🔭 astrophysics

Simulating the convection in red super-giant stars: wobbling jets in common envelope evolution

Using a three-dimensional red supergiant model, this study demonstrates that vigorous envelope convection induces stochastic fluctuations in the angular momentum of accreted mass, leading to the formation of intermittent accretion disks and wobbling jets around compact companions, thereby supporting the hypothesis that such jets are a crucial component of common envelope evolution.

Original authors: Shlomi Hillel (Technion, Israel), Ron Schreier (Technion, Israel), Noam Soker (Technion, Israel)

Published 2026-07-15

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Shlomi Hillel (Technion, Israel), Ron Schreier (Technion, Israel), Noam Soker (Technion, Israel)

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 giant, bloated star—a Red Super-Giant—swelling to a size so vast it would swallow our entire solar system. Inside this cosmic balloon, the gas isn't sitting still; it's churning like a pot of boiling soup, with massive bubbles of hot gas rising and cold gas sinking in a wild, chaotic dance. Now, picture a smaller, dense companion star (like a neutron star or a black hole) spiraling inward through this turbulent soup.

For a long time, scientists thought that as this small star spiraled in, it would scoop up gas and spin it into a neat, flat disk, launching a straight, steady jet of material like a laser beam shooting out from a spinning top. But this new study suggests that reality is much messier, wilder, and more "wobbly."

The Main Discovery: The Wobble
The researchers built a super-detailed 3D computer model of a Red Super-Giant to see exactly what happens to the gas the companion star tries to eat. They found that the gas the companion grabs isn't just moving in a smooth circle. Because the giant star's outer layers are churning with violent convection (that boiling soup effect), the gas hits the companion from all sorts of random directions.

Think of it like trying to catch a ball while standing in a hurricane. Sometimes the wind blows the ball from the left, sometimes from the right, sometimes from above. The study found that these random "wind gusts" from the star's convection are so strong that they can shake the direction of the gas being swallowed by two to three times more than the steady spin caused by the orbit itself.

The Result: A Wobbling Jet
Because the gas is being grabbed from such chaotic directions, the disk that forms around the companion star doesn't stay flat and steady. Instead, it wobbles. Consequently, the jets launched from this disk don't shoot out in a straight line. They wobble back and forth, changing their aim constantly.

The paper calculates that for a companion star orbiting at a distance of 700 R⊙ (solar radii), the gas can have a specific angular momentum (a measure of how much "spin" it has) that fluctuates wildly. In some moments, the random spin from the convection is so strong that it completely flips the direction of the gas flow, turning the jet upside down relative to the orbit.

What This Means for the Universe
The authors suggest that these wobbly jets might explain some of the weird, lumpy shapes we see in the clouds of gas ejected by dying stars (planetary nebulae). Instead of perfect, symmetrical bubbles, we might see "medium-size bubbles" and twisted arcs because the jets were drilling through the star's envelope in a zig-zag pattern, punching holes in different directions.

What the Paper Rules Out (and What It Doesn't)
The study explicitly argues against the idea that these jets are always steady, straight beams. They rule out the notion that the gas flow is smooth and predictable. However, they also admit that their results are based on simulations, not direct observations of a real event happening right now. They did not include the gravity of the companion star or the energy the jets themselves might dump back into the star in this specific run, so they warn that their numbers are an approximation.

They also note that while these wobbly jets are great at forming disks around tiny, dense objects like neutron stars (which are only about 1.4 M⊙ or 1.4 times the mass of our Sun), they might struggle to form stable disks around larger, main-sequence stars. For those bigger stars, the disk might not have enough time to settle down before the direction of the gas changes again.

The Bottom Line
In short, the universe is less like a precision machine and more like a chaotic dance floor. When a small star spirals into a giant, it doesn't just spin a perfect jet; it gets tossed around by the giant's boiling atmosphere, creating a jet that wobbles, flips, and punches holes in the star's envelope in a messy, unpredictable, but fascinating way. The authors suggest that to truly understand these cosmic explosions, we need to stop looking for straight lines and start looking for the wobble.

Technical Summary: Simulating Convection in Red Supergiant Stars and Wobbling Jets in Common Envelope Evolution

Problem Statement
The study addresses the dynamics of mass accretion onto a compact companion (neutron star, black hole, or main-sequence star) spiraling within the envelope of a red supergiant (RSG) during Common Envelope Evolution (CEE). A critical unresolved issue is the source and magnitude of the specific angular momentum of the accreted mass. While orbital motion and radial density gradients provide a fixed-axis angular momentum component, the contribution of vigorous envelope convection introduces a stochastic component. The authors investigate whether the stochastic angular momentum from convection is sufficient to form accretion disks and how its random direction affects the morphology of jets launched by the companion. The study operates under the premise that the "standard" CEE includes jets launched by the companion, a mechanism crucial for explaining the morphologies of many planetary nebulae and post-CEE systems.

