JWST Spectroscopy of SN Ia 2022aaiq and 2024gy: Evidence for Enhanced Central Stable Ni Abundance and a Deflagration-to-Detonation Transition

TL;DR

This work uses JWST NIR/MIR spectroscopy to map stable Ni, radioactive Ni, and IMEs in two normal SN Ia (SN 2022aaiq and SN 2024gy) across 1.7–28 μm, revealing narrow stable Ni cores and a broken-slope Ni profile that point to a two-zone ejecta structure typical of delayed-detonation explosions. Through line-profile fits, emissivity inversions, and comparisons to explosion models, the authors infer a central enhancement of stable Ni and a progenitor mass near the Chandrasekhar limit for SN 2024gy, contrasted with a lower central Ni content suggesting a sub-$M_{ ext{Ch}}$ origin for SN 2022xkq. Radiative-transfer modeling supports the need for enhanced inner $^{58}$Ni to reproduce the observed narrow cores, while the line morphologies and asymmetries are consistent with off-center ignition and viewing-angle effects. The study demonstrates that JWST’s medium-resolution MIR/NIR spectra provide stringent diagnostics of explosion physics, central density, and progenitor mass in SN Ia, and highlights the remaining challenges for fully reproducing narrow Ni cores in current 3D DD/DDT models.

Abstract

We present optical + near-infrared (NIR) + mid-infrared (MIR) observations of the normal Type Ia supernovae (SN Ia) 2022aaiq and 2024gy in the nebular phase, continuously spanning 0.35-28 microns. Medium-resolution JWST spectroscopy reveals novel narrow ($v_{\mathrm{FWHM}}<1500$ km s$^{-1}$) [Ni II] 1.94 and 6.64 micron cores in both events. The MIR [Ni II] 6.64 micron line exhibits a distinct narrow core atop a broader base, indicating a central enhancement of stable Ni. This structure points to high central densities consistent with a near-Chandrasekhar-mass ($M_{Ch}$) progenitor or a high-metallicity sub-$M_{Ch}$ progenitor. From detailed line-profile inversions of SN 2024gy, we derive emissivity profiles for stable iron-group elements (IGEs), radioactive material, and intermediate-mass elements (IMEs), revealing spatially distinct ejecta zones. The [Ni III] 7.35 micron line shows a shallow-to-steep slope transition -- a "broken-slope" morphology -- that matches predictions for delayed detonation explosions with separated deflagration and detonation ashes. We also reanalyze and compare to archival JWST spectra of SN 2021aefx and the subluminous SN 2022xkq. We estimate a stable $^{58}$Ni mass of $\sim0.1$ M$_\odot$ for SN 2024gy, consistent with delayed detonation models, and $\sim0.01$ M$_\odot$ for SN 2022xkq, favoring sub-$M_{Ch}$ scenarios. These results demonstrate that resolved line profiles, now accessible with JWST, provide powerful diagnostics of explosion geometry, central density, and progenitor mass in SN Ia.

Figures (17)

• Figure 1: HST WFC3/IR NIR images of SN 2022aaiq (left) in its elliptical host galaxy, NGC 5631, and SN 2024gy (right) in its spiral host galaxy, NGC 4216. The RGB channels are mapped from F160W, F140W, and F105W images, respectively. The images are 40$^{\prime\prime}$$\times$40$^{\prime\prime}$ and 120$^{\prime\prime}$$\times$120$^{\prime\prime}$, respectively, with a scale bar for reference. The orientation is marked by the compass rose with the longer and shorter arms representing north and east, respectively. A 5$^{\prime\prime}$$\times$5$^{\prime\prime}$ box is centered on each SN with a zoom-in image of that region shown in the upper-right corner.
• Figure 2: Panchromatic optical $+$ NIR $+$ MIR spectra of the normal SN Ia 2024gy (orange), at $+$144 d and $+$337 d post-maximum, and SN 2022aaiq (blue) at $+$125 d and $+$207 d post-maximum. The spectra are scaled and offset, and the ordinate is given in $\nu$F$\nu = \lambda$F$_\lambda$ using an arcsinh scaling, for display purposes.
• Figure 3: Comparison and identifications of prominent lines for JWST/MIRI MRS spectra of normal SN Ia 2022aaiq (blue), 2024gy (orange), 2021aefx (red), and 1991bg-like SN 2022xkq (green). A narrow component of [Ni$\;$] 6.64 $\mu$m is detected in SN 2024gy. Low opacities show the unbinned data. Owing to differences in phase and distance, we scale the spectra and offset for display purposes. A linear (not arcsinh) flux scaling is used.
• Figure 4: Comparison and identifications of prominent lines for the JWST/NIRSpec G235M$+$G395M spectra of normal SN Ia 2022aaiq (blue), and 2024gy (orange). Narrow features from [Ni$\;$] 1.94 $\mu$m are detected in both epochs for SN 2022aaiq and SN 2024gy. A narrow [Ni$\;$] 3.12 $\mu$m spike is also detected in SN 2024gy at $+$337 days. Owing to differences in phase and distance, we scale the spectra and offset for display purposes. An arcsinh flux scaling is also applied to more clearly show weaker lines.
• Figure 5: Line profile fits for [Ar$\;$] 6.98 $\mu$m and [Ar$\;$] 8.99 $\mu$m in SN 2024gy (orange) at $+$144 and $+$337 days, SN 2022aaiq (blue) at $+$125 and $+$207 days, SN 2022xkq (green) at $+$114 days, and SN 2021aefx (red) at $+$418 days. The contribution from Ar is shown in dashed black, and contributions from other nearby Ni lines are shown in dashed gray. The composite fit is shown in solid gray. The [Ar$\;$] and [Ar$\;$] lines are well-fit by slanted flat-topped profiles indicating a mildly asymmetric shell.