SN 2021foa: The "Flip-Flop" Type IIn / Ibn supernova

TL;DR

SN 2021foa exhibits a rare 'flip-flop' spectroscopic evolution from hydrogen to helium and back within 50 days, driven by complex circumstellar material interactions and intense pre-explosion mass loss.

Contribution

This study provides the first detailed analysis of a supernova with such rapid and repeated spectral type transitions, revealing insights into its circumstellar environment and progenitor mass-loss history.

Findings

Spectroscopic evolution from IIn to Ibn and back within 50 days.

Mass-loss rate of approximately 2 M$_{ ext{sun}}$ per year, much higher than typical Type II SNe.

Ejecta mass around 8 M$_{ ext{sun}}$, with dense CSM expelled about 15 years prior.

Abstract

We present a comprehensive analysis of the photometric and spectroscopic evolution of SN~2021foa, unique among the class of transitional supernovae for repeatedly changing its spectroscopic appearance from hydrogen-to-helium-to-hydrogen-dominated (IIn-to-Ibn-to-IIn) within 50 days past peak brightness. The spectra exhibit multiple narrow ($\approx$ 300--600~km~s$^{-1}$) absorption lines of hydrogen, helium, calcium and iron together with broad helium emission lines with a full-width-at-half-maximum (FWHM) of $\sim 6000$~km~s$^{-1}$. For a steady, wind-mass loss regime, light curve modeling results in an ejecta mass of $\sim 8$ M$_{\odot}$ and CSM mass below 1 M$_{\odot}$, and an ejecta velocity consistent with the FWHM of the broad helium lines. We obtain a mass-loss rate of $\approx 2$ M$_{\odot} {\rm yr}^{-1}$. This mass-loss rate is three orders of magnitude larger than derived for normal Type II SNe. We estimate that the bulk of the CSM of SN~2021foa must have been expelled within half a year, about 15 years ago. Our analysis suggests that SN~2021foa had a helium rich ejecta which swept up a dense shell of hydrogen rich CSM shortly after explosion. At about 60 days past peak brightness, the photosphere recedes through the dense ejecta-CSM region, occulting much of the red-shifted emission of the hydrogen and helium lines, which results in observed blue-shift ($\sim -3000$~km~s$^{-1}$). Strong mass loss activity prior to explosion, such as those seen in SN~2009ip-like objects and SN~2021foa as precursor emission, are the likely origin of a complex, multiple-shell CSM close to the progenitor star.