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Pre-supernova O-C shell mergers could produce more $^{44}\mathrm{Ti}$ than the explosion

This study demonstrates that pre-supernova O-C shell mergers, particularly when incorporating 3D hydrodynamic mixing effects, can produce significant amounts of $^{44}\mathrm{Ti}$ that may exceed explosive yields and match observations, highlighting the critical need to account for 3D mixing physics in predicting radioactive isotope production in massive stars.

Original authors: Joshua Issa, Falk Herwig

Published 2026-05-27

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CC BY 4.0

Original authors: Joshua Issa, Falk Herwig

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: A Cosmic "Pre-Party" Before the Explosion

Imagine a massive star as a giant, multi-layered onion. Deep inside, it burns fuel like a nuclear furnace. Usually, scientists think that the most interesting radioactive elements (like Titanium-44, or 44Ti) are only created in the final, violent explosion of the star (a supernova).

However, this paper suggests that a significant amount of this radioactive "glow" might actually be created before the explosion happens. It's like a band playing a massive, loud concert (the supernova), but this paper argues that the band actually rehearsed and played a huge, energetic set just hours before the main show, and that pre-show performance might be louder than the actual concert.

The Event: The "O-C Shell Merger"

About a few hours before a massive star dies, two specific layers of its onion-like structure—the Oxygen layer and the Carbon layer—crash into each other. This is called an O-C shell merger.

Think of these layers as two different soups swirling in a giant pot. When they merge, they don't just sit there; they mix violently. This mixing is chaotic and three-dimensional, like a blender on high speed, rather than a smooth, orderly stir.

The Discovery: Mixing is the Key Ingredient

The researchers used computer simulations to see what happens during this mixing. They found that the speed and style of the mixing change everything.

The Old Way (1D Models): Imagine stirring a pot with a spoon in a straight line. This is how older computer models worked. They predicted a certain amount of radioactive Titanium.
The New Way (3D Physics): Imagine the soup is actually churning, swirling, and splashing in all directions, with hot pockets diving deep and cold pockets shooting up. The paper shows that when you simulate this "real" 3D turbulence, the production of radioactive Titanium changes dramatically.

The Analogy of the Bakers:
Imagine two bakers trying to make a specific cake (Titanium-44).

Baker A follows a strict recipe and stirs slowly. They make a small cake.
Baker B uses a high-powered mixer that swirls the ingredients violently, bringing the best ingredients from the bottom of the bowl to the top instantly. Baker B makes a cake that is huge—sometimes even bigger than what the explosion itself would produce later.

The paper found that depending on how "wild" the mixing is, the amount of Titanium-44 produced before the explosion can vary by a massive factor (over 60,000 times different in some cases!).

Why This Matters: The "Ghost" in the Machine

The paper focuses on a specific star model (15 times the mass of our Sun). Here is what they found:

Pre-Explosion Can Win: In many of their "wild mixing" scenarios, the star produced more radioactive Titanium-44 before it exploded than it would have during the explosion.
It Doesn't Break the Other Rules: Usually, if you make more Titanium, you mess up the amount of another element called Nickel-56 (which powers the brightness of the explosion). But the paper shows that with this specific 3D mixing, you can boost the Titanium without ruining the Nickel. It's like getting a free upgrade on your car's engine without changing the tires.
Solving a Mystery: We see radioactive Titanium in the remains of exploded stars (like the famous Cassiopeia A). Sometimes, the amount we see is hard to explain with standard explosion models. This paper suggests that if the star had a "pre-party" merger with wild mixing, it could explain exactly how much Titanium we see, without needing to invent new explosion physics.

The Takeaway

The main conclusion is that to understand how stars create radioactive elements, we can't just look at the explosion. We have to understand the chaotic, 3D mixing that happens in the hours leading up to the death of the star.

If we ignore this "pre-explosion mixing," we are missing a huge part of the story. The paper argues that the "pre-show" (the merger) might be just as important, or even more important, than the "main show" (the supernova) when it comes to creating the radioactive elements that light up the universe.

Technical Summary: Pre-supernova O-C shell mergers could produce more 44Ti than the explosion

Problem Statement
The radioactive isotope $^{44}$ Ti is a primary tracer of explosive nucleosynthesis in core-collapse supernovae (CCSN), traditionally thought to be synthesized during $\alpha$ -rich freeze-out from nuclear statistical equilibrium in the innermost ejecta. However, constraining the explosive origin of $^{44}$ Ti is challenging because yields are highly sensitive to the mass cut and the co-produced $^{56}$ Ni. Standard 1D models struggle to reproduce observed $^{44}$ Ti yields (e.g., in Cassiopeia A and SN 1987A) without overproducing $^{56}$ Ni, while spherically symmetric explosions underproduce $^{44}$ Ti compared to observations. Recent studies suggest that significant $^{44}$ Ti production may occur prior to the explosion during Oxygen-Carbon (O-C) shell mergers in massive stars. The specific impact of 3D macro-physical mixing conditions on this pre-explosive nucleosynthesis, and whether it can dominate over explosive production, remains to be quantified.

