Massive stars exploding in a He-rich circumstellar medium

X. Flash spectral features in the Type Ibn SN 2019cj and observations of SN 2018jmt

¹ School of Physics and Astronomy, Beijing Normal University, Beijing 100875, PR China
² Department of Physics, Faculty of Arts and Sciences, Beijing Normal University, Zhuhai 519087, PR China
³ INAF - Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁴ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁵ INAF - Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
⁶ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
⁷ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, PR China
⁸ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China
⁹ Las Cumbres Observatory, 6740 Cortona Drive, Suite 102, Goleta, CA 93117-5575, USA
¹⁰ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
¹¹ South African Astronomical Observatory, PO Box 9 Observatory 7935, Cape Town, South Africa
¹² Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹³ Department of Physics, University of the Free State, PO Box 339 Bloemfontein 9300, South Africa
¹⁴ INAF - Osservatorio Astronomico di Roma, via Frascati 33, I-00078 Monte Porzio Catone, Italy
¹⁵ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
¹⁶ Graduate Institute of Astronomy, National Central University, 300 Jhongda Road, 32001 Jhongli, Taiwan
¹⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
¹⁸ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel
¹⁹ The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, SE-10691 Stockholm, Sweden
²⁰ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
²¹ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China
²² Center for Astrophysics | Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
²³ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, USA
²⁴ Department of Physics, Tsinghua University, Beijing 100084, PR China
²⁵ Cardiff Hub for Astrophysics Research and Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
²⁶ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
²⁷ UKIRT Observatory, Institute for Astronomy, 640 N. A’ohoku Place, University Park, Hilo, Hawai’i 96720, USA
²⁸ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
²⁹ Instituto de Astrofísica, Facultad de Ciencias Exactas, Universidad Andres Bello, Av. Fernández Concha 700, Santiago, Chile
³⁰ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
³¹ Space Science Data Center-ASI, Via del Politecnico SNC, 00133 Roma, Italy
³² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
³³ Manipal Centre for Natural Sciences, Manipal Academy of Higher Education, Manipal, 576104 Karnataka, India
³⁴ INAF - Osservatorio Astronomico d’Abruzzo, Via M. Maggini snc, 64100 Teramo, Italy
³⁵ Physics and Astronomy Department, Johns Hopkins University, Baltimore, MD 21218, USA
³⁶ Center for Astrophysics and Cosmology, University of Nova Gorica, Vipavska 11c, 5270 Ajdovščina, Slovenia
³⁷ Institut d’Estudis Espacials de Catalunya (IEEC), 08860 Castelldefels (Barcelona), Spain
³⁸ Millennium Institute of Astrophysics, Nuncio Monseñor Sotero Sanz 100, Of. 104, Providencia, Santiago, Chile
³⁹ Center for Fundamental Physics, School of Mechanics and opticelectrical Physics, Anhui University of Science and Technology, Huainan, Anhui 232001, PR China
⁴⁰ Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuan Hua Road, Nanjing, Jiangsu 210023, PR China
⁴¹ Institute for Frontier in Astronomy and Astrophysics, Beijing Normal University, Beijing 102206, PR China
⁴² Advanced Institute of Natural Sciences, Beijing Normal University, Zhuhai 519087, PR China

^⋆ Corresponding authors; caiyongzhi@ynao.ac.cn; wangxf@mail.tsinghua.edu.cn; zhuxj@bnu.edu.cn

Received: 15 June 2024
Accepted: 22 August 2024

Abstract

We present optical and near-infrared observations of two Type Ibn supernovae (SNe), SN 2018jmt and SN 2019cj. Their light curves have rise times of about ten days, reaching an absolute peak magnitude of M_g(SN 2018jmt) = −19.07 ± 0.37 and M_V(SN 2019cj) = −18.94 ± 0.19 mag, respectively. The early-time spectra of SN 2018jmt are dominated by a blue continuum, accompanied by narrow (600−1000 km s⁻¹) He I lines with the P-Cygni profile. At later epochs, the spectra become more similar to those of the prototypical SN Ibn 2006jc. At early phases, the spectra of SN 2019cj show flash ionisation emission lines of C III, N III, and He II superposed on a blue continuum. These features disappear after a few days, and then the spectra of SN 2019cj evolve similarly to those of SN 2018jmt. The spectra indicate that the two SNe exploded within a He-rich circumstellar medium (CSM) lost by the progenitors a short time before the explosion. We modelled the light curves of the two SNe Ibn to constrain the progenitor and the explosion parameters. The ejecta masses are consistent with either what is expected for a canonical SN Ib (∼2 M_⊙) or for a massive Wolf Rayet star (> ∼4 M_⊙), with the kinetic energy on the order of 10⁵¹ erg. The lower limit on the ejecta mass (> ∼2 M_⊙) argues against a scenario involving a relatively low-mass progenitor (e.g. M_ZAMS ∼ 10 M_⊙). We set a conservative upper limit of ∼0.1 M_⊙ for the ⁵⁶Ni masses in both SNe. From the light curve modelling, we determined a two-zone CSM distribution, with an inner, flat CSM component and an outer CSM with a steeper density profile. The physical properties of SN 2018jmt and SN 2019cj are consistent with those expected from the core collapse of relatively massive envelope-stripped stars.