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

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 ∼ −14 mag) was detected in the weeks preceding the main event, showing a slowly 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 (∼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 (∼2700 km s −1 ) component. At +12 days, metal lines in emission appear and He I lines become very strong, with He I λ 5876 reaching half of the H α luminosity, much higher than in previous SN 2009ip-like objects. We propose that SN 2021foa is a transitional event between the H-rich SN 2009ip-like SNe and the He-rich Type Ibn SNe.