'Zombie Vortices' May Solve Key Mystery Regarding Star Formation

Researchers have identified a possible solution to a mystery that has long frustrated researchers in regards to the formation of stars. Called "zombie vortices," the phenomenon described in the study published in Physical Review Letters explains how variations in gas density led to the instability necessary for generating the whirlpool-like vortices necessary for stars to develop.

Astronomers agree that during the first phase of star gestation, dense clouds of gas collapse into clumps that then spin into a disk with the help of angular momentum. What is unclear, however, is how these disks eventually lose this momentum -- a necessary step in the development of a protostar.

The most popular proposal cites magnetic fields as the force responsible for slowing the disk down, thereby allowing the mass to then spiral inward onto the protostar.

However, this theory is not without its problems. For example, such a scenario would require gas to be ionized, or charged with a free electron, if it is to interact with the magnetic field. The problem is that many regions in a protoplanetary disk are simply too cold for ionization to occur.

"Current models show that because the gas in the disk is too cool to interact with magnetic fields, the disk is very stable," Philip Marcus, a computational physicist who led the research, explained. "Many regions are so stable that astronomers call them dead zones -- so it has been unclear how disk matter destabilizes and collapses onto the star."

Furthermore, Marcus added, current models fail to take into account changes in a protoplanetary disk's gas density based on its height.

And herein lies the key, according to the researchers, who found that when they accounted for density change in their computer models, 3D vortices emerged in the protoplanetary disk, which then gave way to more vortices -- all of which eventually led to the disruption of the disk's angular momentum.

"Because the vortices arise from these dead zones, and because new generations of giant vortices march across these dead zones, we affectionately refer to them as 'zombie vortices,'" Marcus said. "Zombie vortices destabilize the orbiting gas, which allows it to fall onto the protostar and complete its formation."

While promising, work on the theory is not complete. Going forward, Marcus plans on collaborating with other researchers to study the development and effects of zombie vortices in an even more realistic model of the disks that surround protostars.

"Other research teams have uncovered instabilities in protoplanetary disks, but part of the problem is that those instabilities required continual agitations," said Richard Klein, an adjunct professor of astronomy and theoretical astrophysicist at the Lawrence Livermore National Laboratory. "The nice thing about the zombie vortices is that they are self-replicating, so even if you start with just a few vortices, they can eventually cover the dead zones in the disk."