SN 2021foa, a transitional event between a Type IIn (SN 2009ip-like) and a Type Ibn supernova
A. Reguitti, A. Pastorello, G. Pignata, M. Fraser, M. Stritzinger, S. Brennan, Yongzhi Cai, N. Elias–Rosa, D. Fugazza, C. P. Gutiérrez, E. Kankare, R. Kotak, Peter Lundqvist, P. A. Mazzali, S. Moran, I. Salmaso, L. Tomasella, G. Valerin, H. Kuncarayakti
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.