The key to solving a longstanding mystery about thin gas disks rotating around young stars: the motion of a tiny number of charged particles. This is according to a new study from the California Institute of Technology (Caltech).
These rotating gas disks, called accretion disks, last tens of millions of years and are an early phase of solar system evolution. 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 spirals slowly inward toward the star. Astrophysicists recognized long ago that when this inward spiraling transpires, it should cause the radially inner part of the disk to spin increasingly faster, according to the law of the conservation of angular momentum. To understand the basic idea of the conservation of angular momentum, think of spinning figure skaters: when their arms are outstretched, they spin slowly, but as they draw their arms in, they spin faster and faster.
The law of angular momentum conservation states that the angular momentum mystery in a system stays constant, and angular momentum is proportional to velocity times radius. Therefore, if the skater’s radius decreases mystery because they have pulled their arms in, then the only way to keep angular momentum constant is to increase the spin velocity. The inward spiral motion of the accretion disk is analogous to a skater drawing their arms in and as such, the inner part of the accretion disk should spin faster. mystery Astronomical observations do indeed show that the inner part of an accretion disk does spin faster. Curiously, however, it does not spin as fast as predicted by the law of conservation of angular momentum. Scientists have investigated many possible explanations for why accretion disk angular momentum is not conserved over the years. Some hypothesized that friction between the inner and outer rotating parts of the accretion disk might slow down the inner region. Calculations, however, demonstrate that accretion disks have very little internal friction. According to the dominant current hypothesis, magnetic fields cause a phenomenon known as a “magnetorotational instability” that results in the production of magnetic turbulence and gas—effectively forming friction that slows down the rotational speed of inward spiraling gas.
“That concerned me,” says Paul Bellan, professor of applied physics at Caltech. “People always want to blame turbulence for phenomena they do not understand. There’s a big cottage industry right now arguing that turbulence accounts for getting rid of angular momentum in accretion disks.” A decade and a half ago, Bellan began investigating the question by analyzing the trajectories of individual atoms, electrons, and ions in the gas that constitutes an accretion disk. His goal was to determine how the individual particles in the gas behave when they collide with each other, as well as how they move in between collisions, to see if angular momentum loss could be explained without invoking turbulence.
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