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

• Published in: arXiv.org 2022-06
• Main Authors: Reguitti, A, Pastorello, A, Pignata, G, Fraser, M, Stritzinger, M D, Brennan, S J, Y -Z Cai, Elias-Rosa, N, Fugazza, D, Gutierrez, C P, Kankare, E, Kotak, R, Lundqvist, P, Mazzali, P A, Moran, S, Salmaso, I, Tomasella, L, Valerin, G, Kuncarayakti, H
• Format: Article
• Language: English
• Subjects: Brightening; Light curve; Luminosity
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2206.00625

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.