Ground-based and JWST Observations of SN 2022pul: II. Evidence from Nebular Spectroscopy for a Violent Merger in a Peculiar Type Ia Supernova

We present an analysis of ground-based and JWST observations of SN 2022pul, a peculiar “03fg-like” (or “super-Chandrasekhar”) Type Ia supernova (SN Ia), in the nebular phase at 338 d post explosion. Our combined spectrum continuously covers 0.4–14 µm and includes the first mid-infrared spectrum of an 03fg-like SN Ia. Compared to normal SN Ia 2021aefx, SN 2022pul exhibits a lower mean ionization state, asymmetric emission-line profiles, stronger emission from intermediate-mass elements (IMEs; argon and calcium), weaker emission from iron-group elements (IGEs), and the first unambiguous detection of neon in a SN Ia. A strong, broad, centrally peaked [Ne II] 12.81 µm line was predicted as a hallmark of “violent merger” SN Ia models, where dynamical interaction between two sub-MCh white dwarfs causes disruption of the lower mass WD and detonation of the other. The violent merger scenario can explain the large-scale ejecta asymmetries seen between the IMEs and IGEs and the central location of narrow oxygen and broad neon. We modify extant models to add clumping of the ejecta to better reproduce the optical iron emission, and add mass in the innermost region (<2000 km s−1) to account for the observed narrow [O I] λλ6300, 6364 emission. A violent WD–WD merger explains many observations of SN 2022pul, and our results favor this interpretation for the 03fg-like SN Ia subclass.

Kw, Jo, Luc Dessart, T Temim, Andr, Au, Bad, G Br, Ca, Clo, Lindsey A Kwok