The low-luminosity type II SN 2016aqf: A well-monitored spectral evolution of the Ni/Fe abundance ratio

Low-luminosity Type II supernovae (LL SNe II) make up the low explosion energy end of core-collapse SNe, but their study and physical understanding remain limited. We present SN 2016aqf, an LL SN II with extensive spectral and photometric coverage. We measure a V-band peak magnitude of −14.58 mag, a plateau duration of ∼100 d, and an inferred ⁵⁶Ni mass of 0.008 ± 0.002 M☉. The peak bolometric luminosity, L_bol ≈ 10^41.4 erg s⁻¹, and its spectral evolution are typical of other SNe in the class. Using our late-time spectra, we measure the [O I] λλ6300, 6364 lines, which we compare against SN II spectral synthesis models to constrain the progenitor zero-age main-sequence mass. We find this to be 12 ± 3 M☉. Our extensive late-time spectral coverage of the [Fe II] λ7155 and [Ni II] λ7378 lines permits a measurement of the Ni/Fe abundance ratio, a parameter sensitive to the inner progenitor structure and explosion mechanism dynamics. We measure a constant abundance ratio evolution of 0.081⁺₋⁰₀⁰⁰⁹₀₁₀ and argue that the best epochs to measure the ratio are at ∼200-300 d after explosion. We place this measurement in the context of a large sample of SNe II and compare against various physical, light-curve, and spectral parameters, in search of trends that might allow indirect ways of constraining this ratio. We do not find correlations predicted by theoretical models; however, this may be the result of the exact choice of parameters and explosion mechanism in the models, the simplicity of them, and/or primordial contamination in the measured abundance ratio.