Ground-based and JWST Observations of SN 2022pul. I. Unusual Signatures of Carbon, Oxygen, and Circumstellar Interaction in a Peculiar Type Ia Supernova
Authors: Siebert Matthew R. , Kwok Lindsey A. , Johansson Joel , Jha Saurabh W. , Blondin Stéphane , Dessart Luc , Foley Ryan J. , Hillier D. John , Larison Conor , Pakmor Rüdiger , Temim Tea , Andrews Jennifer E. , Auchettl Katie , Badenes Carles , Barna Barnabas , Bostroem K. Azalee , Brenner Newman Max J. , Brink Thomas G. , Bustamante-Rosell María José , Camacho-Neves Yssavo , Clocchiatti Alejandro , Coulter David A. , Davis Kyle W. , Deckers Maxime , Dimitriadis Georgios , Dong Yize , Farah Joseph , Filippenko Alexei V. , Flörs Andreas , Fox Ori D. , Garnavich Peter , Padilla Gonzalez Estefania , Graur Or , Hambsch Franz-Josef , Hosseinzadeh Griffin , Howell D. Andrew , Hughes John P. , Kerzendorf Wolfgang E. , Le Saux Xavier K. , Maeda Keiichi , Maguire Kate , McCully Curtis , Mihalenko Cassidy , Newsome Megan , O’Brien John T. , Pearson Jeniveve , Pellegrino Craig , Pierel Justin D. R. , Polin Abigail , Rest Armin , Rojas-Bravo César , Sand David J. , Schwab Michaela , Shahbandeh Melissa , Shrestha Manisha , Smith Nathan , Strolger Louis-Gregory , Szalai Tamás , Taggart Kirsty , Terreran Giacomo , Terwel Jacco H. , Tinyanont Samaporn , Valenti Stefano , Vinkó József , Wheeler J. Craig , Yang Yi , Zheng WeiKang , Ashall Chris , DerKacy James M. , Galbany Lluís , Hoeflich Peter , Hsiao Eric , de Jaeger Thomas , Lu Jing , Maund Justyn , Medler Kyle , Morrell Nidia , Shappee Benjamin J. , Stritzinger Maximilian , Suntzeff Nicholas , Tucker Michael , Wang Lifan 
Published: December 2023

Abstract
Nebular-phase observations of peculiar Type Ia supernovae (SNe Ia) provide important constraints on progenitor scenarios and explosion dynamics for both these rare SNe and the more common, cosmologically useful SNe Ia. We present observations from an extensive ground- and space-based follow-up campaign to characterize SN 2022pul, a super-Chandrasekhar mass SN Ia (alternatively “03fg-like” SN), from before peak brightness to well into the nebular phase across optical to mid-infrared (MIR) wavelengths. The early rise of the light curve is atypical, exhibiting two distinct components, consistent with SN Ia ejecta interacting with dense carbon–oxygen (C/O)-rich circumstellar material (CSM). In the optical, SN 2022pul is most similar to SN 2012dn, having a low estimated peak luminosity (M

B
= −18.9 mag) and high photospheric velocity relative to other 03fg-like SNe. In the nebular phase, SN 2022pul adds to the increasing diversity of the 03fg-like subclass. From 168 to 336 days after peak B-band brightness, SN 2022pul exhibits asymmetric and narrow emission from [O i] λ
λ6300, 6364 (FWHM ≈ 2000 km s−1), strong, broad emission from [Ca ii] λ
λ7291, 7323 (FWHM ≈ 7300 km s−1), and a rapid Fe iii to Fe ii ionization change. Finally, we present the first ever optical-to-MIR nebular spectrum of an 03fg-like SN Ia using data from JWST. In the MIR, strong lines of neon and argon, weak emission from stable nickel, and strong thermal dust emission (with T ≈ 500 K), combined with prominent [O i] in the optical, suggest that SN 2022pul was produced by a white dwarf merger within C/O-rich CSM.