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JWST Observations of Calcium-Strong Transients: I. Complex Nebular He Emission in SN 2024uj

This paper presents the first JWST observations of the Calcium-Strong Transient SN 2024uj, revealing complex, mixed ejecta and early forbidden calcium emission that strongly favor a thermonuclear explosion of a low-mass, partially helium-rich white dwarf over a massive star core-collapse origin.

Original authors: Saarah Hall, Lindsey A. Kwok, Aravind P. Ravi, Adam A. Miller, Luc Dessart, W. V. Jacobson-Galán, Huei Sears, Moira Andrews, K. Azalee Bostroem, Thomas G. Brink, Joseph Farah, Alexei V. Filippenko, Ry

Published 2026-07-02

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CC BY 4.0

Original authors: Saarah Hall, Lindsey A. Kwok, Aravind P. Ravi, Adam A. Miller, Luc Dessart, W. V. Jacobson-Galán, Huei Sears, Moira Andrews, K. Azalee Bostroem, Thomas G. Brink, Joseph Farah, Alexei V. Filippenko, Ryan J. Foley, Griffin Hosseinzadeh, D. Andrew Howell, Saurabh W. Jha, Conor Larison, Chang Liu, Colin W. Macrie, Kate Maguire, Curtis McCully, Nicolas E. Meza-Retamal, Maryam Modjaz, Megan Newsome, Estefania Padilla Gonzalez, David J. Sand, Steve Schulze, Giacomo Terreran, C. -G. Touchard-Paxton, Stefano Valenti, 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

Imagine the universe as a grand, chaotic theater. Usually, when a star dies in a spectacular explosion called a supernova, we can tell exactly what kind of actor it was: a massive, heavy star collapsing in on itself, or a smaller, dense star (a white dwarf) blowing up like a cosmic firecracker.

But then, there are the "CaSTs" (Calcium-Strong Transients). These are the supernovae that refuse to pick a side. They show up with clues pointing to both types of actors, leaving astronomers scratching their heads.

Enter SN 2024uj, the newest star in this mysterious cast. This paper is the first time we've used the James Webb Space Telescope (JWST)—our most powerful cosmic camera—to get a close-up look at one of these CaSTs. Here is what the team found, explained simply.

1. The Mystery of the "Double-Peaked" Light

Most supernovae have a single, bright flash that slowly fades. SN 2024uj, however, was like a firework that went off twice.

The First Spark: It had a quick, bright flash early on. The team thinks this was caused by a shockwave hitting a layer of material right next to the star, kind of like a car hitting a pile of sand and sending a spray of dust flying.
The Second Spark: A few days later, it brightened again. This second peak was powered by the radioactive decay of nickel, the standard "fuel" for many supernovae.
The Clue: This double-bump pattern suggests the star had a "compact envelope" (a tight layer of material) around it before it exploded, which is a hint that it might be a white dwarf rather than a massive star.

2. The "Forbidden" Calcium Surprise

In the world of physics, some chemical reactions are "forbidden" because they happen so rarely they usually only occur in very empty, low-density spaces (like the deep vacuum of space).

The Surprise: In SN 2024uj, the team saw strong "forbidden" calcium light appearing very early—just days after the explosion.
The Analogy: Imagine walking into a crowded room and seeing a ghost immediately. You'd expect the ghost to only appear once the room empties out.
The Verdict: For a massive star to have this much calcium floating in its outer layers this early is like trying to mix oil and water and having them stay perfectly mixed. It's very hard to explain with a massive star. However, if the star was a white dwarf that exploded, the calcium would naturally be right on the surface, making this "ghost" appear early. This is a strong point for the white dwarf theory.

3. The Helium Helix: A Twisted Story

The most exciting discovery came from the JWST's infrared eyes, which looked at the helium gas in the explosion.

