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

Abstract

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 , 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.

Intermediate-luminosity red transients: Spectrophotometric properties and connection to electron-capture supernova explosions

“…SN 2018zd 26 , which was 26 Hiramatsu et al (2021) estimated a much lower 56 Ni mass (∼8.6 × 10 −3 M ) for SN 2018zd, adopting a very low distance obtained through the standard candle method (SCM). We note however that the SCM distance is discrepant with the significantly larger kinematic estimates (e.g., Zhang et al 2020;Callis et al 2021). also proposed as a possible EC SN, has a significantly higher 56 Ni mass (0.013-0.035 M ; Zhang et al 2020), exceeding that of ILRTs (∼1-5 × 10 −3 M ) by nearly one order of magnitude.…”

“…Subsequent follow-up campaigns allowed us to confirm their classifications as ILRTs. In this work, we also compared ILRTs with other claimed EC SN candidates, such as SN 2018zd (Zhang et al 2018(Zhang et al , 2020Arcavi et al 2018;Hiramatsu et al 2021;Kozyreva et al 2021;Callis et al 2021), SN 2018hwm (Reguitti et al 2021), and SN 2015bf (Lin et al 2021), as well as underluminous SNe IIP (i.e. SN 1999br, SN 2005cs;Pastorello et al 2004Pastorello et al , 2006Pastorello et al , 2009.…”

Low luminosity Type II supernovae -- IV. SN 2020cxd and SN 2021aai, at the edges of the sub-luminous supernovae class

“…We note that a tight relationship between these two quantities was found for SNe IIP, but SN 2008S fits remarkably well in the lower end of the brightness distribution despite being a member of a different class of transients. SN 2018zd appears to belong to the standard IIP events when correcting for the distance reported by Callis et al (2021), while it lies towards the region of transitional objects (like SN 2021aai) when adopting the distance prescribed by Hiramatsu et al (2021).…”

“…3.2) light curves of the three transients mentioned. For SN 2018zd we adopt both distances reported in Hiramatsu et al (2021) and Callis et al (2021). The increase in brightness during the plateau of SN 2020cxd is striking, since it is the only object displaying this behaviour.…”

“…The electron-capture SN scenario, however, can potentially produce transients with different observable properties. The peculiar type II SN 2018zd (Hiramatsu et al 2021) has shown a remarkable compatibility with the ECSN scenario, although no consensus has been reached yet on the nature of this object (Zhang et al 2020;Callis et al 2021).…”