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

JWST spectroscopy of Type Ia supernovae 2022aaiq and 2024gy reveals narrow [Ni II] line cores and distinct ejecta structures that provide strong evidence for enhanced central stable nickel abundance, supporting a near-Chandrasekhar-mass progenitor scenario and a delayed deflagration-to-detonation transition explosion mechanism.

Original authors: Lindsey A. Kwok, Chang Liu, Saurabh W. Jha, Stéphane Blondin, Conor Larison, Adam A. Miller, Mi Dai, Ryan J. Foley, Alexei V. Filippenko, Jennifer E. Andrews, Moira Andrews, Katie Auchettl, Carles Bad

Published 2026-06-12

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Lindsey A. Kwok, Chang Liu, Saurabh W. Jha, Stéphane Blondin, Conor Larison, Adam A. Miller, Mi Dai, Ryan J. Foley, Alexei V. Filippenko, Jennifer E. Andrews, Moira Andrews, Katie Auchettl, Carles Badenes, K. Azalee Bostroem, Thomas G. Brink, Cristine Koelln, Kyle W. Davis, Andreas Flörs, Lluís Galbany, Or Graur, D. Andrew Howell, Sahana Kumar, Réka Könyves-Tóth, Natalie LeBaron, Colin W. Macrie, Keiichi Maeda, Kate Maguire, Curtis McCully, Nicolas E. Meza-Retamal, Estefania Padilla Gonzalez, Rüdiger Pakmor, Jeniveve Pearson, Anthony L. Piro, Abigail Polin, Nabeel Rehemtulla, César Rojas-Bravo, David J. Sand, Chita Sangkachan, Michaela Schwab, Huei Sears, Mridweeka Singh, Bhagya M. Subrayan, Kirsty Taggart, Tea Temim, Jacco H. Terwel, Samaporn Tinyanont, József Vinkó, Xiaofeng Wang, J. Craig Wheeler, Yi Yang, WeiKang Zheng

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Big Picture: Cracking Open a Cosmic Firework

Imagine a Type Ia supernova as a giant, cosmic firework exploding in space. For decades, astronomers have been trying to figure out exactly how these fireworks are built and how they go off. Do they explode from the inside out like a bomb? Or do they burn from the outside in?

This paper uses the James Webb Space Telescope (JWST)—the most powerful space camera we have—to look at two specific supernovae (named SN 2022aaiq and SN 2024gy) long after they exploded. By looking at the "ash" left behind, the team discovered clues that reveal the explosion's secret recipe and how it happened.

The Detective Work: Listening to the "Ash"

When a star explodes, it throws out different layers of material, like the layers of an onion.

Outer layers: Lighter elements (like Silicon and Argon).
Inner layers: Heavier, radioactive elements (like Cobalt).
The Core: The densest center, where the heaviest stable elements (like Nickel) are forged.

Usually, when we look at these explosions with ground-based telescopes, the light from these different layers gets mixed up, like trying to hear a whisper in a noisy crowd. But JWST is so sensitive and clear that it can separate the whispers. It can hear the specific "voice" of Stable Nickel (a heavy metal that doesn't decay) in the mid-infrared part of the spectrum.

The Big Discovery: The "Narrow Core"

The most exciting finding is that in both supernovae, the stable Nickel isn't spread out evenly. Instead, it has a "narrow core."

The Analogy: Imagine a giant, expanding cloud of smoke from a firework.

What we expected: The smoke (Nickel) is spread out in a big, fuzzy cloud.
What we found: Inside that big fuzzy cloud, there is a tiny, incredibly dense, bright bead of smoke right in the very center.

This "bead" moves very slowly compared to the rest of the cloud. This tells the scientists that the center of the explosion was extremely dense and hot, creating a lot of heavy, stable Nickel right at the very heart of the star.

The "Broken Slope": A Tale of Two Explosions

The paper also looked at a different type of Nickel (ionized Nickel) and found a strange shape in its light signature, which the authors call a "broken-slope" profile.

The Analogy: Imagine a hill.

A normal hill: The slope goes up steadily and then down steadily.
The "Broken Slope": The hill has a gentle slope at the bottom, but then suddenly gets very steep halfway up.