The Shape: The helium wasn't just a smooth, round cloud. It was a twisted, lopsided mess. It had a strong, narrow peak moving away from us at a specific speed, and a broad, messy tail stretching out to much higher speeds.
The Comparison: The team compared this weird helium shape to V445 Puppis, a known system where two white dwarfs dance around each other. V445 Puppis has a similar "lopsided" helium signature.
The Mix: The helium was mixed up with the calcium and oxygen in a way that is very difficult to happen in a massive star explosion. It's as if you threw a salad, but the lettuce, tomatoes, and dressing were all stuck in different corners of the bowl, yet somehow blended in the middle. This suggests a violent, messy merger of two small stars.

4. The Dusty Aftermath

The JWST also saw a rising glow in the mid-infrared and detected Carbon Monoxide (CO) gas.

The Significance: This is the first time anyone has found molecular gas (like CO) and dust in a CaST. It's like finding a cozy, dusty attic inside a house that just exploded.
The Implication: This suggests that even though the explosion was violent, it left behind enough cool material to form new molecules and dust grains, much like how a forest fire eventually leads to new growth.

The Final Verdict: Who Did It?

The paper weighs the evidence like a detective closing a case:

The Case Against the Massive Star: The explosion happened far away from any star-forming regions (massive stars are usually born in busy nurseries). The "forbidden" calcium appeared too early. The helium was too strangely shaped and mixed.
The Case For the White Dwarf: The double-peaked light curve fits a white dwarf with a tight envelope. The early calcium fits a white dwarf explosion perfectly. The weird, lopsided helium looks exactly like what we see in systems where two white dwarfs merge.

The Conclusion:
The authors believe SN 2024uj was likely a thermonuclear explosion of a white dwarf, specifically one that was part of a binary system (a pair of stars) where at least one was a helium-rich white dwarf. It wasn't a massive star collapsing; it was a small, dense star blowing up, perhaps after merging with a partner.

While they can't rule out a massive star 100%, the evidence points strongly to the white dwarf theory. This discovery helps astronomers understand that these "CaST" supernovae are likely a diverse family of white dwarf explosions, rather than a single type of event.

Technical Summary: JWST Observations of Calcium-Strong Transients: I. Complex Nebular He Emission in SN 2024uj

Problem and Context
Calcium-Strong Transients (CaSTs) represent a puzzling subclass of supernovae (SNe) characterized by unusually strong [Ca II] emission, faint peak magnitudes ( $\lesssim -16.5$ mag), and rapid evolution to the nebular phase. Their origin remains debated: while their large offsets from host galaxies and low ejecta masses suggest a thermonuclear origin involving white dwarfs (WDs), their early-time optical spectra often resemble Type Ib/c core-collapse supernovae (CCSNe) due to the presence of helium. A critical gap in understanding this class is the lack of late-time near-infrared (NIR) and mid-infrared (MIR) observations, which are necessary to probe the innermost ejecta geometry and nucleosynthesis. SN 2024uj, a CaST with a double-peaked light curve and a significant offset from its host, serves as the primary case study to disentangle these competing progenitor scenarios.

Methodology
The authors present the first James Webb Space Telescope (JWST) observations of a CaST, SN 2024uj.

Observations: Data were acquired at $+150$ days post-second $r$ -band peak. This includes medium-resolution ( $R \approx 1000$ ) NIR spectroscopy (1–5 $\mu$ m) using the NIRSpec instrument and photometry in the F1000W filter using the MIRI instrument. These were paired with ground-based optical spectroscopy (from discovery to $+135$ days) and UV/optical photometry from a network of robotic telescopes and the Swift observatory.
Photometric Analysis: A bolometric light curve was constructed using optical and UV data. The authors modeled the double-peaked structure using a combination of shock-envelope cooling (Piro et al. 2021) and radioactive decay of $^{56}$ Ni to derive explosion parameters ( $M_{\text{ej}}$ , $M_{\text{Ni}}$ , $E_k$ ).
Spectral Analysis: The authors performed line identification in the nebular phase, focusing on [O I], [Ca II], and He I features. They utilized Gaussian fitting and nested sampling (via dynesty) to decompose complex, asymmetric line profiles into distinct kinematic components.
Radiative Transfer Modeling: To test progenitor scenarios, the observed spectra were compared against non-local thermodynamic equilibrium (non-LTE) radiative-transfer simulations from CMFGEN. Models included a stripped He-star core collapse (he5), a He-shell detonation on a low-mass WD (COp45HEp2), and a double-WD merger scenario (fca1).

