'Zombie vortices' key to new star's birth

Last Updated: Wed, Aug 21, 2013 08:40 hrs

Fluid dynamics experts have brought forward a new theory that shows how 'zombie vortices' help in star formation.

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.

Marcus, a professor in the Department of Mechanical Engineering, said that after this last step, a star is born.

The 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.

Marcus said that current models show that because the gas in the disk is too cool to interact with magnetic fields, the disk is very stable, asserting that 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.

Study co-author Pedram Hassanzadeh, who did this work as a UC Berkeley Ph.D. student in mechanical engineering, said that this change in density creates the opening for violent instability.

He said that 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.

Marcus said that 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.

He said that zombie vortices destabilize the orbiting gas, which allows it to fall onto the protostar and complete its formation.

The study has been published in the journal Physical Review Letters. (ANI)

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