A low mass, binary-stripped envelope for the Type IIb SN 2024abfo

¹ DTU Space, Technical University of Denmark, Building 327, Elektrovej, 2800 Kgs. Lyngby, Denmark
² South African Astronomical Observatory, PO Box 9 7935 Observatory, South Africa
³ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁴ Department of Physics, Stellenbosch University, Stellenbosch 7602, South Africa
⁵ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁶ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel

^⋆ Corresponding author.

Received: 10 July 2025
Accepted: 26 October 2025

Abstract

Context. Type IIb supernovae (SNe) are a transitional subclass of stripped-envelope SNe showing hydrogen lines in their spectra that gradually weaken and give way to helium lines reminiscent of SNe Ib. The presence of hydrogen indicates that these SNe retain a non-negligible hydrogen-rich envelope that has been stripped through stellar winds or binary interaction.

Aims. The direct detection of SN progenitors is a valuable way to connect the various SN sub-types with their progenitor stars. SN 2024abfo is the seventh SN IIb with a direct progenitor detection. Our aim is to study the progenitor candidate and the SN itself to determine the evolutionary history of the system.

Methods. In this paper, we astrometrically register our ERIS adaptive optics imaging with archival HST imaging to determine whether the SN position is consistent with the progenitor candidate position. We perform photometry on archival DECam imaging to derive the spectral energy distribution of the progenitor candidate and investigate its temporal variability. We consider single and binary star models to explain the end point of the progenitor candidate in the Hertzsprung-Russell diagram. For the SN, we compare the light curves and spectra with other SNe IIb with progenitor detections. We derive the bolometric light curve and attempt to fit this with a semi-analytic light curve model.

Results. The position of the SN in our adaptive optics imaging agrees with the progenitor position to within 20 mas. The progenitor spectral energy distribution is consistent with an A3V star with a radius of ∼120 R_⊙, a temperature of ∼8800 K, and a luminosity of log(L/L_⊙)∼4.9. Single star models predict an initial mass in the range of 12–16 M_⊙, while the most probable binary model is a 12 + 1.2 M_⊙ system with an initial period of 1.73 years. We also find significant evidence of variability of the progenitor candidate in the years prior to core collapse. SN 2024abfo is the least luminous SN IIb with a direct progenitor detection. At late times, the r-band light curve decays more slowly than the comparison SNe, which may be due to increased γ-ray trapping, although this requires further investigation. Similar to SN 2008ax, SN 2024abfo does not show a prominent double-peaked light curve. Our semi-analytic light curve modelling shows that this may be due to a very low mass of hydrogen (≲0.006 M_⊙) in the outer envelope. Spectrally, SN 2024abfo is most similar to SN 2008ax at early times, while at later times (∼80 days) it appears to show persistent Hα absorption compared to the comparison sample.

Conclusions. We prefer a binary system to explain SN 2024abfo and its progenitor, but we are unable to rule out single star models. We recommend late-time observations to search for a binary companion and signatures of circumstellar medium interaction. The absence of these features would support the hypothesis that SN 2024abfo resulted from a system that underwent a period of binary mass transfer well before (≳1000 yr) the explosion, resulting in a low-mass (≲0.01 M_⊙) hydrogen-rich envelope.