SN 2021foa, a transitional event between a Type IIn (SN 2009ip-like) and a Type Ibn supernova

TL;DR

SN 2021foa exhibits characteristics bridging Type IIn and Type Ibn supernovae, with unique spectral features and light curve behavior, providing insights into supernova transitional phenomena.

Contribution

This paper reports the first detailed observation of SN 2021foa, a supernova that displays features of both Type IIn and Type Ibn, highlighting a transitional event.

Findings

Light curve similar to SN 2009ip-like events with two main brightening phases.

Spectral evolution shows increasing He I lines and complex line profiles.

SN 2021foa may represent a transitional supernova between H-rich and He-rich types.

Abstract

We present photometric and spectroscopic data of the unusual interacting supernova (SN) 2021foa. It rose to an absolute magnitude peak of $M_r=-18$ mag in 20 days. The initial light curve decline shows some luminosity fluctuations before a long-lasting flattening. A faint source ($M_r\sim -14$ mag) was detected in the weeks preceding the main event, showing a slow-rising luminosity trend. The $r$-band absolute light curve is very similar to those of SN 2009ip-like events, with a faint and shorter duration brightening (`Event A') followed by a much brighter peak (`Event B'). The early spectra of SN 2021foa show a blue continuum with narrow ($v_{FWHM}\sim$400 km s$^{-1}$) H emission lines, that, two weeks later, reveal a complex profile, with a narrow P Cygni on top of an intermediate-width ($v_{FWHM}\sim$2700 km s$^{-1}$) component. At +12 days metal lines in emission appear, while \Hei lines become very strong, with \Hei~$\lambda$5876 reaching half of the \Ha luminosity, much higher than in previous SN 2009ip-like objects. We propose SN 2021foa to be a transitional event between the H-rich SN 2009ip-like SNe and the He-rich Type Ibn SNe.