SN2017ckj light curves and spectra

Both the ^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 H_alpha_ and HeII emission lines. The earliest spectrum exhibits flash ionization features, from which we estimated a progenitor mass-loss rate of ~3.4x10^-4^M_{sun}_/yr.

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 H_alpha_ emission profile, also indicating a relatively massive envelope of its progenitor. Aside from the H_alpha_ feature, the nebular spectrum exhibits prominent emission lines of OI, CaII, [CaII], and MgI], which are consistent with the prototypical SN1993J.