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

TL;DR

SN 2017ckj is a luminous, peculiar Type IIb supernova that shows a linear late-time decline and a very short $V$-band rise time. By combining six months of multi-band photometry, early flash-ionised spectroscopy, and a two-component bolometric light-curve model, the study derives a relatively large hydrogen envelope ($M_{ m env} \approx 0.4\ M_\odot$) and a high $^{56}$Ni mass ($M_{\rm Ni} \approx 0.21\ M_\odot$), with a progenitor radius of $R_0 \approx 575\ R_\odot$ and explosion epoch $t_0 \approx 57837.1$ MJD. The early spectra reveal flash-ionisation features implying a significant pre-explosion mass loss, while late-time spectra show persistent H$\alpha$ emission and typical nebular lines, suggesting incomplete envelope stripping and a stratified ejecta. Collectively, SN 2017ckj supports a progenitor scenario involving an extended, hydrogen-rich envelope in a relatively massive IIb star, likely formed via binary evolution, and highlights the need for high-cadence monitoring to capture complex light-curve morphologies and CSM interactions in IIb SNe.

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 $\rm H_α$ 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 $\rm H_α$ emission profile, also indicating a relatively massive envelope of its progenitor. Aside from the $\rm H_α$ feature, the nebular spectrum exhibits prominent emission lines of \Oi, \Caii, [\Caii], and \Mgi], which are consistent with the prototypical SN1993J.

Figures (15)

• Figure 1: Image of the location of SN 2017ckj, taken on 29 March 2017 by the Copernico telescope with the $B$ filter. The orientation and scale of the images are reported.
• Figure 2: Multi-band light curves of SN 2017ckj. A dashed vertical line is used to visually represent the reference epoch (MJD = 57843.9), which corresponds to the $i$-band observation maximum. The linear fit is applied to the light curve in each band, as indicated by the dashed lines. The epochs of our spectra are marked with vertical solid red lines on the top. In most cases, the errors associated with the magnitudes are smaller than the plotted symbol sizes.
• Figure 3: Absolute $V$-band light curve of SN 2017ckj compared to other SNe IIb. The ATLAS absolute $c$-band data of SN 2017ckj are also plotted with grey prismatic dots, as the $V$-band data is missing from 15 to 40 days. The subplot (upper right) displays the initial 30 days of absolute light curves. Note that DES14X2fna and SN 2013cu lack the $V$-band observations, therefore, we substitute with $r$-band data.
• Figure 4: Colour evolution of SN 2017ckj based on the estimated explosion epoch, compared with those of a sample of SNe IIb. The colour curves are corrected for Galactic and host galaxy extinction.
• Figure 5: Pseudo-bolometric light curve of SN 2017ckj, along with several other well-observed SNe IIb over the first 200 d. All SNe have been corrected for reddening as well as time dilation.