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High mass accretion rates onto evolved stripped-envelope massive stars by jet-induced mass removal

This study demonstrates that jet-induced mass removal from the outer inflated envelopes of accreting stripped-envelope stars significantly suppresses stellar expansion, enabling these stars to maintain deep potential wells and power intermediate-luminosity optical transients like luminous red novae.

Original authors: Yotham Cohen, Ealeal Bear, Noam Soker

Published 2026-03-02

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CC BY 4.0

Original authors: Yotham Cohen, Ealeal Bear, Noam Soker

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 Star That Refuses to Get Fat

Imagine a star as a person trying to eat a massive buffet. Usually, if you force-feed a person too much food too quickly, they get huge, their stomach expands, and they become sluggish. In the world of stars, when a star "eats" (accretes) a lot of mass quickly, it swells up into a giant red balloon. This is bad news for the physics of the situation because a swollen star has a weak gravitational grip, making it hard to keep the "eating" process going or to generate the massive energy explosions astronomers are trying to explain.

This paper proposes a clever solution: The Star Diet.

The authors suggest that if a star is being fed while simultaneously having its "fat" (outer layers) shaved off by powerful jets, it can eat a huge amount of food without getting fat. This allows it to stay compact, powerful, and ready to launch a spectacular explosion.

The Characters in Our Story

The Star (The Wolf-Rayet): Think of this star as a bodybuilder who has already stripped off their skin and fat. It's a dense, hot, "stripped-envelope" star (specifically a Wolf-Rayet star). It's small and tough.
The Food (The Accretion Disk): Another star or cloud of gas is dumping a massive amount of hydrogen-rich gas onto our bodybuilder. This happens through a spinning disk of gas, like a conveyor belt feeding the star.
The Jets (The Shaving Machine): As the gas spins into the star, it launches powerful jets of energy (like twin fire hoses shooting out the top and bottom). These jets don't just push gas away; they act like a high-precision laser trimmer, slicing off the outer, puffy layers of the star that are trying to expand.

The Problem: The "Swelling" Trap

In the past, scientists simulated what happens when a star eats too much.

The Old Way: You feed the star. The star gets hot, expands, and puffs up like a balloon.
The Result: Because the star is now huge and puffy, its gravity is weak. It can't hold onto the new gas well, and it can't generate the intense energy needed to power the bright flashes of light (transients) we see in the sky. It's like trying to run a marathon while wearing a 500-pound backpack.

The Solution: The "Pulse and Shave" Method

The authors simulated a new scenario they call the "Jetted Mass Removal Accretion Scenario."

Instead of just feeding the star, they simulated a cycle:

Feed: Pour a bunch of gas onto the star.
Shave: Immediately use the jets to blast away a significant chunk of that new gas (and the outer layers of the star).

The Analogy:
Imagine you are filling a bucket with water (the star).

Normal scenario: You turn on the hose full blast. The bucket overflows, and the water spills everywhere. The bucket gets heavy and unstable.
This paper's scenario: You turn on the hose, but at the same time, you have a powerful vacuum cleaner sucking out the top 80% of the water.
The Result: The bucket stays relatively small and stable, but the flow of water is massive. The energy released by the water rushing in and being blasted out is enormous.

What They Found

By running these simulations on a computer (using a code called MESA), they discovered:

Staying Small: Even though the star was eating at a very high rate, the "shaving" jets kept the star from expanding. Instead of growing to the size of a giant red balloon, it stayed relatively small (only expanding by a factor of 2 to 5, rather than 10 or more).
Deep Gravity Well: Because the star stayed small, it kept a "deep gravity well." This means it has a strong gravitational pull, which allows it to release a massive amount of energy as it eats.
The Power Source: This setup creates the perfect conditions for Intermediate-Luminosity Optical Transients (ILOTs). These are cosmic fireworks—bright flashes of light that are brighter than a normal star but dimmer than a supernova. The paper suggests these flashes are powered by the collision of the star's jets with the surrounding gas.

Why Does This Matter?

For decades, astronomers have seen these bright flashes in the sky (like Luminous Red Novae) and wondered, "What powers them?"

Some thought it was just two stars crashing together.
This paper argues that it's a star eating a massive meal while being trimmed by jets.

This is a "positive feedback loop." The jets help the star stay small, which keeps the gravity strong, which allows the jets to keep launching, which keeps the star small. It's a self-sustaining engine for cosmic explosions.

The Takeaway

The universe is full of stars that are trying to eat too much too fast. This paper shows that if those stars have a "diet plan" involving powerful jets that trim their outer layers, they can stay compact, powerful, and capable of putting on a spectacular light show for the rest of the galaxy to see.

In short: Jets don't just blow things away; they act as a cosmic diet plan, keeping stars lean enough to power the universe's most dazzling fireworks.

1. Problem Statement

The paper addresses a critical bottleneck in modeling high-mass accretion onto evolved, stripped-envelope massive stars (specifically Wolf-Rayet or WR stars).

