JWST Low-resolution MIRI Spectral Observations of SN 2021aefx: High-density Burning in a Type Ia Supernova

We present a JWST/MIRI low-resolution mid-infrared (MIR) spectroscopic observation of the normal Type Ia supernova (SN Ia) SN 2021aefx at +323 days past rest-frame B-band maximum light. The spectrum ranges from 4 to 14 mu m and shows many unique qualities, including a flat-topped [Ar iii] 8.991 mu m profile, a strongly tilted [Co iii] 11.888 mu m feature, and multiple stable Ni lines. These features provide critical information about the physics of the explosion. The observations are compared to synthetic spectra from detailed non-local thermodynamic equilibrium multidimensional models. The results of the best-fitting model are used to identify the components of the spectral blends and provide a quantitative comparison to the explosion physics. Emission line profiles and the presence of electron capture elements are used to constrain the mass of the exploding white dwarf (WD) and the chemical asymmetries in the ejecta. We show that the observations of SN 2021aefx are consistent with an off-center delayed detonation explosion of a near-Chandrasekhar mass (M (Ch)) WD at a viewing angle of -30 degrees relative to the point of the deflagration to detonation transition. From the strengths of the stable Ni lines, we determine that there is little to no mixing in the central regions of the ejecta. Based on both the presence of stable Ni and the Ar velocity distributions, we obtain a strict lower limit of 1.2 M (circle dot) for the initial WD, implying that most sub-M (Ch) explosions models are not viable models for SN 2021aefx. The analysis here shows the crucial importance of MIR spectra in distinguishing between explosion scenarios for SNe Ia.

Automated summary

We present low-resolution mid-infrared spectroscopic observations of the normal Type Ia supernova SN 2021aefx. The spectrum shows unique features such as flat-topped [Ar iii] 8.991 μm profile, tilted [Co iii] 11.888 μm feature, and multiple stable Ni lines, providing critical information about the explosion physics. Comparisons with synthetic spectra suggest an off-center delayed detonation explosion of a near-Chandrasekhar mass white dwarf at a viewing angle of -30 degrees. The analysis highlights the importance of mid-infrared spectra in distinguishing between explosion scenarios for Type Ia supernovae.