A new study from Caltech suggests that the motion of a few charged particles may solve a longstanding mystery.
The accretion disks are an early phase of the evolution of the solar system. Imagine a ring as large as the solar system if they contained a small fraction of the mass of the star around which they swirl. The gas in accretion disks is slowly moving towards the star.
When this inward spiraling occurs, it should cause the radially inner part of the disk to spin faster. When a skater's arms are extended, they spin slowly, but as they draw their arms in, they spin faster.
The law of angular momentum conserves that the momentum in a system stays the same. Increasing the spin velocity is the only way to keep the skater's radius constant.
The inner part of the accretion disk should spin faster because it's like a skater drawing their arms in. An accretion disk's inner part does spin faster according to astronomy. It does not spin as fast as it could.
Researchers have looked at many possible explanations for why accretion disk inertia is not conserved. The inner region of the accretion disk may be slowed down by some thought. The calculations show that accretion disks don't have much internalfriction. Magnetic fields create what is called a "magnetorotational instability" that causes gas and magnetic turbulence, which slows down the speed of inward spiraling gas.
Professor of applied physics Paul Bellan was concerned by that. People want to blame turbulence for things they don't know. There's a cottage industry that believes turbulence is the reason for the disappearance of angular momentum.
Bellan began investigating the question by analyzing the trajectory of individual atoms, electrons, and ion in the gas that constitutes an accretion disk. To determine how the individual particles in the gas behave when they collide with each other, as well as how they move in betweencollisions, was his goal.
He explained in a series of papers and lectures that charged particles are affected by both gravity and magnetic fields, whereas neutral atoms are not. He thought the difference was important.
Caltech graduate student YangZhang attended one of those talks after taking a course in which he learned how to create simulations of molecule as they collide with each other to produce the random distribution of velocities in ordinary gases After talking to Paul, I decided that the simulations might be extended to charged particles colliding with neutral particles.
Bellan andZhang created a computer model of a spinning disk. The model factored in the effects of gravity and a magnetic field as well as the 40,000 neutral and 1,000 charged particles in the simulation disk. Bellan says that the model had just the right amount of detail to capture all of the essential features.
The simulations showed that neutral atoms and a small number of charged particles would cause positive and negative cations to spiral inward towards the center of the disk. The neutral particles spiral inward to the center.
A careful analysis of the underlying physics at the subatomic level shows that the interaction between charged particles and magnetic fields is not a classical one.
There is an additional quantity that is dependent on the charge on a particle and the magnetic field. There is no difference between the two for neutral particles. The extra magnetic quantity is different for charged particles.
Because electrons are negative and cations are positive, the inward motion of ion and outward motion of electrons increases. When neutral particles collide with charged particles and move inward, there is an increase in the charge of the particles.
It's a small difference, but it makes a huge difference on a solar system-wide scale.
Bellan says that the disk becomes a gigantic battery with a positive terminal near the disk center and a negative terminal at the edge because of the inward motion of cations and electrons. There would be electric currents flowing away from the disk both above and below it. Astronomers have observed jets for over a century and they are thought to be associated with accretion disks.
Their paper was published in The Astrophysical Journal.
More information: Yang Zhang et al, Neutral-charged-particle Collisions as the Mechanism for Accretion Disk Angular Momentum Transport, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac62d5 Journal information: Astrophysical Journal
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