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 mill
New research from Caltech proposes a new solution to a longstanding mystery about thin gas disks rotating around young stars.
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.
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.
As he explained over the years in a series of papers and lectures that were focused on “first principles”—the fundamental behavior of the constituent parts of accretion disks—charged particles are affected by both gravity and magnetic fields, whereas neutral atoms are only affected by gravity. This difference, he suspected, was key.
The computer simulation showed collisions between neutral atoms and a much smaller number of charged particles would cause positively charged ions, or cations, to spiral inward toward the center of the disk, while negatively charged particles spiral outward toward the edge. Neutral particles, meanwhile, lose angular momentum and, like the positively charged ions, spiral inward to the center.
Because electrons are negative and cations are positive, the inward motion of ions and outward motion of electrons, which are caused by collisions, increases the canonical angular momentum of both. Neutral particles lose angular momentum as a result of collisions with the charged particles and move inward, which balances out the increase in the charged-particle canonical angular momentum.
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