The Expansion Problem: When a non-degenerate star accretes mass at high rates, it typically expands significantly. This expansion deepens the star's potential well, reducing the gravitational energy released per unit of accreted mass. If the star expands too much, it may prevent the formation of an accretion disk or shut down the accretion process entirely.
The Feedback Gap: While negative feedback (jets expelling mass and reducing accretion) is well-studied in other contexts (e.g., galaxy clusters), the authors investigate a positive feedback mechanism. They propose that jets launched by an accretion disk can remove the high-entropy outer layers of the accreting star simultaneously with mass addition. This removal prevents the star from expanding, maintaining a deep potential well and allowing the accretion process to continue at high rates, thereby powering energetic transient events.
Specific Context: Previous studies focused on main-sequence stars. This paper extends the investigation to evolved stripped-envelope stars (WR stars), which are relevant to the powering of Intermediate-Luminosity Optical Transients (ILOTs) and Luminous Red Novae (LRNe).

2. Methodology

The authors utilize the Modules for Experiments in Stellar Astrophysics (MESA) code (version 24.03.1) in single-star mode to simulate a three-stage evolutionary process.

Stage A1: Pre-Common Envelope Evolution

A $20 M_\odot$ Zero-Age Main Sequence (ZAMS) star is evolved until it becomes a Red Supergiant (RSG) with a Carbon-Oxygen core.
Standard stellar winds are applied (Dutch scaling factor 0.8).
The stage ends at $9 \times 10^6$ years with a radius of $773.2 R_\odot$ and mass of $17.16 M_\odot$ .

Stage A2: Stripping the Envelope (Binary Interaction Mimicry)

The authors manually remove the hydrogen-rich envelope and a significant portion of the helium envelope to mimic a Common Envelope Evolution (CEE) event where a companion spirals in.
Result: A hydrogen-deficient WR star is formed with a mass of $\approx 6.03 M_\odot$ and a compact radius of $0.67 R_\odot$ .

Stage A3: Jet-Induced Mass Removal Accretion

The Pulse Method: To mimic the physical process where an accretion disk adds mass at the equator while jets remove mass from the poles/outer envelope, the accretion is divided into hundreds of discrete pulses.
Pulse Mechanics:
- Phase 1 (Addition): Mass is added to the star (simulating disk accretion).
- Phase 2 (Removal): A fraction ( $\alpha$ ) of the added mass is removed from the outer, high-entropy layers (simulating jet-induced mass loss).
Parameters: The simulations vary the net accretion rate, the number of pulses ( $N_p$ ), and the removal efficiency ratio $\alpha = \dot{M}_{rm} / \dot{M}_{add}$ .
Comparison: Results are compared against control simulations (labeled with subscript '0') where mass is added continuously without removal.

3. Key Contributions

Extension to Evolved Stars: This is the first study to apply the "jetted mass removal accretion scenario" to evolved, stripped-envelope (WR) stars, rather than just main-sequence stars.
Quantification of Expansion Suppression: The paper quantifies how removing a fraction of accreted mass (specifically high-entropy outer layers) drastically reduces stellar expansion.
Disk Survival Analysis: The authors demonstrate that even as the star inflates slightly, the density of the outer envelope remains low enough that the accretion disk can survive and continue launching jets, creating a self-sustaining positive feedback loop.

4. Key Results

Suppression of Expansion:
- In control simulations (no mass removal), accreting $\approx 0.015 M_\odot$ caused the star to expand from $0.67 R_\odot$ to $10 R_\odot$ .
- With jet-induced mass removal (where $\alpha$ ranges from 0.6 to 0.9), the expansion was reduced by a factor of 2 to 5.
- For the highest efficiency case ( $\alpha = 0.9$ ), the final radius was only $\approx 2.76 R_\odot$ (compared to $10 R_\odot$ without removal).
Entropy Profiles:
- Mass addition creates a high-entropy outer layer.
- Mass removal (simulating jets) strips this high-entropy zone, causing the star to contract back toward its initial radius. This cycle prevents the star from reaching a state where the potential well is too shallow to support high-energy accretion.
Energy Release:
- By maintaining a smaller radius ( $R \sim 1-2 R_\odot$ ), the star retains a deep gravitational potential well.
- The accretion power ( $\dot{E}_{acc}$ ) is calculated to be $\approx 6.6 \times 10^6 L_\odot$ , which is tens of times the star's intrinsic luminosity.
- This excess energy is released via the collision of jets with circumstellar material, potentially powering luminous transients.
Disk Stability:
- Density profiles show that the inflated envelope has low density ( $\rho \sim 10^{-6} \text{ g cm}^{-3}$ ), comparable to the accretion disk density. This confirms that the disk can penetrate the outer envelope without being destroyed, allowing the jet mechanism to persist.

5. Significance and Implications

Powering ILOTs and LRNe: The results strongly support the hypothesis that Intermediate-Luminosity Optical Transients (ILOTs) and Luminous Red Novae can be powered by the accretion of a non-degenerate star (specifically a WR star in this model) via an accretion disk. The jet-induced mass removal mechanism solves the "expansion problem" that previously made such high-accretion scenarios difficult to sustain.
Bipolar Morphology: The model naturally explains the observed bipolar ejecta in these transients, as the energy release is driven by jets rather than spherical explosions.
Binary Evolution: The study provides a viable evolutionary pathway for systems involving massive stars and companions, suggesting that mass transfer onto stripped cores can be highly energetic and transient-producing.
Theoretical Validation: The work validates the "positive feedback" cycle where jets facilitate further accretion by preventing the donor/accretor from expanding, a mechanism crucial for understanding the most energetic non-supernova stellar transients.

In summary, the paper demonstrates that jet-induced mass removal is a critical regulatory mechanism that allows evolved massive stars to accrete at high rates without expanding uncontrollably, thereby enabling the release of massive amounts of gravitational energy required to power luminous optical transients.