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SN 2021zny: an early flux excess combined with late-time oxygen emission suggests a double white dwarf merger event

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Kozyreva,  Alexandra
Stellar Astrophysics, MPI for Astrophysics, Max Planck Society;

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Citation

Dimitriadis, G., Maguire, K., Karambelkar, V. R., Lebron, R. J., Liu, C., Kozyreva, A., et al. (2023). SN 2021zny: an early flux excess combined with late-time oxygen emission suggests a double white dwarf merger event. Monthly Notices of the Royal Astronomical Society, 521(1), 1162-1183. doi:10.1093/mnras/stad536.


Cite as: https://hdl.handle.net/21.11116/0000-000D-DFD0-D
Abstract
We present a photometric and spectroscopic analysis of the ultraluminous and slowly evolving 03fg-like Type Ia SN 2021zny. Our observational campaign starts from ∼5.3 h after explosion (making SN 2021zny one of the earliest observed members of its class), with dense multiwavelength coverage from a variety of ground- and space-based telescopes, and is concluded with a nebular spectrum ∼10 months after peak brightness. SN 2021zny displayed several characteristics of its class, such as the peak brightness (MB = −19.95 mag), the slow decline (Δm15(B) = 0.62 mag), the blue early-time colours, the low ejecta velocities, and the presence of significant unburned material above the photosphere. However, a flux excess for the first ∼1.5 d after explosion is observed in four photometric bands, making SN 2021zny the third 03fg-like event with this distinct behaviour, while its +313 d spectrum shows prominent [O i] lines, a very unusual characteristic of thermonuclear SNe. The early flux excess can be explained as the outcome of the interaction of the ejecta with ∼0.04M of H/He-poor circumstellar material at a distance of ∼1012 cm, while the low ionization state of the late-time spectrum reveals low abundances of stable iron-peak elements. All our observations are in accordance with a progenitor system of two carbon/oxygen white dwarfs that undergo a merger event, with the disrupted white dwarf ejecting carbon-rich circumstellar material prior to the primary white dwarf detonation.