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

Kwok, Lindsey and Siebert, Matthew and Johansson, Joel and Jha, Saurabh W and Blondin, Stéphane and Dessart, Luc and Foley, Ryan and Hillier, Desmond John and Larison, Conor and Pakmor, Ruediger and Temim, Tea and Andrews, J.E. and Auchettl, Katie and Badenes, Carles and Barna, Barnabás and Bostroem, K. Azalee and Newman, Max J. B. and Brink, Thomas G. and Rosell, María José Bustamante and Camacho-Neves, Yssavo and Clocchiatti, Alejandro and Coulter, David and Davis, Kyle and Deckers, Maxime and Dimitriadis, Georgios and Dong, Yize and Farah, Joseph R. and Filippenko, Alexei and Flörs, Andreas and Fox, Ori and Garnavich, Peter and Gonzalez, Estefania Padilla and Graur, Or and Hambsch, Franz-Josef and Hosseinzadeh, Griffin and Howell, D. Andrew and Hughes, John P. and Kerzendorf, Wolfgang and Saux, Xavier Le and Maeda, Keiichi and Maguire, Kate and McCully, Curtis and Mihalenko, Cassidy and Newsome, Megan and O'Brien, John and Pearson, Jeniveve and Pellegrino, Craig and Pierel, Justin R. and Polin, Abigail and Rest, Armin and Rojas-Bravo, Cesar and Sand, David and Schwab, Michaela and Shahbandeh, Melissa and Shrestha, Manisha and Smith, Nathan and Strolger, Louis-Gregory and Szalai, Tamas and Taggart, Kirsty and Terreran, Giacomo and Terwel, Jacco and Tinyanont, Samaporn and VALENTI, Stefano and Vinko, Jozsef and Wheeler, J. Craig and Yang, Yi and Zheng, Weikang and Ashall, Chris and DerKacy, James M and Galbany, Lluís and Hoeflich, Peter and de Jaeger, Thomas and Lu, Jing and Maund, Justyn and Medler, Kyle and Morrell, Nidia Irene and Shappee, Benjamin and Stritzinger, Maximilian and Suntzeff, Nicholas B. and Tucker, Michael and Wang, Lifan (2024) Ground-based and JWST Observations of SN 2022pul. II. Evidence from Nebular Spectroscopy for a Violent Merger in a Peculiar Type Ia Supernova. The Astrophysical Journal, 966 (1): 135. ISSN 0004-637X

Abstract

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 days postexplosion. Our combined spectrum continuously covers 0.4–14 μm and includes the first mid-infrared spectrum of a 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 the 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] line at 12.81 μm was previously predicted as a hallmark of "violent merger" SN Ia models, where dynamical interaction between two sub-MCh white dwarfs (WDs) causes disruption of the lower-mass WD and detonation of the other. The violent merger scenario was already a leading hypothesis for 03fg-like SNe Ia; in SN 2022pul it 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 reproduce the optical iron emission better, 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 of the observations of SN 2022pul, and our results favor this model interpretation for the subclass of 03fg-like SNe Ia.