SN2017ckj: A linearly declining Type IIb supernova with a relatively 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 $M_{\mathrm{V}}=-18.49\pm 0.18\mathrm{mag}$. 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 ${}^{56}\mathrm{Ni}$ synthesized by SN2017ckj of $M_{\mathrm{Ni}}=0.21_{-0.03}^{+0.05}{M}_{\odot }$, with a massive H-rich envelope of $M_{\mathrm{env}}=0.4_{-0.1}^{+0.1}{M}_{\odot }$. Both the ${}^{56}\mathrm{Ni}$ 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 ${\mathrm{H}}_{\hat{\mathrm{I}}}\pm$ and \Heii emission lines. The earliest spectrum exhibits flash ionization features, from which we estimated a progenitor mass-loss rate of $\sim 3\times 10^{-4}{M}_{\odot }{\mathrm{yr}}^{-1}$. 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 ${\mathrm{H}}_{\hat{\mathrm{I}}}\pm$ emission profile, also indicating a relatively massive envelope of its progenitor. Aside from the ${\mathrm{H}}_{\hat{\mathrm{I}}}\pm$ feature, the nebular spectrum exhibits prominent emission lines of \Oi, \Caii, [\Caii], and \Mgi], which are consistent with the prototypical SN1993J.

19 pages, 15 figures, published in A&A