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Comparative Study of Two Luminous Red Novae I. Progenitor Modeling and Dust Formation

This paper investigates the binary progenitors, ejected gas masses, and dust formation of the luminous red novae AT2021biy and AT2021blu by combining MESA stellar evolution modeling with infrared observations, revealing that donor stars were massive and that only a small fraction of the ejected gas condensed into dust.

Original authors: M. Wavasseur, N. Blagorodnova, M. A. Gómez-Muñoz, O. R. Pols

Published 2026-06-23

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CC BY 4.0

Original authors: M. Wavasseur, N. Blagorodnova, M. A. Gómez-Muñoz, O. R. Pols

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 two massive stars locked in a cosmic dance, orbiting each other so closely that they are about to crash. This paper is a detective story investigating two specific cosmic crashes that happened in 2021, named AT2021biy and AT2021blu. These events are called "Luminous Red Novae" (LRNs)—think of them as the universe's version of a dramatic, slow-motion car crash that results in a massive explosion of light and gas, but not as violent as a supernova.

Here is the breakdown of what the astronomers did and what they found, using simple analogies:

1. The Setup: A Tangled Dance

Most massive stars don't live alone; they live in pairs. In these pairs, one star (the "donor") is usually older and puffier than its partner. As the donor expands, it starts spilling its outer layers onto the partner, like a balloon leaking air into a smaller, tighter balloon.

Usually, this leak is slow and steady. But sometimes, the leak becomes a gush. The donor star can't hold itself together, and it dumps a huge amount of gas onto its partner all at once. This is called unstable mass transfer. It's like trying to pour a bucket of water into a cup that's already full; the water spills everywhere, and the system goes into chaos.

2. The Investigation: Reconstructing the Crash

The team wanted to know: How big were these stars? How fast were they spinning? And how much stuff got thrown out during the crash?

To figure this out, they used three different "detective tools":

Tool A: The Blueprint (Computer Models)
They used a super-computer program called MESA to simulate thousands of different binary star scenarios. They looked for the specific "blueprints" that matched the stars' appearance before the crash (using old photos from the Hubble Space Telescope).
- The Filter: They didn't just look for any match; they filtered for systems that were destined to crash. They applied a "stability test" to see which stars would inevitably spill their guts.
- The Result: They found that the star in AT2021biy was a giant, about 18–23 times the mass of our Sun. The star in AT2021blu was slightly smaller, about 14 times the Sun's mass.
Tool B: The Energy Bill (The Crash Physics)
When the two stars spiral into each other, they lose orbital energy (like a spinning top slowing down). This lost energy has to go somewhere—it usually blows the outer layers of the star off into space.
The team did the math on this "energy budget." They asked: If we take all the energy released by the spiraling stars, how much gas could we theoretically blow away?
- The Result: They calculated a range. For the big star (AT2021biy), they could have ejected anywhere from a tiny bit of gas to nearly 8 suns' worth of material. For the smaller one (AT2021blu), it was between a tiny bit and about 4 suns' worth.
Tool C: The Dust Trail (Infrared Cameras)
When gas is thrown out into space, it cools down and turns into dust (like soot from a fire). This dust glows in infrared light (heat), which the NEOWISE telescope can see.
The team measured how much dust was left behind years after the crash.
- The Result: They found a surprisingly small amount of dust. In fact, the dust mass was 1 to 100,000 times smaller than the total amount of gas they calculated was ejected.

3. The Big Discovery: The "Soot" Problem

This is the most interesting part of the paper. The team realized that while a lot of gas was thrown out (like a massive cloud of smoke), only a tiny fraction of that gas actually turned into solid dust (like a few specks of soot).

The Analogy: Imagine a massive bonfire. You throw a whole log into the fire, and it explodes into a huge cloud of smoke and ash. If you only collect the ash, you might think you only threw in a tiny twig. But the paper shows that the "smoke" (gas) was huge, and the "ash" (dust) was just a tiny leftover.

They also found that the dust wasn't just forming from the crash itself; it seemed to be interacting with "pre-existing" dust that was already floating around the stars before the crash. It's like the crash happened in a room that was already dusty, and the explosion just stirred it up and made it glow brighter.

4. The Conclusion

The paper successfully combined three different ways of looking at the problem (computer models, energy math, and dust counting) to get a clear picture of these cosmic events.

The Stars: They were massive, unstable giants in a binary pair.
The Crash: They merged after a chaotic, unstable spill of gas.
The Aftermath: A massive amount of gas was ejected, but very little of it turned into dust.

The authors admit their methods have limits (like assuming the stars are perfect spheres, which they aren't), but their combined approach gives the best estimate we have so far for how these stellar mergers work. They didn't find a way to use this for anything practical on Earth; it's purely about understanding how stars live, die, and crash into each other in the universe.

