SN 2021foa: the bridge between SN IIn and Ibn

Abstract

We present the long-term photometric and spectroscopic analysis of a transitioning SN~IIn/Ibn from $-$10.8 d to 150.7 d post $V$-band maximum. SN~2021foa shows prominent He {\sc i} lines comparable in strength to the H$\hat{I}\pm$ line around peak, placing SN~2021foa between the SN~IIn and SN~Ibn populations. The spectral comparison shows that it resembles the SN~IIn population at pre-maximum, becomes intermediate between SNe~IIn/Ibn and at post-maximum matches with SN~IIn 1996al. The photometric evolution shows a precursor at $-$50 d and a light curve shoulder around 17d. The peak luminosity and color evolution of SN 2021foa are consistent with most SNe~IIn and Ibn in our comparison sample. SN~2021foa shows the unique case of a SN~IIn where the narrow P-Cygni in H$\hat{I}\pm$ becomes prominent at 7.2 days. The H$\hat{I}\pm$ profile consists of a narrow (500 -- 1200 km s${}^{-1}$) component, intermediate width (3000 -- 8000 km s${}^{-1}$) and broad component in absorption. Temporal evolution of the H$\hat{I}\pm$ profile favours a disk-like CSM geometry. Hydrodynamical modelling of the lightcurve well reproduces a two-component CSM structure with different densities ($\ddot{I}$ $\propto$ r${}^{-2}$ -- $\ddot{I}$ $\propto$ r${}^{-5}$), mass-loss rates (10${}^{-3}$ -- 10${}^{-1}$ M${}_{\odot }$ yr${}^{-1}$) assuming a wind velocity of 1000 km s${}^{-1}$ and having a CSM mass of 0.18 M${}_{\odot }$. The overall evolution indicates that SN~2021foa most likely originated from a LBV star transitioning to a WR star with the mass-loss rate increasing in the period from 5 to 0.5 years before the explosion or it could be due to a binary interaction.

Accepted for publication in MNRAS; 21 pages, 14 figures, 4 tables