor main-sequence mass of 13–15 M⊙. SN 2012ec is unique among hydrogen-rich SNe in showing a distinct line of stable nickel [Ni ii] λ7378. This line is produced by 58Ni, a nuclear burning ash whose abundance is a sensitive tracer of explosive burning conditions. Using spectral synthesis modelling, we use the relative strengths of [Ni ii] λ7378 and [Fe ii] λ7155 (the progenitor of which is 56Ni) to derive a Ni/Fe production ratio of 0.20 ± 0.07 (by mass), which is a factor 3.4 ± 1.2 times the solar value. High production of stable nickel is confirmed by a strong [Ni ii] 1.939 μm line. This is the third reported case of a core-collapse SN producing a Ni/Fe ratio far above the solar value, which has implications for core-collapse explosion theory and galactic chemical evolution models.