The bright long-lived Type II SN 2021irp powered by aspherical circumstellar material interaction

I. Revealing the energy source with photometry and spectroscopy

¹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, FI-20014 Turku, Finland
² Cosmic Dawn Center (DAWN)
³ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 København N, Denmark
⁴ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁵ Aalto University Department of Electronics and Nanoengineering, P.O. BOX 15500 FI-00076 AALTO, Finland
⁶ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
⁷ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
⁸ Finnish Centre for Astronomy with ESO (FINCA), FI-20014 University of Turku, Finland
⁹ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
¹⁰ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122 Padova, Italy
¹¹ UCD School of Physics, L.M.I. Main Building, Beech Hill Road, Dublin 4 D04 P7W1, Ireland
¹² School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
¹³ The Oskar Klein Centre, Department of Astronomy, Stockholm University, Albanova University Center, SE 106 91 Stockholm, Sweden
¹⁴ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, P.R. China
¹⁵ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, P.R. China
¹⁶ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, P.R. China
¹⁷ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan
¹⁸ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, I-23807 Merate (LC), Italy
¹⁹ Amanogawa Galaxy Astronomy Research Center (AGARC), Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima, Kagoshima 890-0065, Japan

^⋆ Corresponding author: thmire@utu.fi

Received: 17 January 2025
Accepted: 18 July 2025

Abstract

Context. Some core-collapse supernovae (CCSNe) are too luminous and radiate too much total energy to be powered by the release of thermal energy from the ejecta and radioactive-decay energy from the synthesised ⁵⁶Ni/⁵⁶Co. A source of additional power is the interaction between the supernova (SN) ejecta and the massive circumstellar material (CSM). This is an important power source in Type IIn SNe, which show narrow spectral lines arising from the unshocked CSM, but not all interacting SNe show such narrow lines.

Aims. We present photometric and spectroscopic observations of the hydrogen-rich SN 2021irp, which is both luminous, with M_o < −19.4 mag, and long-lived, remaining brighter than M_o = −18 mag for ∼250 d. We show that an additional energy source is required to power such a SN, and we determine the nature of the source. We also investigate the properties of the pre-existing and newly formed dust associated with the SN.

Methods. Photometric observations show that the luminosity of the SN is an order of magnitude higher than typical Type II SNe and persists for much longer. We detect an infrared excess attributed to dust emission. Spectra show multi-component line profiles, an Fe II pseudo-continuum, and a lack of absorption lines, all typical features of Type IIn SNe. We detect a narrow (< 85 kms⁻¹) P Cygni profile associated with the unshocked CSM. An asymmetry in emission line profiles indicates dust formation occurring from 250–300 d. Analysis of the SN blackbody radius evolution indicates asymmetry in the shape of the emitting region.

Results. We identify the main power source of SN 2021irp as extensive interaction with a massive CSM, and that this CSM is distributed asymmetrically around the progenitor star. The infrared excess is explained with emission from newly formed dust although there is also some evidence of an IR echo from pre-existing dust at early times.