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Exploring the effect of mixing in Low-Luminosity Type IIp Supernovae by modeling SN 2024abfl

This paper demonstrates that suppressing ejecta mixing during shock breakout in models of the low-luminosity Type IIp supernova SN 2024abfl successfully reproduces its unique light curve featuring a flat plateau followed by a steep luminosity drop, offering a new perspective on the mechanisms distinguishing low-luminosity from typical Type IIp supernovae.

Original authors: Akshith Karri, Abigail Polin, Paul Duffell

Published 2026-07-10

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CC BY 4.0

Original authors: Akshith Karri, Abigail Polin, Paul Duffell

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 a supernova as a massive, glowing campfire that a dying star lights off before it collapses. Usually, when these fires burn out, they don't just vanish instantly; they fade away slowly, like embers cooling down over a long, gentle evening. But SN 2024abfl is a very different kind of fire. It burned steadily for about 126 days, holding a flat, bright plateau, and then—snap—it plunged into darkness, dropping in brightness by 2.4 magnitudes in just 5.5 days. It was like someone didn't just let the fire die out; they threw a bucket of ice water on it.

The authors of this paper wanted to figure out why this specific supernova, SN 2024abfl, decided to shut off so abruptly. They built a digital laboratory using powerful computer codes called MESA and STELLA to simulate how these stellar explosions work. Think of MESA as the architect that designs the star's life story, and STELLA as the special effects engine that watches the explosion happen and calculates how the light looks from Earth.

Here is the big mystery they tackled: In most Type II supernovae, the "embers" (the radioactive material inside the explosion) get mixed up with the outer layers of gas as the star explodes. This mixing is like stirring a pot of soup; it spreads the heat and the glowing bits around, making the fade-out smooth and gradual. But for SN 2024abfl, the authors ran simulations where they turned off the stirring. They simulated an explosion where the radioactive material stayed deep inside and didn't get mixed into the outer layers.

The result? When they stopped the mixing in their computer models, the supernova light curve looked exactly like the real SN 2024abfl. It held that flat, steady plateau and then dropped off a cliff, just like the real thing. The authors suggest that this lack of mixing is likely the reason for the star's unique behavior. Without the "stirring," the outer layers cooled down quickly once the hydrogen stopped glowing, and the light switched almost immediately to the dim glow of the radioactive tail, creating that steep, sharp drop.

They also tested other variables to see what else could cause this. They found that if you change the size of the star or the energy of the explosion, you can make the light brighter or dimmer, or make the plateau last longer or shorter. For instance, a smaller star (with a radius of 1.65 AU, which is about 355 times the size of our Sun) and a specific explosion energy of 1.4 × 10^50 ergs helped create the right conditions. But the "secret sauce" that made the drop so steep was definitely the lack of mixing.

It is important to note that the authors aren't saying the star was completely unmixed in reality; they just suggest that it needed significantly less mixing than usual to create the shape we saw. They also admit that they didn't account for every single detail, like how gas surrounding the star might have interacted with the explosion early on. However, their simulations strongly point to this "low-mixing" scenario as the best explanation for the weird, sharp drop in SN 2024abfl's light.

So, while we can't say for sure that this is the final answer without more data, the computer models give us a very strong hint: SN 2024abfl was a star that exploded without stirring its pot, letting the heat stay trapped deep inside until the very end, causing it to go dark much faster than its neighbors.

Technical Summary: Exploring the Effect of Mixing in Low-Luminosity Type IIp Supernovae by Modeling SN 2024abfl

Problem Statement
Low-luminosity Type IIp supernovae (LLSNe) are characterized by peak magnitudes $\ge -15.5$ and plateau magnitudes between $-13.5$ and $-15.5$ in the V band. SN 2024abfl presents a unique case within this class, exhibiting a remarkably flat $\sim 126$ -day plateau followed by an unusually steep luminosity drop of $\sim 2.4$ magnitudes over just $\sim 5.5$ days before transitioning to a faint radioactive tail. This morphology contrasts with well-studied LLSNe like SN 2003Z and SN 2005cs, which display more gradual transitions. Interpreting these observables is complicated by significant uncertainties regarding the distance to the host galaxy, NGC 2146, leading to degeneracies in inferred physical parameters such as progenitor mass, radius, and explosion energy. The primary challenge addressed is determining whether the distinct light curve morphology of SN 2024abfl can be reproduced by specific progenitor and explosion configurations, particularly focusing on the role of ejecta mixing.

