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Why doesn’t the interior of the solar system rotate faster? An old mystery has a possible new solution

Why doesn't the interior of the solar system rotate faster?  An old mystery has a possible new solution

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The movement of a small number of charged particles could solve a long-standing mystery about thin gas disks revolving around young stars, according to a new study from Caltech.

These features, called accretion disks, last for tens of millions of years and are an early phase in the evolution of the solar system. They contain a small fraction of the mass of the star around which they swirl; imagine a Saturn-like ring as big as the solar system. They are called accretion disks because the gas in these disks slowly spins inward toward the star.

Scientists realized long ago that when this inward spiral occurs, it should spin the radially inner part of the disk faster, according to the law of conservation of angular momentum. To understand conservation of angular momentum, think of spinning figure skaters: when their arms are extended, they spin slowly, but when they retract their arms, they spin faster.

The angular momentum is proportional to the velocity multiplied by the radius, and the law of conservation of angular momentum states that the angular momentum in a system remains constant. So, if the skater’s radius decreases because he has retracted his arms, the only way to keep the angular momentum constant is to increase the rotational speed.






Simulation results. (a), (b) The trajectories of the particles of a system with ions and electrons. (c), (d) The trajectories of the particles of a reference system having only neutrals. (e), (f) The neutral radial drift velocity profile and the ion and electron density fraction of the system in (a), (b). Credit: The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac62d5

The inward spiraling motion of the accretion disk is akin to a skater tucking in their arms – and as such, the inner part of the accretion disk should spin faster. Indeed, astronomical observations show that the inner part of an accretion disk rotates faster. Curiously, however, it does not spin as fast as predicted by the law of conservation of angular momentum.

Over the years, researchers have investigated many possible explanations for why the angular momentum of the accretion disk is not conserved. Some thought that the friction between the inner and outer rotating parts of the accretion disk could slow down the inner region. However, calculations show that accretion disks have negligible internal friction. The main current theory is that magnetic fields create what is called “magnerotational instability” which generates gaseous and magnetic turbulence, thereby forming friction which slows the rate of rotation of the inwardly spiraling gas.

“It worried me,” says Paul Bellan, professor of applied physics. “People always want to blame turbulence for phenomena they don’t understand. There’s a big cottage industry right now that argues that turbulence explains the suppression of angular momentum in accretion disks.”

A decade and a half ago, Bellan began to study the question by analyzing the trajectories of atoms, electrons and individual ions in the gas which constitutes an accretion disk. His goal was to determine how individual particles in the gas behave when they collide, as well as how they move between collisions, to see if the loss of angular momentum can be explained without invoking turbulence.

As he has explained over the years in a series of articles and lectures focusing on “first principles” – the fundamental behavior of the building blocks of accretion disks – charged particles (i.e. electrons and ions) are affected by both gravity and magnetic fields. , while neutral atoms are only affected by gravity. This difference, he suspected, was the key.

Yang Zhang, a Caltech graduate student, attended one of these conferences after taking a course in which he learned how to create simulations of molecules as they collide with each other to produce the random distribution of velocities in ordinary gases, such as the air we breathe. “I approached Paul after the conference, we discussed it and finally decided that the simulations could be extended to charged particles colliding with neutral particles in magnetic and gravitational fields,” Zhang said.

Ultimately, Bellan and Zhang created a computer model of a spinning, superthin virtual accretion disk. The simulated disk contained about 40,000 neutral particles and about 1,000 charged particles that could collide with each other, and the model also took into account the effects of gravity and a magnetic field. “This model had just the right amount of detail to capture all the essential features,” says Bellan, “because it was large enough to behave like billions and billions of neutral particle, electron, and electron collisions. ions orbiting a star in a magnetic field.”

Computer simulation showed that collisions between neutral atoms and a much smaller number of charged particles would result in positively charged ions, or cations, spiraling inward toward the center of the disk, while negatively charged particles ( electrons) coil outwards towards the edge. Neutral particles, on the other hand, lose their angular momentum and, like positively charged ions, spin towards the center.

Careful analysis of the underlying physics at the subatomic level – in particular, the interaction between charged particles and magnetic fields – shows that angular momentum is not conserved in the classical sense, although something called “angular momentum canonical” is indeed preserved.

The canonical angular momentum is the sum of the original ordinary angular momentum plus an additional quantity that depends on the charge of a particle and the magnetic field. For neutral particles, there is no difference between ordinary angular momentum and canonical angular momentum, so worrying about canonical angular momentum is unnecessarily complicated. But for charged particles – cations and electrons – the canonical angular momentum is very different from the ordinary angular momentum because the additional magnetic quantity is very large.

Because electrons are negative and cations are positive, the inward motion of ions and the outward motion of electrons, which are caused by collisions, increase the canonical angular momentum of both. Neutral particles lose angular momentum as a result of collisions with charged particles and move inwards, compensating for the increased canonical angular momentum of charged particles.

It’s a small distinction, but one that makes a huge difference on a solar system scale, says Bellan, who argues that this subtle accounting satisfies the canonical law of conservation of angular momentum for the sum of all particles in the entire disk; only about one particle in a billion needs to be charged to explain the observed loss of angular momentum of neutral particles.

Also, says Bellan, the inward movement of the cations and the outward movement of the electrons causes the disk to become something like a gigantic battery with a positive terminal near the center of the disk and a negative terminal at the edge. of the disc. Such a battery would drive electric currents away from the disk both above and below the plane of the disk. These currents would power astrophysical jets that shoot out of the disk in both directions along the axis of the disk. Indeed, the jets have been observed by astronomers for over a century and are known to be associated with accretion disks, although the force behind them has long been a mystery.

Bellan and Yang’s article was published in The Astrophysical Journal May 17.


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More information:
Yang Zhang et al, Neutral Charged Particle Collisions as an Accretion Disc Angular Momentum Transport Mechanism, The Astrophysical Journal (2022). DOI: 10.3847/1538-4357/ac62d5

Provided by California Institute of Technology

Quote: Why doesn’t the interior of the solar system rotate faster? Ancient Mystery Has New Possible Solution (2022, July 6) Retrieved July 7, 2022 from https://phys.org/news/2022-07-solar-faster-mystery-solution.html

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