A remarkable amount of science consists of theorists saying, “Hey, we should see this thing!” and observers either peering through the sky or searching through data archives to say “yay” or “nay” to those theorists.

The nays are the much more interesting situations.

For instance, at the freshman physics level of explanation, solar systems form as a cloud of material collapses and spins faster and faster. It is completely straightforward, totally makes sense, and predicts the inner regions of solar systems should rotate far faster than we observe. We can see this happening in our solar system, and we can see it in the gas motions of young solar systems still forming.

And we don’t generally have a “graduate school” level of explanation that can explain why things rotate slowly. Somehow, somewhere in that mix of gas, dust, light, and energy, some force or friction is dumping angular momentum and allowing for the slow roll of systems like ours.

The leading excuse for this behavior has been magnetic fields causing turbulence, and magnetic fields are complicated enough that most folks aren’t wanting to do the maths and modeling needed to confirm or deny this idea. But one researcher was willing to look deeper. Paul Bellen explains: 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.

Working from first principles with graduate student Yang Zhang, Bellen has run massive computer simulations that look at all the interactions in an accretion disk, including charged particles and neutral atoms, to see how they respond to magnetic and gravitational forces. Bellen explains further: This model had just the right amount of detail to capture all of the essential features because it was large enough to behave just like trillions upon trillions of colliding neutral particles, electrons, and ions orbiting a star in a magnetic field.

And in this massive simulation, they found, and I quote from the Caltech press release: …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 (electrons) spiral outward toward the edge. Neutral particles, meanwhile, lose angular momentum and, like the positively charged ions, spiral inward to the center.

This means that particles of different charges act in different ways, and using straightforward, freshman physics, angular momentum doesn’t work. Instead, a more complex “canonical angular momentum” theory has to be used that takes into account charge and the effects of electromagnetism. According to Bellen: …the inward motion of cations and outward motion of electrons results in the disk becoming something like a gigantic battery with a positive terminal near the disk center and a negative terminal at the disk edge. Such a battery would drive electric currents that flow away from the disk both above and below the plane of the disk. These currents would power astrophysical jets that shoot out from the disk in both directions along the disk axis. Indeed, jets have been observed by astronomers for over a century and are known to be associated with accretion disks, though the force behind them has long been a mystery.

And now, we understand the jets, the slow roll of solar systems, and so much more because two people were willing to work the maths, write the simulations, and publish their work in The Astrophysical Journal.

This is your reminder that there are still questions waiting to be answered that just require someone to be willing to do the detailed work that most people don’t want to do.

More Information

Caltech press release

“Neutral-charged-particle Collisions as the Mechanism for Accretion Disk Angular Momentum Transport,” Yang Zhang and Paul M. Bellan, 2022 May 17, The Astrophysical Journal