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Magnetic-fluid experiment sheds light on astrophysical accretion discs

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Researchers in the US have designed an experiment that attempts to simulate the complex dynamics of astrophysical accretion discs more closely than ever before. Yin Wang and colleagues at Princeton University did this by adapting previous experimental techniques to avoid unwanted flows in their simulated disc, while more closely representing the magneto-rotational instability that is believed to emerge in real accretion discs.

Accretion discs are swirling vortices of matter that form as massive objects such as black holes and newly forming stars gather gas and dust from their interstellar surroundings. The influx of this material leads to planet formation and produces the intense radiation that is emitted from the vicinity of some black holes.

For gas and dust to move nearer to the massive object, it must transfer angular momentum to the outer edge of the disc – and an explanation of how this happens has eluded astronomers. One leading theory is that this transfer is driven by turbulent flows in the disc. To explore this idea, previous studies have used a Taylor Couette setup in which a fluid fills the gap between two concentric cylinders that can be rotated independently.

Astrophysics in the lab

By rotating the outer cylinder more slowly than the inner cylinder, and carefully controlling their respective motions, researchers can closely recreate the motions of evolving accretion discs as closely as possible. Their aim here is to determine whether turbulent flows could really be responsible for their angular momentum transfer.

However, beyond the clear limitation that these motions are not driven by gravity, the fluid must also be contained vertically by upper and lower caps. This introduces secondary flows to the fluid, with no analogue in real accretion discs. One recent study done in Paris reduced the influence of these unwanted flows by applying a vertical magnetic field to a liquid metal disc – more closely recreating the electrical conductivity of real accretion discs. However, the Parisian team did not fully recreate the desired turbulent flows.

One possible driver for turbulence in accretion discs is magneto-rotational instability (MRI): which could better explain how a differentially-rotating, electrically conducting fluid can be destabilized by a magnetic field. This concept has been widely studied theoretically, but still hasn’t been confirmed in Taylor Couette experiments because of difficulties in setting the appropriate parameters.

Conductive liquid

Wang’s team have addressed this challenge by using a fluid called galinstan, which is a liquid alloy of gallium, indium and tin that is about twice as viscous as water, and some 100 million times more conductive of electricity. To eliminate secondary flows, they also implemented a pair of electrically conducting caps, which rotated independently at speeds intermediate to the inner and outer cylinders.

As they applied a vertical magnetic field along the cylinders’ rotation axis, the researchers measured the fluid’s magnetic Reynolds number, which characterizes how a magnetic field interacts with a conducting fluid. Crucially, they observed this value passing a certain threshold: beyond which the strength of the magnetic field passing through the inner cylinder began to increase nonlinearly – indicating that MRI had been triggered.

Simulations have also been able to reproduce this behaviour, so the team’s observations are an important step forward in researchers’ ability to reproduce accretion disc dynamics in real experiments; and ultimately, in answering the long-standing mystery surrounding the transfer of angular momentum in accretion discs.

The research is described in Physical Review Letters.

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