How low can you go? SN 2018zd as a low-mass Fe core-collapse supernova

Kavli Affiliate: Subo Dong

| First 5 Authors: E. Callis, M. Fraser, A. Pastorello, Subo Dong, S. J. Brennan

| Summary:

We present spectroscopy and photometry of SN 2018zd, a Type IIP core-collapse
supernova with signatures of interaction with circumstantial material in its
earliest spectra. High ionization lines, the earmark of shock breakout, are not
seen in the earliest spectral epoch, and are only seen in a single spectrum at
4.9 d after explosion. The strength and brevity of these features imply a
confined circumstellar material shell in the immediate vicinity of the
progenitor. Once the narrow emission lines disappear, SN 2018zd evolves
similarly to a Type IIP SN, although the blue colour and enhanced plateau
magnitude of SN 2018zd suggests an additional source of luminosity throughout
the plateau phase. While SN 2018zd has previously been proposed as an
electron-capture SN, we suggest that it is an Fe core-collapse from a low mass
red supergiant progenitor. Differences in interpretation for SN 2018zd arise in
part due to the large uncertainty on the distance to the host-galaxy NGC 2146,
which we re-derive here to be $15.6^{+6.1}_{-3.0}$ Mpc. We find the ejected
$^{56}$Ni mass for SN 2018zd to be 0.017 M$_{odot}$, significantly higher than
models of ECSNe predict. We also find the Ni/Fe ratio in SN 2018zd to be much
lower that would be expected for an ECSN.

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