This shape is a fingerprint of a specific type of explosion called a Delayed Detonation.

Phase 1 (The Slow Burn): The explosion starts as a slow, subsonic fire (deflagration) that puffs up the star. This creates the "gentle slope" and the dense core.
Phase 2 (The Boom): The fire suddenly turns into a supersonic shockwave (detonation) that blows the rest of the star apart. This creates the "steep slope" and the outer layers.

The fact that they see this "broken slope" means the explosion didn't just go off all at once; it had a two-step process.

Comparing the Suspects

The team compared these two "normal" supernovae to two others:

SN 2021aefx: Another normal one that also showed signs of this two-step explosion.
SN 2022xkq: A "sub-luminous" (dimmer) one. This one was different. It didn't have the dense central bead or the broken slope. It looked more like a single, simpler explosion. This suggests it came from a smaller star that exploded differently, perhaps without the slow-burn phase.

What This Means for the "Recipe"

The paper concludes that for the bright, normal supernovae (like 2022aaiq and 2024gy):

The Star: It was likely a white dwarf star that was very close to a critical mass limit (called the Chandrasekhar mass).
The Explosion: It started with a slow burn in the center (creating that dense Nickel bead), then exploded violently.
The Center: The center was so dense that it made a lot of stable Nickel, which is now sitting right in the middle of the expanding debris.

Summary

Think of these supernovae as a complex cake.

Old theories thought the cake was mixed evenly.
This paper says: "No, look! There's a super-dense, heavy chocolate chip right in the very center, and the cake was baked in two stages: first a slow rise, then a sudden pop."

By using JWST to see the "chocolate chip" (Stable Nickel) clearly, the astronomers have confirmed that these cosmic explosions are more complex and layered than we previously thought, involving a specific sequence of burning that leaves a unique fingerprint in the light.

Technical Summary: JWST Spectroscopy of SN Ia 2022aaiq and 2024gy

Problem and Motivation
Type Ia supernovae (SN Ia) are thermonuclear explosions of white dwarfs (WDs) whose nucleosynthetic yields depend sensitively on the progenitor density and explosion mechanism. The abundance of stable iron-group elements (IGEs), particularly stable nickel (Ni), serves as a critical diagnostic: it requires the highest densities and temperatures, distinguishing between near-Chandrasekhar-mass ( $M_{\text{Ch}}$ ) and sub- $M_{\text{Ch}}$ progenitors. Historically, the analysis of stable Ni has been hindered by line blending in optical spectra and atmospheric telluric absorption in the near-infrared (NIR). While previous studies identified [Ni II] 7378 Å and 1.94 µm features, robust detection of narrow components indicative of central density enhancements has been difficult. The James Webb Space Telescope (JWST) offers the capability to observe mid-infrared (MIR) stable Ni diagnostics ([Ni II] 6.64 µm, [Ni III] 7.35 µm, etc.) with high signal-to-noise ratio (S/N) and minimal blending, providing a new window into the chemical structure and explosion geometry of SN Ia.

Methodology
The authors present medium-resolution optical, NIR, and MIR spectroscopy of two normal SN Ia, 2022aaiq and 2024gy, obtained during the nebular phase (125–337 days post-peak) using JWST's NIRSpec and MIRI Medium Resolution Spectrograph (MRS). The dataset spans 0.35–28 µm. For comparison, the study includes re-analyzed archival JWST data of the normal SN 2021aefx and the subluminous, SN 1991bg-like SN 2022xkq.

Key methodological steps include:

Data Reduction: A consistent reduction workflow was applied to all MIRI/MRS data, utilizing the JWST pipeline and AstroBkgInterp for background subtraction optimized for point sources.
Line Identification and Fitting: Spectral lines were identified using predictions from Blondin et al. (2023). The authors employed simultaneous fitting of blended spectral regions using functional forms (Lorentzian, super-Gaussian, slanted flat-top) to model line profiles.
Kinematic Analysis: Velocity offsets ( $v_{\text{off}}$ ) and full widths at half-maximum (FWHM) were derived to map the spatial distribution of elements.
Emissivity Inversion: Assuming spherical symmetry and homologous expansion, the authors inverted line profiles to derive emissivity profiles $j(v)$ , visualizing the 3D distribution of stable Ni, radioactive Co, and intermediate-mass elements (IMEs).
Radiative Transfer Experiments: Using the CMFGEN code on spherically averaged Delayed Detonation (DDT) models (specifically the N100L ddt model), the authors performed controlled experiments to test how modifications to central density and stable Ni abundance affect emergent line profiles.
Luminosity Estimation: Stable Ni luminosities were calculated by integrating fluxes across optical, NIR, and MIR bands, corrected for distance and extinction, and extrapolated to a common phase (+130 days) to compare SN 2024gy and SN 2022xkq.

