Brett Smith for redOrbit.com – Your Universe Online
For years, most astronomers have agreed on the basic steps that lead to star formation, except one – how a cloud of swirling gas can slow down enough to concentrate into something capable of nuclear fusion.
A new study from the University of California, Berkeley has found evidence of “zombie vortices” within a gestating star leading to a final push that gives birth to a new star.
According to prevailing theories, stars begin as dense clouds of gas that slowly collapse into clumps. These clumps begin to spin into one or more disks, referred to as protostars. For a protostar to become larger, the disk has to lose some of its spin so that the gas can spiral inward, eventually creating enough mass to ignite through a nuclear reaction.
“After this last step, a star is born,” said Philip Marcus, a professor in the Department of Mechanical Engineering and co-author of the new Berkeley study published in the journal Physical Review Letters.
In the study, the Berkeley researchers focused on how the spinning disk of gas loses its angular momentum in order to kick off this last step. One theory posits that magnetic forces destabilize the disks enough to slow momentum. However, the Berkeley team asserts that the gas needs to be charged to interact with a magnetic field and parts of a protoplanetary disk are too cold to accept a charge.
“Current models show that because the gas in the disk is too cool to interact with magnetic fields, the disk is very stable,” Marcus said. “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.”
Unlike the newly developed Berkeley model, prevailing models do not account for changes in a disk’s gas density based upon its height, the researchers said.
“This change in density creates the opening for violent instability,” said co-author Pedram Hassanzadeh, a geophysical fluid dynamics expert currently with Harvard University.
When the Berkeley team accounted for density change in their computer models, vortices emerged within the disk. These vortices spawned more vortices, culminating in 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.”
The researchers noted that these types of vortices are already found throughout nature, from Jupiter’s Great Red Spot to tornadoes that are spun off violent storms.
The Berkeley researchers said they plan to apply their findings to more detailed computer models that include velocities, temperatures and densities of known protostar disks.
“Other research teams have uncovered instabilities in protoplanetary disks, but part of the problem is that those instabilities required continual agitations,” said Richard Klein, a theoretical astrophysicist at the Berkeley’s Lawrence Livermore National Laboratory who is working with the study researchers on the next round of models. “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.”