UNITED STATES: The movement of a small number of charged particles holds the key to unlocking a long-standing puzzle about thin gas discs orbiting around nascent stars. A recent California Institute of Technology (Caltech) study supports this.
These long-lasting rotating gas discs, also known as accretion discs, constitute a formative stage in the solar system’s development. Imagine a Saturn-like ring the size of the solar system; they are made up of a tiny portion of the star’s mass. Due to the way the gas in these discs spirals progressively inward toward the star, they are known as accretion discs.
Astrophysicists have known for a long time that this inward spiralling should result in the disk’s radially inner portion spinning faster and faster due to the conservation of angular momentum. Think of spinning figure skaters to grasp the fundamental concept of the conservation of angular momentum: when their arms are spread, they spin slowly, but as they pull their arms in, they spin faster and faster.
According to the law of angular momentum conservation, angular momentum in a system is proportional to velocity times radius and remains constant. The only option to maintain angular momentum is to increase spin velocity if the skater’s radius drops due to drawing their arms in.
The inner portion of the accretion disc should spin more quickly because the inward spiral motion of the accretion disc is comparable to a skater pulling their arms in. According to astronomical studies, the inner region of an accretion disc actually spins more quickly. Interestingly, it does not spin as quickly as the law of conservation of angular momentum predicts.
Angular momentum of the accretion disc
Over the years, researchers have looked at a wide range of potential reasons why the angular momentum of the accretion disc is not conserved.
Some suggested that the accretion disc’s inner section would slow down due to friction between the inner and outer revolving regions. However, calculations show that accretion discs’ internal friction is relatively low.
The central current hypothesis states that magnetic fields result in a “magnetorotational instability” phenomenon that generates magnetic turbulence and gas, thereby generating friction that reduces the rotational speed of inward spiralling gas.
Paul Bellan, a professor of applied physics at Caltech, adds, “That worried me. People frequently seek to attribute turbulence to events they do not fully comprehend. There is a sizable cottage business currently contending that angular momentum loss in accretion discs is caused by turbulence.”
Bellan started looking into the issue 15 years ago by tracing the motions of individual atoms, electrons, and ions in the gas that makes up an accretion disc. To investigate if angular momentum loss could be
described without mentioning turbulence, he set out to understand how the individual particles in the gas behave when they clash with one another and how they travel in the intervals between collisions.
Charged particles (i.e., electrons and ions) are affected by both gravity and magnetic fields. In contrast, neutral atoms are only affected by gravity, he explained over the years in a series of papers and lectures that were focused on “first principles”—the fundamental behaviour of the constituent parts of accretion discs. He believed that this distinction was crucial.
Yang Zhang, a graduate student at Caltech, attended one of those seminars after learning how to simulate molecules colliding to produce the random distribution of velocities in common gases, such as the air we breathe, in a course. After the discussion, Zhang adds, “I spoke with Paul about it, and we debated it, and we came to the conclusion that the simulations might be expanded to include charged particles striking neutral particles in magnetic and gravitational fields.”
In the end, Bellan and Zhang developed a computer simulation of a rotating, fragile accretion disc. Around 40,000 neutral and 1,000 charged particles that may collide were present in the simulated disc, and the model also took the effects of gravity and a magnetic field into account.
“Because it was massive enough,” according to Bellan. “It behaved much like trillions upon trillions of colliding neutral particles, electrons, and ions orbiting a star in a magnetic field. The model had just the perfect degree of detail to capture all of the crucial properties,” he added.
The computer simulation demonstrated that positively charged ions, or cations, would spiral inward toward the disk’s centre. In contrast, negatively charged particles (electrons) would curve outward toward the edge in collisions between neutral atoms and a far lower number of charged particles. While the positively charged ions spiral inward to the centre, neutral particles lose angular momentum.
Although “canonical angular momentum” is maintained, the underlying physics at the subatomic level—mainly, the interaction between charged particles and magnetic fields—shows that angular momentum is not conserved in the classical sense.
The sum of the initial ordinary angular momentum and an additional quantity influenced by the charge on a particle and the magnetic field is known as canonical angular momentum. Canonical angular momentum is unnecessary to worry about because ordinary angular momentum and canonical angular momentum are identical for neutral particles.
However, because the added magnetic amount is so tremendous for charged particles like cations and electrons, the canonical angular momentum differs significantly from the ordinary angular momentum.
The inward motion of ions and the outward motion of electrons, which are brought about by collisions, increase the canonical angular momentum of both since electrons are negative and cations are positive. Collisions between the neutral and charged particles cause the neutral particles to lose angular momentum and travel inward, which cancels out the rise in the canonical angular momentum of the charged particles.
Bellan contends that this subtle accounting satisfies the law of conservation of canonical angular momentum for the sum of all particles in the entire disc; only about one in a billion particles need to be charged to explain the observed loss of angular momentum of the neutral particles. This seemingly insignificant distinction, he claims, makes a significant difference on a solar system-wide scale.
Bellan adds that the disc resembles a massive battery with a positive terminal in the disc centre and a negative terminal at the disc periphery due to the inward motion of cations and the outward motion of electrons. Such a battery would propel electric currents that flow away from the disc from above and below its plane.
Astrophysical jets that emerge from the disc in both directions along the disc axis would be propelled by these currents. Accretion discs are associated with jets, which have been spotted by astronomers for more than a century, though their origin has long remained a mystery.
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