SN 2022joj: A Peculiar Type Ia Supernova Possibly Driven by an Asymmetric Helium-shell Double Detonation

Liu, Chang and Miller, Adam and Boos, Samuel and Shen, Ken J. and Townsley, Dean M. and Schulze, Steve and Harvey, Luke and Maguire, Kate and Johansson, Joel and Brink, Thomas G. and Burgaz, Umut and Dimitriadis, Georgios and Filippenko, Alexei and Hall, Saarah and Hinds, K-Ryan and Hoffman, Andrew and Karambelkar, Viraj and Kilpatrick, Charles and Perley, Daniel and Pichay, Neil and Sears, Huei and Sollerman, Jesper and Stein, Robert and Terwel, Jacco and Zheng, Weikang and Graham, Matthew J. and Kasliwal, Mansi and Lacroix, Leander and Purdum, Josiah and Rusholme, Benjamin and Wold, Avery (2023) SN 2022joj: A Peculiar Type Ia Supernova Possibly Driven by an Asymmetric Helium-shell Double Detonation. The Astrophysical Journal, 958 (2): 178. ISSN 0004-637X

Abstract

We present observations of SN 2022joj, a peculiar Type Ia supernova discovered by the Zwicky Transient Facility. SN 2022joj exhibits an unusually red g ZTF − r ZTF color at early times and a rapid blueward evolution afterward. Around maximum brightness, SN 2022joj shows a high luminosity ( M g ZTF , max ≃ − 19.7 mag), a blue broadband color (g ZTF − r ZTF ≃ −0.2 mag), and shallow Si ii absorption lines, consistent with those of overluminous, SN 1991T-like events. The maximum-light spectrum also shows prominent absorption around 4200 Å, which resembles the Ti ii features in subluminous, SN 1991bg-like events. Despite the blue optical-band colors, SN 2022joj exhibits extremely red ultraviolet minus optical colors at maximum luminosity (u − v ≃ 0.6 mag and uvw1 − v ≃ 2.5 mag), suggesting a suppression of flux at ∼2500-4000 Å. Strong C ii lines are also detected at peak. We show that these unusual spectroscopic properties are broadly consistent with the helium-shell double detonation of a sub-Chandrasekhar mass (M ≃ 1 M ⊙) carbon/oxygen white dwarf from a relatively massive helium shell (M s ≃ 0.04-0.1 M ⊙), if observed along a line of sight roughly opposite to where the shell initially detonates. None of the existing models could quantitatively explain all the peculiarities observed in SN 2022joj. The low flux ratio of [Ni ii] λ7378 to [Fe ii] λ7155 emission in the late-time nebular spectra indicates a low yield of stable Ni isotopes, favoring a sub-Chandrasekhar mass progenitor. The significant blueshift measured in the [Fe ii] λ7155 line is also consistent with an asymmetric chemical distribution in the ejecta, as is predicted in double-detonation models.