🔭 astrophysics

Circumbinary disk formation through AGB star winds

Through smoothed-particle hydrodynamics simulations, this study demonstrates that wind-binary interactions, particularly wind Roche-lobe overflow driven by slow winds and massive companions, naturally generate diverse circumbinary disks around post-AGB binaries, with their morphology and evolution critically dependent on binary parameters like eccentricity and mass ratio.

Original authors: Shunquan Huang, Rebecca G. Martin

Published 2026-07-07

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CC BY 4.0

Original authors: Shunquan Huang, Rebecca G. Martin

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 dying star, an "Asymptotic Giant Branch" (AGB) star, nearing the end of its life. It's like a giant, bloated balloon that is slowly leaking air (stellar wind) into space. Usually, this wind just blows away into the void. But what happens if this dying star has a partner—a companion star orbiting nearby?

This paper uses computer simulations to figure out how that wind interacts with the partner star and whether it can build a giant, spinning ring of gas (a disk) around both of them. Think of it as trying to build a sandcastle while a hurricane is blowing, but the "sand" is gas and the "wind" is coming from a dying star.

Here is the breakdown of what the researchers found, using simple analogies:

1. The Two Main Scenarios: The "Fast Wind" vs. The "Slow Leak"

The outcome depends entirely on two things: how fast the wind is blowing and how heavy the companion star is.

Scenario A: The Fast Wind / Light Partner (The "Spiral Garden Hose")
If the dying star blows a very fast wind, or if the partner star is relatively light, the wind moves too quickly to be caught. The companion star acts like a rock in a fast-flowing river. It doesn't stop the water; it just deflects it.
- The Result: The wind creates a beautiful, swirling spiral pattern (like a garden hose spinning on the lawn), but the gas mostly escapes into space. No stable disk forms. The gas is "unbound," meaning it has enough energy to fly away forever.
Scenario B: The Slow Wind / Heavy Partner (The "Roof Gutter")
If the wind is slower and the companion star is heavy (massive), the situation changes. The companion is so heavy that its gravity grabs the slow-moving wind before it can speed up.
- The Result: The wind gets trapped, forming a small, spinning disk around the companion star (like water swirling around a drain). This small disk acts as a bridge. It grabs the wind, spins it up, and then flings it outward through a specific "gate" (called the L2 point) into a much larger ring that surrounds both stars. This is how a circumbinary disk is born.

2. The Shape of the Disk: The "Eccentricity" Factor

Once the big ring forms, its shape depends on how the two stars orbit each other.

Circular Orbits (The "Round Track"): If the two stars orbit in a perfect circle, the resulting gas disk is relatively round and grows steadily outward, like a ripple in a pond.
Eccentric Orbits (The "Egg-Shaped Track"): If the stars orbit in a stretched-out, egg-shaped path, the disk becomes wild.
- When the stars get close to each other, they kick the gas out at high speeds.
- This creates a disk that is stretched out (eccentric) and sits further away from the stars. It's like throwing a ball in a circle vs. throwing it in a long, oval arc; the oval arc lands much further out.

3. The Weight of the Companion: The "Vacuum Cleaner" Effect

The researchers also tested what happens if the companion star is very light.

Heavy Companion: Acts like a powerful vacuum cleaner. It sucks up the wind efficiently, builds a strong inner ring, and quickly feeds the outer ring. The outer ring becomes dense and grows fast.
Light Companion: Acts like a weak vacuum. It struggles to catch the wind. The outer ring forms much more slowly and is much thinner (less dense).
The Surprise: Even with a very weak companion, a large, wide ring can still form eventually. It just takes longer and is made of thinner gas. This suggests that you don't need a "perfect" setup to get a big ring; even a messy, low-density start can grow into a massive structure over time.

4. How the Ring Moves: The "Traffic Jam"

The paper also looked at how the gas moves inside these rings. Usually, we think of friction (viscosity) as the thing that moves gas around. However, the simulations showed that the gas isn't moving because of simple friction.

Instead, the movement is driven by shockwaves and spiral arms created by the binary stars themselves. Imagine a crowded dance floor where people aren't just sliding past each other; they are bumping into each other, creating waves of movement that push the crowd around. The "bumps" (shocks) and the "waves" (spirals) generated by the two stars are the main engines moving the gas, not just simple friction.

Summary

In short, this paper explains that when a dying star blows wind on a partner star:

Fast wind/Light partner = A spiral pattern that flies away (no disk).
Slow wind/Heavy partner = A small ring forms around the partner, which then feeds a giant ring around both stars.
The shape of the stars' orbit determines if the giant ring is round or stretched out.
The mass of the partner determines how thick and fast the ring grows, but even a light partner can eventually build a huge ring.

The study confirms that these interactions are a natural way for the universe to create the beautiful, spinning disks of gas we see around many dying binary star systems.

Technical Summary: Circumbinary Disk Formation through AGB Star Winds

Problem Statement
Circumbinary disks are frequently observed around post-asymptotic giant branch (post-AGB) star binaries, yet the mechanisms governing their formation and long-term evolution remain poorly understood. While previous studies have explored mass transfer via common-envelope phases or Roche-lobe overflow (RLOF) in semi-detached systems, the specific role of wind-binary interactions in generating these disks over long timescales has not been fully characterized. Existing simulations often run for fewer than $\sim100$ orbital periods, limiting their ability to capture the secular evolution of disk morphology. Furthermore, the transition between different mass transfer regimes—specifically Bondi-Hoyle (BH) accretion versus wind Roche-lobe overflow (WRLOF)—and how binary parameters (eccentricity and mass ratio) influence the resulting disk structure requires systematic investigation.

