Letter to the Editor

Evidence for bipolar explosions in Type IIP supernovae

¹ Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
e-mail: t.nagao90@gmail.com
² Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
³ Aalto University Department of Electronics and Nanoengineering, PO BOX 15500 00076 Aalto, Finland
⁴ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁵ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
⁶ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Turku 20014, Finland
⁷ Okayama Observatory, Kyoto University, 3037-5 Honjo, Kamogatacho, Asakuchi, Okayama 719-0232, Japan
⁸ Hiroshima Astrophysical Science Center, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima, Hiroshima 739-8526, Japan
⁹ Department of Physical Science, Hiroshima University, Kagamiyama 1-3-1, Higashi-Hiroshima 739-8526, Japan
¹⁰ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
¹¹ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, 44122 Ferrara, Italy
¹² INFN, Sezione di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
¹³ INAF, Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini snc, 64100 Teramo, Italy
¹⁴ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
¹⁵ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
¹⁶ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
¹⁷ Amanogawa Galaxy Astronomy Research Center (AGARC), Graduate School of Science and Engineering, Kagoshima University, 1-21-35 Korimoto, Kagoshima, Kagoshima 890-0065, Japan
¹⁸ Department of Multi-Disciplinary Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro, Tokyo 153-8902, Japan

Received: 30 March 2024
Accepted: 19 June 2024

Abstract

Aims. Recent observations of core-collapse supernovae (SNe) suggest aspherical explosions. Globally, aspherical structures in SN explosions are thought to encode information regarding the underlying explosion mechanism. However, the exact explosion geometries from the inner cores to the outer envelopes are poorly understood.

Methods. Here, we present photometric, spectroscopic, and polarimetric observations of the Type IIP SN 2021yja and discuss its explosion geometry in comparison to those of other Type IIP SNe that show large-scale aspherical structures in their hydrogen envelopes (SNe 2012aw, 2013ej and 2017gmr).

Results. During the plateau phase, SNe 2012aw and 2021yja exhibit high continuum polarization characterized by two components with perpendicular polarization angles. This behavior can be interpreted as being due to a bipolar explosion, where the SN ejecta is composed of a polar (energetic) component and an equatorial (bulk) component. In such a bipolar explosion, an aspherical axis created by the polar ejecta would dominate at early phases, while the perpendicular axis along the equatorial ejecta would emerge at late phases after the photosphere in the polar ejecta has receded. Our interpretation of the explosions in SNe 2012aw and 2021yja as bipolar is also supported by other observational properties, including the time evolution of the line velocities and the line shapes in the nebular spectra. The polarization of other Type IIP SNe that show large-scale aspherical structures in the hydrogen envelope (SNe 2013ej and 2017gmr) is also consistent with the bipolar-explosion scenario, although this is not conclusive.