Methodology
The authors utilize a newly constructed three-dimensional (3D) hydrodynamical model of an RSG star, based on the code FLASH with a Cartesian grid. The model, derived from a 15 $M_{\odot}$ zero-age-main-sequence star evolved via MESA, incorporates two specific numerical ingredients to mimic realistic stellar behavior:

Photospheric Emission Mimicry: Grid cells with densities below the initial photospheric density ( $\rho_{ph} \approx 2 \times 10^{-9}$ g cm $^{-3}$ ) are cooled to 1000 K to simulate radiative cooling at the surface.
Nuclear Energy Production Mimicry: Energy is deposited in a shell ( $201 R_{\odot} < r < 230 R_{\odot}$ ) to match the luminosity of the 1D model ( $L \approx 7 \times 10^4 L_{\odot}$ ), sustaining stellar pulsation.

The simulation considers an undisturbed, non-rotating RSG model without the gravitational influence or energy deposition of the companion, isolating the convective properties of the envelope. The authors calculate the specific angular momentum of mass accreted by a companion (mass $M_2$ ) orbiting at radii $a = 450 R_{\odot}$ and $a = 700 R_{\odot}$ . They decompose the total specific angular momentum ( $\vec{j}_{acc}$ ) into two components:

Fixed-axis component ( $\vec{j}_O$ ): Arising from the orbital motion and the radial density gradient, calculated using Bondi-Hoyle-Lyttleton (BHL) accretion theory with a density gradient parameter $\eta = 0.1$ .
Stochastic component ( $\vec{j}_R$ ): Calculated by summing the angular momentum of convective blobs within a spherical volume ( $R_{acc} = 0.5 R_{BHL}$ to $0.75 R_{BHL}$ ) centered on the companion's orbit over the accretion timescale.

Key Results

Amplitude of Fluctuations: The stochastic angular momentum component ( $\vec{j}_R$ ) exhibits large fluctuations. In the realistic simulation with energy deposition, the amplitude of these fluctuations reaches approximately 2 to 3 times the magnitude of the fixed-axis component ( $\vec{j}_O$ ).
Directional Variability: The direction of the total specific angular momentum ( $\vec{j}_{acc}$ ) varies significantly. The random component frequently causes the total angular momentum vector to deviate from the orbital plane, with the angle $\alpha_w$ varying widely. In many instances, the stochastic component is large enough to effectively "flip" the direction of the accreted angular momentum relative to the orbital axis.
Disk Formation:
- Compact Companions (NS/Black Holes): The total specific angular momentum is easily sufficient to form intermittent accretion disks around neutron stars and black holes, even with the stochastic variations.
- Main-Sequence Companions: The angular momentum is only marginally sufficient, or insufficient, to form stable accretion disks around main-sequence stars. Disks may form only during brief periods of large $j_R$ fluctuations.
Timescales: The fluctuation period of the angular momentum direction is on the order of 19 to 35 days, depending on the orbital radius and accretion radius. This is comparable to or shorter than the viscous relaxation time of the outer accretion disk, suggesting the disk may not have time to relax between fluctuations.

Significance and Claims
The paper claims that the convection-driven stochastic angular momentum is a dominant factor in the CEE accretion process, often exceeding the contribution from orbital motion. The primary implications are:

Wobbling Jets: Because the accretion disk's angular momentum vector fluctuates rapidly and significantly, the jets launched by the companion will not be aligned with a single symmetry axis. Instead, they will be "wobbling," launching in varying directions.
Jet Interaction with Envelope: These wobbling jets are less likely to drill a straight path through the envelope compared to fixed-axis jets. However, the random orientation increases the probability that one jet in a pair will be launched radially outward, potentially breaking out of the envelope and shaping the ejected mass into complex structures (arcs, filaments, and medium-sized bubbles) rather than simple bipolar lobes.
Feedback Mechanism: The study supports the "jetted mass removal accretion scenario," where jets remove high-entropy gas, facilitating further accretion. The wobbling nature of these jets complicates the feedback cycle, as jets may heat regions of the envelope not yet reached by the companion or break out asymmetrically.
Standard CEE Definition: The authors reiterate that the standard CEE model must include jets launched by the companion during the Grazing Envelope Evolution (GEE), the CEE itself, and/or the exit phase. This study provides a physical basis for the variability of these jets, emphasizing that the "wobbling" is an intrinsic consequence of envelope convection.

The study concludes that while full feedback simulations including the companion's gravity and jet injection are computationally intensive and left for future work, the current results strongly support the necessity of including wobbling jets in models of CEE and GEE to explain the observed morphologies of interacting binary systems and planetary nebulae.

More like this