The hydrogen-free circumstellar interaction in the Type Ib supernova 2021efd: A clue to the mechanism of the helium-layer stripping

¹ Department of Physics and Astronomy, University of Turku FI-20014 Turku, Finland
² Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7 E-38711 Breña Baja, Spain
³ Aalto University Metsähovi Radio Observatory Metsähovintie 114 02540 Kylmälä, Finland
⁴ Aalto University Department of Electronics and Nanoengineering PO BOX 15500 FI-00076 AALTO, Finland
⁵ National Astronomical Observatory of Japan, National Institutes of Natural Sciences 2-21-1 Osawa Mitaka Tokyo 181-8588, Japan
⁶ Finnish Center of Astronomy with ESO (FINCA), 20014 University of Turku Vesilinnantie 5 Turku, Finland
⁷ Department of Physics and Astronomy, Aarhus University, Denmark Aarhus University Ny Munkegade 120 DK-8000 Aarhus C, Denmark
⁸ Department of Astronomy, Kyoto University Kitashirakawa-Oiwake-cho Sakyo-ku Kyoto 606-8502, Japan
⁹ Institute for Advanced Study in Physics, Zhejiang University Hangzhou 310027, China
¹⁰ Institute for Astronomy, School of Physics, Zhejiang University Hangzhou 310027, China
¹¹ The Oskar Klein Centre, Department of Astronomy, Stockholm University AlbaNova SE-10691 Stockholm, Sweden
¹² Observatories of the Carnegie Institution for Science 813 Santa Barbara St. Pasadena CA 91101, USA
¹³ Instituto de Astrofísica de La Plata (UNLP – CONICET) Paseo del Bosque S/N 1900 La Plata, Argentina
¹⁴ Facultad de Ciencias Astronómicas y Geofísicas (UNLP) Paseo del Bosque S/N 1900 La Plata, Argentina
¹⁵ Kavli Institute for the Physics and Mathematics of the Universe (WPI),The University of Tokyo Institutes for Advanced Study, The University of Tokyo Kashiwa Chiba 277-8583, Japan
¹⁶ Carnegie Observatories, Las Campanas Observatory Casilla 601 La Serena, Chile
¹⁷ INAF – Osservatorio Astronomico di Padova Vicolo dell’Osservatorio 5 I-35122 Padova, Italy
¹⁸ INAF – Osservatorio Astronomico di Brera Via E. Bianchi 46 I-23807 Merate (LC), Italy
¹⁹ INAF–Osservatorio Astronomico di Capodimonte Salita Moiariello 16 80131 Napoli, Italy
²⁰ School of Sciences, European University Cyprus Diogenes Street Engomi 1516 Nicosia, Cyprus
²¹ Department of Particle Physics and Astrophysics Weizmann Institute of Science 234 Herzl St. Rehovot, Israel
²² Division of Physics, Mathematics, and Astronomy, California Institute of Technology Pasadena CA 91125, USA
²³ Kavli Institute for Particle Astrophysics and Cosmology, Stanford University Stanford CA 94305-4085, USA
²⁴ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC 08860 Castelldefels Barcelona, Spain
²⁵ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n E-08193 Barcelona, Spain
²⁶ Institute for Astronomy, University of Hawai’i at Manoa 2680 Woodlawn Dr. Hawai’i HI 96822, USA
²⁷ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), Northwestern University 1800 Sherman Ave. Evanston IL 60201, USA
²⁸ IPAC, California Institute of Technology 1200 E. California Blvd Pasadena CA 91125, USA

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Received: 17 September 2025
Accepted: 9 December 2025

Abstract

Context. Stripped-envelope supernovae (SESNe), including Type IIb, Ib, and Ic supernovae (SNe), originate from the explosions of massive stars whose outer envelopes have been largely removed during their lifetimes. The main stripping mechanism for the hydrogen (H) envelope in the progenitors of SESNe is often considered to be interaction with a binary companion, but the stripping mechanism for the helium (He) layer is unclear.

Aims. We study the process of the He-layer stripping in the progenitors of SESNe. This is closely related to the origin of their diverse observational properties.

Methods. We conducted photometric and spectroscopic observations of the Type Ib SN 2021efd, which shows signs of interaction with H-free circumstellar material (CSM). At early phases, its photometric and spectroscopic properties resemble those of typical Type Ib SNe. Around 30 days after the r-band light curve (LC) peak until at least ∼770 days, the luminosity of the multiband LCs is higher than that of regular SESNe and has at least three distinct peaks. The LC evolution is similar to that of SN 2019tsf, whose previously unpublished spectrum at 400 days is also presented here. The nebular spectrum of SN 2021efd shows narrow emission lines (∼1000 km s⁻¹) in various species, such as O I, Ca II, Mg II, He I, [O I], [Ca II], and [S II]. Based on the observations, we studied the properties of the ejecta and CSM of SN 2021efd.

Results. Our observations suggest that SN 2021efd is a Type Ib SN that interacts with the CSM with the following parameters: The estimated ejecta mass, explosion energy, and ⁵⁶Ni mass are 2.2 M_⊙, 9.1 × 10⁵⁰ erg, and 0.14 M_⊙, respectively, and the estimated CSM mass, composition, and distribution are at least a few times 0.1 M_⊙, H free, and clumpy, respectively. Based on the estimated ejecta properties, we conclude that this event is a transitional SN whose progenitor was experiencing He-layer stripping at the epoch of the explosion and was on the way to becoming a carbon-oxygen star (as the progenitors of Type Ic SNe) from a He star (as the progenitors of Type Ib SNe). The estimated CSM properties suggest that the progenitor had some episodic mass ejections at a rate of ∼5 × 10⁻³ − 10⁻² M_⊙ yr⁻¹ for the last decade and slightly lower before this final phase at least from ∼200 years before the explosion for the assumed CSM velocity of 100 km s⁻¹. For the case of ∼1000 km s⁻¹, the necessary mass-loss rate would be higher by a factor of ten, and the timescales would be shorter by a factor of ten.