Methodology
The authors employ smoothed-particle hydrodynamics (SPH) simulations using the phantom code to model the interaction between an AGB star's wind and a binary companion. The system consists of an AGB star ( $M_1$ ) and a companion ( $M_2$ ), treated as sink particles with defined accretion radii ( $R_{s,1} = 1.0$ AU, $R_{s,2} = 0.4$ AU). The simulations are based on the AC Her system parameters but are scaled to be scale-free depending on the specific energy of injected particles.

Key methodological features include:

Wind Injection: Gas particles are injected at $R_1 = 1.1$ $R_{1} = 1.1$ AU from the AGB star. The study investigates two injection regimes defined by the specific energy ratio $\eta$ $η$ :
- $\eta > 1$ : Fast winds where all particles are unbound (BH regime).
- $\eta < 0$ : Slow winds where particles are bound, leading to WRLOF.
- $0 \leq \eta \leq 1$ : Intermediate regimes.
Injection Geometry: Both uniform and Gaussian (equatorially biased) injection distributions are tested, with the Gaussian model adopted for detailed disk studies to enhance resolution near the mid-plane.
Thermodynamics: A local isothermal equation of state is primarily used ( $H/R = 0.1$ ), with a comparative simulation using a binary isothermal equation of state to test thermal assumptions.
Viscosity: An effective Shakura-Sunyaev viscosity parameter of $\alpha \sim 0.1$ is imposed, though the study explicitly analyzes whether physical structures (shocks/spirals) dominate angular momentum transport over this imposed viscosity.
Duration: Simulations run for over 600 binary orbital periods ( $\sim4500$ years), significantly exceeding the duration of prior works.

Key Results
The simulations reveal that the morphology of the outflow is determined by the interplay between wind velocity and companion mass, categorized by the specific energy ratio $\eta$ :

Bondi-Hoyle Regime (Fast Winds/Low Mass Companions): When $\eta \gg 1$ , the wind remains largely unbound. The companion gravitationally focuses the wind, creating a persistent spiral density pattern. The gas flow is dominated by radial velocity, and material tends to escape the binary system directly without forming a stable circumbinary disk.
WRLOF Regime (Slow Winds/High Mass Companions): When $\eta < 0$ (or intermediate), the slow, dense wind fills the donor's Roche lobe. A circumsingle disk forms around the companion, significantly enhancing the effective cross-section for mass capture. This disk facilitates efficient angular momentum transfer, allowing material to escape through the outer Lagrange point ( $L_2$ ) and accumulate into a coherent circumbinary disk.
Impact of Binary Eccentricity ( $e_b$ ):
- Circular/Mildly Eccentric ( $e_b = 0, 0.2$ ): Disks form at small radii ( $\sim10$ AU) and expand outward. The disk maintains moderate eccentricity ( $e_d \approx 0.2$ ) which gradually circularizes over time due to viscous dissipation competing with binary torques.
- Highly Eccentric ( $e_b = 0.4$ ): The companion approaches the AGB star closely at periastron, allowing a large fraction of material to be injected directly into highly eccentric circumbinary orbits. This results in a disk concentrated at larger semi-major axes ( $\sim30$ AU) with high eccentricity ( $e_d \approx 0.6$ ).
Impact of Mass Ratio ( $q$ ): Lower mass ratios (lighter companions) reduce the efficiency of wind capture and disk formation. While the resulting circumbinary disks are less dense and grow more slowly, they can still form extended structures. Notably, even when the circumsecondary disk is not resolved (in the lowest $q$ case), an extended, nearly circular circumbinary disk still develops, suggesting that high initial density is not strictly required for wide disk formation.
Angular Momentum Transport: Analysis of the effective stress ( $\alpha_{eff}$ ) indicates that angular momentum transport within the circumbinary disk is dominated by spiral structures and shocks generated by the binary-wind interaction. These physical stresses are comparable to or exceed the imposed artificial viscosity, implying that the imposed viscosity does not dominate the disk's evolution.

Significance and Claims
The paper claims that wind-binary interactions, particularly in the WRLOF regime, can naturally generate the diverse morphologies of circumbinary disks observed in AGB and post-AGB systems without requiring a common-envelope phase. The study demonstrates that:

Long-term simulations are necessary to capture the transition from spiral structures to stable disks and to observe the secular evolution of disk eccentricity and extent.
Binary parameters (eccentricity and mass ratio) sensitively control the disk's radial extent and eccentricity, offering a potential explanation for the variety of observed post-AGB disk structures.
The angular momentum of the resulting circumbinary disk is supplied entirely by the companion, naturally leading to co-planar disk formation.

The authors explicitly note that while their model explains co-planar disks, it does not account for the polar-aligned circumbinary disks observed in systems like AC Her, leaving the origin of such polar angular momentum as an open question. Additionally, the study acknowledges that simplifications in thermodynamics (isothermal assumption) and the neglect of explicit dust-radiation coupling may influence detailed evolution, though the global disk formation mechanism is expected to remain robust.

1. The Two Main Scenarios: The "Fast Wind" vs. The "Slow Leak"

2. The Shape of the Disk: The "Eccentricity" Factor

3. The Weight of the Companion: The "Vacuum Cleaner" Effect

4. How the Ring Moves: The "Traffic Jam"

Summary

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