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

ABSTRACT 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 SN2016aqf, an LL SN II with extensive spectral and photometric coverage. We measure a V-band peak magnitude of −14.58mag, a plateau duration of ∼100d, and an inferred 56Ni mass of 0.008 ± 0.002M. The peak bolometric luminosity, Lbol ≈ 1041.4 ergs−1, and its spectral evolution are typical of other SNe in the class. Using our late-time spectra, we measure the [OI] λλ6300, 6364 lines, which we compare against SN II spectral synthesismodelstoconstraintheprogenitorzero-agemain-sequencemass.Wefindthistobe12±3M.Ourextensivelate-time spectral coverage of the [FeII] λ7155 and [NiII] λ7378 lines permits a measurement of the Ni/Fe abundance ratio, a parameter sensitivetotheinnerprogenitorstructureandexplosionmechanismdynamics.Wemeasureaconstantabundanceratioevolution of 0.081+0.009 −0.010 and argue that the best epochs to measure the ratio are at∼200–300d 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. Key words: techniques: photometric–spectroscopic telescopes–supernovae: individual: SN2016aqf–transients: supernovae.