SN2017ckj: A linearly declining Type IIb supernova with a relatively massive hydrogen envelope

TL;DR

SN2017ckj is a peculiar Type IIb supernova with a linear decline in light curves, a relatively massive hydrogen envelope, and higher nickel mass than typical, providing insights into its progenitor and explosion characteristics.

Contribution

This study presents detailed optical observations and modeling of SN2017ckj, revealing its unique properties and progenitor characteristics compared to typical SNe IIb.

Findings

Linear late-time light curve decline

Higher nickel and envelope mass than typical SNe IIb

Spectral features indicating a massive hydrogen envelope

Abstract

We present optical observations of the Type IIb supernova (SN) 2017ckj, covering approximately 180 days after the explosion. Its early-time multi-band light curves display no clear evidence of a shock-cooling tail, resembling the behavior of SN2008ax. The $V$-band light curve exhibits a short rise time of about 5 days and reaches an absolute fitted peak magnitude of $M_{\rm V}=-18.49\pm0.18\mathrm{mag}$. The late-time multi-band light curves reveal a linear decline. We modelled the bolometric light curve of SN2017ckj to constrain the progenitor and the explosion parameters. We estimated a total mass of $\rm ^{56}Ni$ synthesized by SN2017ckj of $M_{\rm Ni} = 0.21^{+0.05}_{-0.03}\ M_\odot$, with a massive H-rich envelope of $M_{\rm env} = 0.4^{+0.1}_{-0.1}\ M_\odot$. Both the $\rm ^{56}Ni$ mass and the envelope mass of SN2017ckj are higher than those of typical SNe IIb, in agreement with its peculiar light curve evolution. The early-time spectra of SN2017ckj are dominated by a blue continuum, accompanied by narrow $\rm H_\alpha$ and \Heii emission lines. The earliest spectrum exhibits flash ionization features, from which we estimated a progenitor mass-loss rate of $\sim 3\times10^{-4}M_\odot \mathrm{yr}^{-1}$. At later epochs, the spectra develop broad P-Cygni profiles and become increasingly similar to those of SNe IIb, especially SN2018gk. The late-time spectrum at around 139 days does not show a distinct decline in the strength of $\rm H_\alpha$ emission profile, also indicating a relatively massive envelope of its progenitor. Aside from the $\rm H_\alpha$ feature, the nebular spectrum exhibits prominent emission lines of \Oi, \Caii, [\Caii], and \Mgi], which are consistent with the prototypical SN1993J.