JWST MIRI/Medium Resolution Spectrograph (MRS) Observations and Spectral Models of the Underluminous Type Ia Supernova 2022xkq

Abstract

We present a JWST mid-infrared (MIR) spectrum of the underluminous Type Ia Supernova (SN Ia) 2022xkq, obtained with the medium-resolution spectrometer on the Mid-Infrared Instrument (MIRI) ∼130 days post-explosion. We identify the first MIR lines beyond 14 μm in SN Ia observations. We find features unique to underluminous SNe Ia, including the following: isolated emission of stable Ni, strong blends of [Ti ii], and large ratios of singly ionized to doubly ionized species in both [Ar] and [Co]. Comparisons to normal-luminosity SNe Ia spectra at similar phases show a tentative trend between the width of the [Co iii] 11.888 μm feature and the SN light-curve shape. Using non-LTE-multi-dimensional radiation hydro simulations and the observed electron capture elements, we constrain the mass of the exploding WD. The best-fitting model shows that SN 2022xkq is consistent with an off-center delayed-detonation explosion of a near-Chandrasekhar mass WD ( M WD ≈1.37 M ⊙) of high central density (ρ c ≥ 2.0 × 109 g cm−3) seen equator-on, which produced M(56Ni) =0.324 M ⊙ and M(58Ni) ≥0.06 M ⊙. The observed line widths are consistent with the overall abundance distribution; and the narrow stable Ni lines indicate little to no mixing in the central regions, favoring central ignition of subsonic carbon burning followed by an off-center deflagration-to-detonation transition beginning at a single point. Additional observations may further constrain the physics revealing the presence of additional species including Cr and Mn. Our work demonstrates the power of using the full coverage of MIRI in combination with detailed modeling to elucidate the physics of SNe Ia at a level not previously possible.

Type Ia Supernova Progenitor Properties and their Host Galaxies

“…Here, the main analysis is based on the framework of spherical DD models (Khokhlov 1991), which has been employed to analyze many observations of Phillips normal SNe Ia, i.e., those that follow the luminosity decline relation (Phillips 1993), including early-time optical and NIR spectra, LCs, and color-magnitude diagrams (Wang et al 2003a;Hoeflich et al 2017). Moreover, JWST observations show narrow stable 58 Ni in the nebular spectra (Gerardy et al 2007;Telesco et al 2015;Hoeflich et al 2021;DerKacy et al 2023DerKacy et al , 2024Ashall et al 2024), which all indicate high-mass explosions with little mixing by a passive flow of EC elements in a preexisting turbulent field prior to the runaway (Höflich & Stein 2002;Zingale et al 2011) rather than multiple-spot ignitions and Rayleigh-Taylor (RT) instabilities as assumed by Seitenzahl et al (2013) in their version of DD models. However, from agreement on one explosion scenario with individual objects, one cannot exclude the existence of alternative scenarios (see above).…”