‘Zombie vortices’ may be key step in star formation

Aug. 20, 2013 — A new theory by fluid dynamics experts at the University of California, Berkeley, shows how “zombie vortices” help lead to the birth of a new star.Reporting Aug. 20 in the journal Physical Review Letters, a team led by computational physicist Philip Marcus shows how variations in gas density lead to instability, which then generates the whirlpool-like vortices needed for stars to form.Astronomers accept that in the first steps of a new star’s birth, dense clouds of gas collapse into clumps that, with the aid of angular momentum, spin into one or more Frisbee-like disks where a protostar starts to form. But for the protostar to grow bigger, the spinning disk needs to lose some of its angular momentum so that the gas can slow down and spiral inward onto the protostar. Once the protostar gains enough mass, it can kick off nuclear fusion.”After this last step, a star is born,” said Marcus, a professor in the Department of Mechanical Engineering.What has been hazy is exactly how the cloud disk sheds its angular momentum so mass can feed into the protostar.Destabilizing forcesThe leading theory in astronomy relies on magnetic fields as the destabilizing force that slows down the disks. One problem in the theory has been that gas needs to be ionized, or charged with a free electron, in order to interact with a magnetic field. However, there are regions in a protoplanetary disk that are 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,” said Marcus. “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.”The researchers said current models also fail to account for changes in a protoplanetary disk’s gas density based upon its height.”This change in density creates the opening for violent instability,” said study co-author Pedram Hassanzadeh, who did this work as a UC Berkeley Ph.D. student in mechanical engineering. When they accounted for density change in their computer models, 3-D vortices emerged in the protoplanetary disk, and those vortices spawned more vortices, leading to the eventual disruption of the protoplanetary 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,'” said Marcus. …

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