SN 2021foa: The “Flip-flop” Type IIn/Ibn Supernova
/ Authors
D. Farias, C. Gall, G. Narayan, S. Rest, V. Villar, C. Angus, K. Auchettl, K. Davis, R. Foley, A. Gagliano
and 22 more authors
J. Hjorth, L. Izzo, C. Kilpatrick, H. .. L. Perkins, E. Ramirez-Ruiz, C. Ransome, A. Sarangi, R. Yarza, D. Coulter, D. Jones, N. Khetan, A. Rest, M. Siebert, J. Swift, Kirsty Taggart, S. Tinyanont, P. Wrubel, T. D. Boer, K. Clever, A. Dhara, H. Gao, C.-C. Lin
/ 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 (≈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 ∼6000 km s−1. For a steady, wind mass-loss regime, light-curve modeling results in an ejecta mass of ∼8 M ⊙ and circumstellar material (CSM) mass below 1 M ⊙, and an ejecta velocity consistent with the FWHM of the broad helium lines. We obtain a mass-loss rate of ≈2 M ⊙ yr−1. This mass-loss rate is 3 orders of magnitude larger than derived for normal Type II supernovae. We estimate that the bulk of the CSM of SN 2021foa must have been expelled within half a year, about 12 yr ago. Our analysis suggests that SN 2021foa had a helium-rich ejecta that 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 redshifted emission of the hydrogen and helium lines, which results in an observed blueshift (∼−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.
Journal: The Astrophysical Journal