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

II. Estimating the CSM mass and geometry with polarimetry and light curve modeling

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 Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁷ INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁸ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, E-08193 Barcelona, Spain
⁹ UCD School of Physics, L.M.I. Main Building, Beech Hill Road, Dublin 4 D04 P7W1, Ireland
¹⁰ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
¹¹ Finnish Centre for Astronomy with ESO (FINCA), FI-20014 University of Turku, Turku, Finland
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

^⋆ Corresponding author: thmire@utu.fi

Received: 17 January 2025
Accepted: 6 August 2025

Abstract

Context. There is evidence of interaction between supernova (SN) ejecta and massive circumstellar material (CSM) among various types of SNe. The mass-ejection mechanisms that produce a massive CSM are unclear. Therefore, studying interacting SNe and their CSM can shed light on these mechanisms and the final stages of stellar evolution.

Aims. We aim to study the properties of the CSM in the bright, long-lived, hydrogen-rich (Type II) SN 2021irp, which is interacting with a massive aspherical CSM.

Methods. We present imaging and spectro-polarimetric observations of SN 2021irp. Modelling its polarisation and bolometric light curve allowed us to derive the CMS mass and distribution.

Results. SN 2021irp shows a high intrinsic polarisation of ∼0.8%. This high continuum polarisation suggests an aspherical photosphere created by an aspherical CSM interaction. Based on the bolometric light curve evolution and the high polarisation, SN 2021irp can be understood as a typical Type II SN interacting with a CSM disc with a corresponding mass-loss rate and half-opening angle of ∼0.035–0.1 M_⊙ yr⁻¹ and ∼30–50°, respectively. The total CSM mass we derived is ≳2 M_⊙. We suggest that this CSM disc was created by some process related to binary interaction and that SN 2021irp is the end product of a typical massive star (i.e. with a ZAMS mass of ∼8 − 18 M_⊙) that has a separation and/or mass ratio with its companion star which has led to an extreme mass ejection within decades of explosion. We propose that the particular spectroscopic properties of SN 2021irp and similar SNe can be explained through a a Type II SNe interacting with a massive disc CSM.