Key Results

Explosion Properties: SN 2024uj exhibits a double-peaked light curve. The first peak is attributed to shock cooling of a compact envelope ( $M_{\text{env}} \approx 0.14 M_\odot$ , $R_{\text{env}} \approx 29 R_\odot$ ), while the second peak is powered by $^{56}$ Ni decay. The derived ejecta mass is low ( $M_{\text{ej}} \approx 0.61 M_\odot$ ) with a low kinetic energy ( $E_k \approx 0.17 \times 10^{51}$ erg) and a small $^{56}$ Ni mass ( $\approx 1.36 \times 10^{-2} M_\odot$ ).
Early Forbidden [Ca II]: Strong [Ca II] $\lambda\lambda$ 7291, 7324 emission emerges as early as $+2$ days after maximum light. Radiative-transfer modeling of a massive stripped He-star (even with artificially enhanced surface Calcium) fails to reproduce this early forbidden emission, whereas thermonuclear models naturally produce it.
Complex Nebular Helium: The JWST/NIRSpec spectrum reveals highly asymmetric, multicomponent He I emission at 1.083 and 2.058 $\mu$ m. The He distribution extends to velocities $\gtrsim 5000$ km s $^{-1}$ , with a strong, narrow peak at $\sim +1500$ km s $^{-1}$ and a redshifted high-velocity component at $\sim 5740$ km s $^{-1}$ .
Elemental Distribution: The [Ca II] and [O I] lines are relatively centrally located, while the He I emission is significantly redshifted and spatially distinct. This implies a complex, anisotropic mixing of elements that is difficult to produce in standard massive star explosions.
Molecular and Dust Emission: The spectrum shows a rising continuum beyond 3 $\mu$ m and a detection at 10 $\mu$ m, indicative of dust. Furthermore, the authors report the first detection of molecular CO fundamental band emission (4.5–5.1 $\mu$ m) in a CaST.
Progenitor Constraints: The massive star core-collapse scenario is disfavored due to the inability of models to reproduce early [Ca II] and the specific asymmetric He I profile. The data are more consistent with a thermonuclear origin involving a low-mass, partially He-rich WD, potentially within an AM CVn-like binary system or a WD merger.

Significance and Claims
The paper claims that SN 2024uj provides compelling evidence for a thermonuclear origin for at least a subset of CaSTs, specifically those with Ib-like spectral features. The authors argue that the combination of a remote explosion site, early forbidden [Ca II] emission, low ejecta mass, and the specific complex geometry of the He/Ca/O distribution strongly points toward a binary WD merger or an AM CVn-like progenitor system rather than a massive star core collapse.

Specifically, the authors highlight:

The early emergence of forbidden [Ca II] as a discriminator that massive star models cannot naturally explain without unphysical modifications.
The asymmetric, multicomponent He I profile as a signature of anisotropic thermonuclear explosions or interaction with a shocked companion, distinct from typical stripped-envelope CCSNe.
The detection of CO and dust, suggesting that faint thermonuclear SNe can be efficient dust producers, a finding that challenges previous assumptions about dust formation in such events.

The authors conclude that while a CCSN origin cannot be entirely ruled out, the weight of evidence favors a thermonuclear explosion involving at least one low-mass, He-rich WD. They suggest this interpretation may extend to the broader subclass of Ib-like CaSTs, reinforcing the idea that low-mass WDs with helium can produce strong spectroscopic He signatures via mechanisms such as double detonations or He-shell burning. The paper emphasizes the need for further hydrodynamical and radiative-transfer modeling to fully reproduce the observed ejecta distributions and pre-explosion mass loss in these systems.

1. The Mystery of the "Double-Peaked" Light

2. The "Forbidden" Calcium Surprise

3. The Helium Helix: A Twisted Story

4. The Dusty Aftermath

The Final Verdict: Who Did It?

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