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

Abstract

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 μm and shows many unique qualities, including a flat-topped [Ar iii] 8.991 μm profile, a strongly tilted [Co iii] 11.888 μm 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° 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 ⊙ 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.

Origin of High-velocity Ejecta, Excess Emission, and Redward Color Evolution in the Infant Type Ia Supernova 2021aefx

“…We adopt the latter because it is from a much higher resolution (R ∼ 40,000) spectrum and therefore more accurate; though, we note that the scatter associated with E(B − V ) host estimated using the method of Poznanski et al (2012) can be as high as 68% (Phillips et al 2013). The mean redshift of the fitted Na I D lines is z = 0.005284 ± 0.000005, which implies an SN peculiar velocity of (80.0 ± 3.6) km s −1 in the frame of the host galaxy, consistent with the measured host galaxy rotation velocity of (65 ± 60) km s −1 at the SN location (Elagali et al 2019) that was adopted in previous studies (DerKacy et al 2023). Hereafter, we adopt z = 0.005284 for the SN redshift since it is measured from the source position and more accurate.…”

“…The off-center ignited Chandrasekhar-mass explosion mechanism, on the other hand, predicts Fe II and Ni II emission features whose Doppler shifts depend on the motion of the ejecta core with respect to the rest frame of the SN (Maeda et al 2010a(Maeda et al , 2010bLi et al 2021). We observe a redshift in both the Fe II and Ni II features, with velocities of 1020 and 1310 km s −1 , respectively, indicative of an asymmetric explosion where the ejecta core is receding from the observer, consistent with JWST observations of SN 2021aefx in the nebular phase (DerKacy et al 2023;Kwok et al 2023).…”

An Asymmetric Double-degenerate Type Ia Supernova Explosion with a Surviving Companion Star

“…Nebular NIR and mid-infrared (MIR) spectra provide unique constraints on velocity distributions that cannot be obtained from optical spectra (Maguire et al 2018). Some features at these longer wavelengths are isolated and thus easier to model (DerKacy et al 2023;Kwok et al 2023). Furthermore, DerKacy et al (2023 found that the velocity extent of stable Ni features in JWST MIR spectra of the normal SN Ia 2021aefx is consistent with high-density burning, indicating a near-M Ch mass WD.…”

Direct Analysis of the Broad-Line SN 2019ein: Connection with the Core-Normal SN 2011fe

“…Xi et al (2022) found that the high velocities of SN 2019ein are due to asymmetries, however SN 2019ein shows no evidence for polarization (Patra et al 2022) indicating that there are no very strong asymmetries. In the delayed detonation framework, CN SNe Ia appear to arise from off-center detonations (DerKacy et al 2023). In looking for the progenitor/explosion scenario one should note that BL occur preferentially in luminous galaxies (Folatelli et al 2013;Morrell et al 2023).…”

“…The sub-Chandrasekhar helium detonation has been attributed to SN 2019ein Xi et al (2022), SN 2020jgb (Liu et al 2022), and SN 2016dsg (Dong et al 2022). The near Chandrasekhar mass scenario has received a boost from recent JWST observations of SN 2021aefx (DerKacy et al 2023;Kwok et al 2023).…”