Methodology
The authors utilize the NuGrid dataset, specifically a $15\,M_\odot$ stellar model with metallicity $Z=0.02$ , which exhibits an O-C shell merger shortly before core collapse. They post-process the onset of this merger using the multi-zone code mppnp with a nuclear network of 1470 isotopes over a 110-second simulation.

To investigate the dependence on mixing conditions, the authors implement 24 distinct scenarios based on 3D hydrodynamic simulation results:

Mixing Profiles: A baseline Mixing Length Theory (MLT) profile and modified profiles incorporating convective downturns and boosting factors (1 $\times$ , 3 $\times$ , 10 $\times$ , and 50 $\times$ ) inspired by 3D hydro simulations.
Entrainment Rates: Simulations vary the entrainment rate of C-shell material into the O-shell at $0$, $4\times10^{-5}$ , $4\times10^{-4}$ , and $4\times10^{-3}\,M_\odot\,\text{s}^{-1}$ .
Quenching Effects: Four profiles include convective dips to emulate energy feedback and partial merging.

The study calculates pre-explosive yields and compares them against a "rapid" and "delayed" mass cut prescription. Results are expressed as an Overproduction Factor ( $OPR_{15}$ ) relative to the standard pre-explosive yield of the $15\,M_\odot$ model. These pre-explosive yields are then compared to a broad range of observational data (Cassiopeia A, SN 1987A) and theoretical models (1D, 2D, and 3D CCSN simulations) from the literature.

Key Contributions and Results

Nucleosynthesis Pathways: The study identifies a common reaction chain for $^{44}$ Ti production in O-C mergers: $^{38}\text{Ar}(\alpha, \gamma)^{42}\text{Ca}(n, \gamma)^{43}\text{Ca}(p, \gamma)^{44}\text{Sc}(p, n)^{44}\text{Ti}$ . In high-mixing scenarios (50 $\times$ boosted), additional pathways involving $^{39}\text{K}$ and $^{35}\text{Cl}$ contribute significantly. The destruction of $^{44}$ Ti is dominated by $^{44}\text{Ti}(\gamma, \alpha)^{40}\text{Ca}$ at the shell base.
Sensitivity to Mixing: $^{44}$ Ti production is highly sensitive to mixing speed and entrainment. Without entrainment, production is low. While MLT and quenched scenarios show decreased production compared to the baseline, models with the highest entrainment rates and boosted 3D-inspired mixing profiles show a significant increase in $^{44}$ Ti. Production is monotonic with mixing speed for 3D-inspired scenarios.
Pre-explosive vs. Explosive Yields: The pre-explosive production of $^{44}$ Ti in specific mixing scenarios can exceed the explosive production of the same stellar model. The predicted pre-explosive yields for $^{44}$ Ti exhibit a spread of 4.78–4.81 dex depending on the mass cut, significantly larger than the spread for other isotopes.
Comparison to Observations: The range of predicted pre-explosive yields for $^{44}$ Ti in this work is comparable to or larger than the explosive yields of all massive star models in the NuGrid dataset and many literature models. Specifically, the Cassiopeia A $^{44}$ Ti yield ( $2.6 \pm 0.6) \times 10^{-4}\,M_\odot$ ) falls within the range of the authors' 3 $\times$ and 10 $\times$ boosted mixing scenarios with the fastest entrainment rate.
Isotope Specificity: While $^{56}$ Ni production is dominated by the explosion and remains largely unaffected by pre-explosive mixing in 1D models, other radioactive species ( $^{22}$ Na, $^{51}$ Cr, $^{59,60}$ Fe, $^{65}$ Zn) also show significant pre-explosive contributions depending on mixing conditions.

Significance and Claims
The paper concludes that quantitative predictions of $^{44}$ Ti and other radioactive species require an understanding of the 3D hydrodynamic mixing conditions present during O-C shell mergers. The authors demonstrate that:

Pre-explosive O-C shell mergers can produce $^{44}$ Ti in amounts that rival or exceed explosive yields, potentially explaining observed yields without requiring fine-tuned explosive parameters.
3D mixing physics can enhance $^{44}$ Ti in 1D models without modifying $^{56}$ Ni yields, offering a potential solution to the discrepancy where 1D models overproduce $^{56}$ Ni to match $^{44}$ Ti.
The asymmetric and clumpy distribution of $^{44}$ Ti in remnants like Cassiopeia A may be a signature of the pre-explosive origin in an O-C shell merger, preserving the non-radial flow features of the merger.

The authors maintain a modest stance, noting that their yields are "exploratory rather than predictive" because the simulations neglect feedback effects on temperature and density evolution that would require full stellar evolution calculations. Furthermore, they emphasize that 3D simulations of the explosion itself are still necessary to fully capture nucleosynthesis and match observations of both $^{44}$ Ti and $^{56}$ Ni, even if a shell merger occurred. Future work must address the relationship between merger-induced perturbations and the subsequent explosion dynamics.

The Big Picture: A Cosmic "Pre-Party" Before the Explosion

The Event: The "O-C Shell Merger"

The Discovery: Mixing is the Key Ingredient

Why This Matters: The "Ghost" in the Machine

The Takeaway

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