Technical Summary: Comparative Study of Two Luminous Red Novae (I. Progenitor Modeling and Dust Formation)

Problem Statement
Luminous Red Novae (LRNe) are astrophysical transients associated with unstable mass transfer in binary systems, often leading to common-envelope (CE) evolution and stellar mergers. While these events are linked to the formation of compact binaries and gravitational wave sources, the physical properties of their progenitors and the mass budgets of their ejecta remain poorly constrained. Previous studies have often treated the precursor, outburst, and remnant phases in isolation or relied on single-star models. A unified analysis connecting binary progenitor properties to the observed transient and remnant, while explicitly incorporating mass-transfer instability criteria, has been lacking. This study addresses the specific cases of AT2021biy and AT2021blu, aiming to constrain their binary progenitor masses and orbital parameters, estimate the ejected gas mass, and infer the dust mass formed in the remnant phase.

Methodology
The authors employed a multi-faceted approach combining stellar evolution modeling, energy-budget formalism, light-curve analysis, and infrared dust modeling:

Progenitor Modeling (MESA): Two grids of binary stellar evolution tracks were generated using the MESA code (v11554). The models were constrained by pre-outburst photometry from the Hubble Space Telescope (AT2021biy) and ground-based/ZTF/Pan-STARRS data (AT2021blu). The study explicitly applied mass-transfer instability criteria (Quasi-Adiabatic, Outer Lobe Overflow, and Dynamical) to select progenitors capable of merging on timescales consistent with archival data. The models assumed fully non-conservative mass transfer, with half the mass lost via isotropic re-emission and half via a circumbinary toroid.
Energy-Budget Estimation: Based on the MESA models, the authors estimated the envelope mass ejected during the CE phase. They calculated both lower and upper limits by comparing the orbital energy released during inspiral (from the onset of Outer Lobe Overflow to the merger of the companion with the donor's helium core) against the binding energy of the envelope and the bulk kinetic energy of the ejecta.
Light-Curve Comparison: The energy-budget estimates were compared against predictions from various analytical and semi-analytical light-curve models (e.g., Kasen & Woosley, Matsumoto & Metzger) to assess consistency across different physical assumptions (recombination, shock interaction).
Dust Mass Inference: Mid-infrared (NEOWISE) and Spitzer data were modeled using the radiative transfer code dusty (via pydusty) to derive dust masses and temperatures up to ~3 years post-outburst. This provided an independent lower limit on the total ejecta mass.

Key Results

Progenitor Constraints: The study constrained the donor masses to $M_d = 18 - 23 M_\odot$ for AT2021biy and $M_d = 14 \pm 0.5 M_\odot$ for AT2021blu. The viable progenitors were found to have high mass ratios ( $q \gtrsim 3$ for AT2021biy; $q \gtrsim 10$ for AT2021blu) and initial orbital separations of $300-700 R_\odot$ and $200-300 R_\odot$ , respectively.
Ejecta Mass Estimates:
- Energy Budget: The lower-limit estimates for the ejected envelope mass range from $0.03 - 2.98 M_\odot$ (AT2021biy) and $0.02 - 0.14 M_\odot$ (AT2021blu). Upper limits span $1.72 - 7.82 M_\odot$ and $1.8 - 3.8 M_\odot$ , respectively.
- Consistency: Comparison with light-curve models revealed the highest degree of consistency within the intermediate mass-ratio regime ( $3 \lesssim q \lesssim 10$ for AT2021biy and $5 < q \lesssim 15$ for AT2021blu).
Dust Formation: Measured dust masses are 1–5 orders of magnitude lower than the estimated ejected envelope masses, indicating that only a small fraction of the expelled gas condenses into dust. The dust mass evolution is consistent with shock-driven interactions with pre-existing circumstellar material (CSM), supporting the MESA prediction of significant pre-outburst mass loss.
Sensitivity: The inferred ejecta masses are highly sensitive to the assumed ejecta velocity. The authors note that adopting a velocity-stratified profile (where inner layers move slower than outer layers) reduces the bulk kinetic energy, potentially increasing the lower-limit mass estimates by up to a factor of two compared to single-velocity assumptions.

Significance and Claims
The paper claims to present the first unified study connecting the binary progenitor, the outburst, and the remnant of LRNe while explicitly incorporating mass-transfer instability criteria into the modeling. By narrowing the parameter space through instability constraints (specifically the Outer Lobe Overflow criterion), the authors provide tighter constraints on progenitor masses and mass ratios than previous single-star or population synthesis studies.

The study highlights that the "delayed dynamical instability" observed in massive donors (where mass transfer runs away after a period of quasi-stable overflow) is consistent with the detection of slow-moving gas in the vicinity of these events. Furthermore, the discrepancy between the large ejected gas masses and the relatively small dust masses reinforces the view that LRNe are inefficient dust factories, with the observed dust likely forming in pre-existing CSM rather than the bulk of the merger ejecta.

The authors acknowledge limitations, including the assumption of spherical symmetry in MESA, the point-mass approximation for the companion, and the assumption of perfect energy conversion efficiency ( $\alpha_{CE}=1$ ). They posit that their estimates serve as indicative values for the ideal case and that future work incorporating 3D hydrodynamic simulations and self-consistent companion evolution will be necessary to refine these mass estimates.

1. The Setup: A Tangled Dance

2. The Investigation: Reconstructing the Crash

3. The Big Discovery: The "Soot" Problem

4. The Conclusion

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