When Discs Dance: How Misaligned Binary Stars Create Unusual Spiral Arms
In this study, Sahl Rowther and collaborators explore how misaligned circumbinary discs, discs of gas and dust orbiting around two stars, can produce unusual spiral structures. Using advanced three-dimensional computer simulations, they focus on “nearly broken” discs, where an inner disc and outer disc are tilted relative to each other but still connected. In this configuration, the connection points (called nodes) can launch leading spiral arms, spirals that point in the opposite direction from more typical, trailing spirals. These leading spirals are stationary relative to the stars, disappearing when the disc either becomes completely aligned or fully breaks apart.
Methods
To run these experiments, the authors used a hydrodynamics code called Phantom, with some simulations also coupled to mcfost, a radiative transfer code that models the effect of starlight on the disc’s temperature. They examined multiple setups: a standard “fiducial” disc model, a higher-resolution version to check numerical accuracy, models that included the disc’s own gravity, and both high- and low-mass discs. The simulations showed that the key ingredient for leading spirals was a moderate misalignment between the inner and outer discs, too much or too little tilt, and the spirals vanished.
How Leading Spirals Form
The results reveal that the spirals form due to angular momentum transfer between the connected inner and outer discs. This transfer excites radial motions at the two connecting nodes, creating the leading spirals in the outer disc (and corresponding trailing spirals in the inner disc). Unlike spirals formed by planets or gravitational instability, these spirals originate from a source that does not rotate with the disc. When the authors removed the inner disc in a test run, the spirals quickly disappeared, confirming its vital role in sustaining them.
Independence from Disc Physics
Interestingly, the team found that leading spirals appeared regardless of whether they included realistic heating from starlight or the disc’s self-gravity, showing that the effect is independent of detailed disc physics. In low-mass discs, the misalignment evolves more slowly, leading to a pattern where the spirals come and go over time. When realistic temperature effects were added in these lighter discs, shadows cast by the tilted inner disc could also launch trailing spirals, particularly at high misalignments when the discs were disconnected. This means that both leading and trailing spirals can coexist and interact.
Comparison with Previous Work
Rowther’s work also compares these results with earlier studies. Leading spirals may have been missed before due to lower simulation resolution, shorter runtimes, or disc setups that prevented their formation. Some past simulations do show hints of these structures, suggesting they might be more common than recognized. The study also clarifies how shadow-driven trailing spirals seen in previous research differ from the leading spirals found here, the latter depend on direct physical connection between inner and outer discs, while the former can arise purely from temperature differences caused by shadows.
Observational Implications
From an observational perspective, the findings imply that spiral arm direction alone cannot reveal a disc’s rotation, since both leading and trailing spirals can occur. Misaligned discs could thus produce large-scale spirals and gaps without the need for massive planets or gravitational instability. For more massive discs where dust is well-coupled to the gas, these features might even be visible in high-resolution observations, paired with shadows from the inner disc in scattered light images.
Conclusion
In short, the paper uncovers a new spiral-forming mechanism in binary star systems, one that turns out to be a subtle dance between the inner and outer disc, producing patterns unlike those caused by planets or instability.
Source: Rowther