Key Results

Narrow Stable Ni Cores: Both SN 2022aaiq and SN 2024gy exhibit novel, narrow emission cores ( $v_{\text{FWHM}} < 1500$ km s $^{-1}$ ) in the [Ni II] 1.94 µm and 6.64 µm lines. These narrow cores sit atop broader emission bases, a structure clearly resolved in the MIR [Ni II] 6.64 µm line. This indicates a concentrated reservoir of stable Ni in the innermost ejecta.
Broken-Slope [Ni III] Profile: The [Ni III] 7.35 µm line in SN 2024gy displays a "broken-slope" morphology, transitioning from a shallow inner slope to a steeper outer slope. This feature matches predictions for Delayed Detonation (DDT) models where deflagration and detonation ashes occupy distinct spatial zones.
Ionization Stratification: The width of Ni emission lines depends on ionization state. In SN 2024gy, [Ni III] is broader than [Ni II], which is broader than the purely narrow [Ni I] (detected at +337 days). This suggests recombination occurs most efficiently in the densest central regions.
Kinematic Stratification: IMEs (Ar, S) extend to higher velocities ( $\sim$ 20,000 km s $^{-1}$ ) than IGEs (Ni, Co; $\sim$ 8,000–13,000 km s $^{-1}$ ). In SN 2022xkq, the separation is even more pronounced, with stable Ni confined to $\sim$ 4,000 km s $^{-1}$ .
Stable Ni Mass Comparison: SN 2024gy produced approximately 5–10 times more stable Ni mass than the subluminous SN 2022xkq. This favors a near- $M_{\text{Ch}}$ scenario for SN 2024gy and a sub- $M_{\text{Ch}}$ scenario for SN 2022xkq.
Radiative Transfer Constraints: Standard DDT models underpredict the strength of the narrow [Ni II] core. Artificially enhancing the stable Ni abundance in the innermost ejecta ( $v < 1000$ km s $^{-1}$ ) by $\sim 10^{-3} M_{\odot}$ (a tenfold increase over the baseline model) successfully reproduces the observed narrow-core morphology without affecting other lines.
Asymmetries: Line profiles, particularly [Ar III] 8.99 µm and [Co III] 11.88 µm, show asymmetries and velocity offsets consistent with off-center ignition and viewing-angle effects.

Significance and Claims
The paper claims that resolved line profiles accessible via JWST provide powerful, direct diagnostics of SN Ia explosion geometry, central density, and progenitor mass. The detection of narrow stable Ni cores atop broad bases provides evidence for enhanced central stable Ni abundance, consistent with high central densities found in near- $M_{\text{Ch}}$ progenitors or high-metallicity sub- $M_{\text{Ch}}$ progenitors. The "broken-slope" [Ni III] profile offers strong morphological evidence for a Delayed Detonation (DDT) explosion mechanism, supporting the separation of deflagration and detonation ashes.

However, the authors note that current 3D DDT models do not naturally reproduce the extreme central concentration of stable Ni required to explain the narrow cores, suggesting a need for mechanisms such as the confinement of deflagration ashes or core recompression prior to detonation. While the data strongly support a near- $M_{\text{Ch}}$ DDT origin for SN 2024gy, the origin of the subluminous SN 2022xkq remains inconclusive, potentially pointing to a sub- $M_{\text{Ch}}$ double-detonation scenario with specific conditions (e.g., $^{22}$ Ne settling) that are not yet fully captured by existing models. The study underscores that JWST MIR spectroscopy is essential for distinguishing between explosion channels that were previously indistinguishable due to line blending.