SN 2017ckj: A linearly declining type IIb supernova with a relatively massive hydrogen envelope

¹ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, China
² International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, China
³ University of Chinese Academy of Sciences, Beijing 100049, China
⁴ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
⁶ INAF – Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini Snc, 64100 Teramo, Italy
⁷ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield Dublin 4, Ireland
⁸ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁹ Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
¹⁰ Fabra Observatory, Royal Academy of Sciences and Arts of Barcelona (RACAB), 08001 Barcelona, Spain
¹¹ Institute for Space Studies of Catalonia (IEEC), Campus UPC, 08860 Castelldefels (Barcelona), Spain
¹² Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Vesilinnantie 5, Quantum, 20014 Turku, Finland
¹³ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
¹⁴ School of Sciences, European University Cyprus, Diogenes Street, Engomi 1516 Nicosia, Cyprus
¹⁵ Physics and Astronomy Department Galileo Galilei, University of Padova, Vicolo dell’Osservatorio 3, I-35122 Padova, Italy
¹⁶ School of Electronic Science and Engineering, Chongqing University of Posts and Telecommunications, Chongqing 400065, P.R. China
¹⁷ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹⁸ Cosmic Dawn Center (DAWN)
¹⁹ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
²⁰ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
²¹ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
²² Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
²³ School of Physics and Astronomy, Beijing Normal University, Beijing 100875, P.R. China
²⁴ Department of Physics, Faculty of Arts and Sciences, Beijing Normal University, Zhuhai 519087, P.R. China

^★ Corresponding authors: caiyongzhi@ynao.ac.cn; wangbo@ynao.ac.cn

Received: 15 August 2025
Accepted: 26 October 2025

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 behaviour of SN 2008ax. The V-band light curve exhibits a short rise time of about 5 days and reaches an absolute fitted peak magnitude of M_V = −18.49 ± 0.18 mag. The late-time multi-band light curves reveal a linear decline. We modelled the bolometric light curve of SN 2017ckj to constrain the progenitor and the explosion parameters. We estimated a total mass of ⁵⁶Ni synthesised by SN 2017ckj of M_Ni = 0.21^+0.05_−0.03 M_⊙, with a massive H-rich envelope of M_env = 0.4^+0.1_−0.1 M_⊙. Both the ⁵⁶Ni mass and the envelope mass of SN 2017ckj are higher than those of typical SNe IIb, in agreement with its peculiar light curve evolution. The early-time spectra of SN 2017ckj are dominated by a blue continuum, accompanied by narrow Hα and He II emission lines. The earliest spectrum exhibits flash ionisation features, from which we estimated a progenitor mass-loss rate of ∼3 × 10⁻⁴ M_⊙ yr⁻¹. At later epochs, the spectra develop broad P-Cygni profiles and become increasingly similar to those of SNe IIb, especially SN 2018gk. The late-time spectrum at around 139 days does not show a distinct decline in the strength of its Hα emission profile, also indicating a relatively massive envelope of its progenitor. Aside from the Hα feature, the nebular spectrum exhibits prominent emission lines of O I, Ca II, [Ca II], and Mg I], which are consistent with the prototypical SN 1993J.