SN 2018is: A low-luminosity Type IIP supernova with narrow hydrogen emission lines at early phases

¹ Instituto de Astrofísica, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
² Millennium Institute of Astrophysics (MAS), Nuncio Monsenor Sòtero Sanz 100, Providencia, Santiago RM, Chile
³ Aryabhatta Research Institute of observational sciencES, Manora Peak, Nainital 263001, India
⁴ Department of Physics and Astronomy, University of California, Davis, 1 Shields Avenue, Davis, CA 95616-5270, USA
⁵ Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA
⁶ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
⁷ INAF Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
⁸ Instituto de Alta Investigación, Universidad de Tarapacá, Casilla 7D, Arica, Chile
⁹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, DK-8000 Aarhus C, Denmark
¹⁰ Department of Astronomy, The Oskar Klein Center, Stockholm University, AlbaNova, 106 91 Stockholm, Sweden
¹¹ Indian Institute of Astrophysics, Koramangala 2nd Block, Bangalore 560034, India
¹² Gemini Observatory, 670 North A’ohoku Place, Hilo, HI 96720-2700, USA
¹³ The School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel
¹⁴ CIFAR Azrieli Global Scholars program, CIFAR, Toronto, ON M5G 1M1, Canada
¹⁵ Institute for Astronomy, University of Hawai’i at Manoa, 2680 Woodlawn Dr., Hawai’i, HI 96822, USA
¹⁶ Department of Physics, Virginia Tech, Blacksburg, VA 24061, USA
¹⁷ South African Astronomical Observatory, PO Box 9 Observatory 7935, Cape Town, South Africa
¹⁸ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
¹⁹ Department of Physics, University of the Free State, P.O. Box 339 Bloemfontein 9300, South Africa
²⁰ SURF, Science Park 140, 1098 XG Amsterdam, The Netherlands
²¹ Department of Physics and Astronomy, Michigan State University, 567 Wilson Road, East Lansing, MI 48824, USA
²² European Southern Observatory, Alonso de C’ordova 3107, Casilla 19, Santiago 19001, Chile
²³ Department of Astronomy, School of Physics, Peking University, Yiheyuan Rd. 5, Haidian District, Beijing 100871, China
²⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Yi He Yuan Road 5, Hai Dian District, Beijing 100871, PR China
²⁵ National Astronomical Observatories, Chinese Academy of Science, 20A Datun Road, Chaoyang District, Beijing 100101, China
²⁶ UCD School of Physics, L.M.I. Main Building, Beech Hill Road, Dublin 4 D04 P7W1, Ireland
²⁷ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen, Denmark
²⁸ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
²⁹ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
³⁰ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, Cambridge, MA 02138, USA
³¹ Department of Astronomy & Astrophysics, University of California, San Diego, 9500 Gilman Drive, MC 0424, La Jolla, CA 92093-0424, USA
³² Las Cumbres Observatory, 6740 Cortona Dr, Suite 102, Goleta, CA 93117-5575, USA
³³ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
³⁴ Department of Physics, Florida State University, 77 Chieftan Way, Tallahassee, FL 32306, USA

^⋆ Corresponding author; rdastidr@gmail.com

Received: 6 October 2024
Accepted: 2 January 2025

Abstract

We present a comprehensive photometric and spectroscopic study of the Type IIP supernova (SN) 2018is. The V band luminosity and the expansion velocity at 50 days post-explosion are −15.1 ± 0.2 mag (corrected for A_V = 1.34 mag) and 1400 km s⁻¹, classifying it as a low-luminosity SN II. The recombination phase in the V band is shorter, lasting around 110 days, and exhibits a steeper decline (1.0 mag per 100 days) compared to most other low-luminosity SNe II. Additionally, the optical and near-infrared spectra display hydrogen emission lines that are strikingly narrow, even for this class. The Fe II and Sc II line velocities are at the lower end of the typical range for low-luminosity SNe II. Semi-analytical modelling of the bolometric light curve suggests an ejecta mass of ∼8 M_⊙, corresponding to a pre-supernova mass of ∼9.5 M_⊙, and an explosion energy of ∼0.40 × 10⁵¹ erg. Hydrodynamical modelling further indicates that the progenitor had a zero-age main sequence mass of 9 M_⊙, coupled with a low explosion energy of 0.19 × 10⁵¹ erg. The nebular spectrum reveals weak [O I] λλ6300,6364 lines, consistent with a moderate-mass progenitor, while features typical of Fe core-collapse events, such as He I, [C I], and Fe I, are indiscernible. However, the redder colours and low ratio of Ni to Fe abundance do not support an electron-capture scenario either. As a low-luminosity SN II with an atypically steep decline during the photospheric phase and remarkably narrow emission lines, SN 2018is contributes to the diversity observed within this population.