Methodology
The authors employed a custom computational pipeline integrating MESA (Modules for Experiments in Stellar Astrophysics) and STELLA (a radiative-transfer and hydrodynamics code) to model the photometric evolution of SN 2024abfl. The workflow consisted of three main steps:

Progenitor Evolution: One-dimensional stellar models were evolved from the Pre-Main Sequence to Core Collapse using MESA. Initial parameters varied included mass, metallicity, wind schemes, and the mixing length parameter ( $\alpha$ ).
Explosion and Shock Breakout: The resulting progenitor structures were exploded using a modified version of the ccsn IIp test suite. This stage allowed for the modification of explosion energy and synthesized $^{56}$ Ni mass. Crucially, the pipeline included options to toggle specific mixing mechanisms: hydrodynamics, Rayleigh–Taylor instability (RTI) mixing, and boxcar smoothing.
Light Curve Synthesis: STELLA was used to simulate shock breakout and evolve the ejecta for up to 400 days, generating synthetic bolometric light curves. These were compared against observational data from M. Gerard et al. (2026), derived using the Light Curve Fitting Package.

Key Results
The study produced a model that successfully reproduces the overall morphology of SN 2024abfl, including the flat plateau and the sharp transition to the radioactive tail. The best-fit model parameters are:

Progenitor: $10 M_{\odot}$ with solar metallicity and a pre-collapse radius of $1.65$ AU ( $\approx 355 R_{\odot}$ ).
Explosion: Energy of $1.4 \times 10^{50}$ ergs.
Nucleosynthesis: Synthesized $^{56}$ Ni mass of $0.01 M_{\odot}$ .

Parameter sensitivity studies revealed that:

Explosion Energy: Higher energies result in brighter light curves with shorter plateau durations, while lower energies produce dimmer events with extended plateaus.
Progenitor Radius: Smaller radii lead to lower luminosities and shorter plateau durations due to reduced radiating surface area and thermal energy reservoirs.

The Role of Mixing
The most significant finding concerns the mechanism driving the steep luminosity drop. Standard Type II supernovae typically exhibit a gradual transition from the plateau to the radioactive tail, attributed to efficient ejecta mixing that redistributes radioactive material and thermal energy. In contrast, the models for SN 2024abfl demonstrated that suppressing mixing mechanisms (specifically disabling RTI and boxcar smoothing) during shock breakout yields a substantially steeper decline at the end of the plateau.

Unmixed models produced a rapid luminosity drop approximately 125 days post-explosion, closely matching the observed $\sim 2.4$ mag decline of SN 2024abfl.
The authors interpret this as evidence that the hydrogen envelope in SN 2024abfl experienced little contribution from deeper ejecta during the recombination phase, allowing the luminosity to transition almost directly to $^{56}$ Ni-powered emission.

Significance and Claims
The paper presents this work as a "proof-of-concept" demonstrating that suppressed ejecta mixing can explain the unique light curve morphology of SN 2024abfl without requiring new progenitor mass, radius, or explosion energy parameters beyond those previously proposed. The authors modestly claim that while real ejecta are unlikely to be completely unmixed, significantly less mixing than in typical Type IIp events is necessary to reproduce the observed plateau-to-tail transition.

The study concludes that the degree of ejecta mixing is a critical factor in the morphology of the plateau-to-tail transition. While the results align with general trends observed in previous theoretical studies regarding explosion energy and progenitor radius, the authors acknowledge that significant degeneracies remain in the parameter space. They suggest that future work combining multi-epoch photometric and spectroscopic modeling, potentially including nebular phase constraints, is required to break these degeneracies and fully understand the physics governing ejecta mixing in Type II supernovae.

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