Astronomy

From: http://www.ornl.gov/info/press_releases/get_press_release.cfm?ReleaseNumber=mr20070105-00

OAK RIDGE, Tenn., Jan. 5, 2007 — A team of scientists using Oak Ridge National Laboratory supercomputers has discovered the first plausible explanation for a pulsar's spin that fits the observations made by astronomers. Anthony Mezzacappa of the Department of Energy lab's Physics Division and John Blondin of North Carolina State University explain their results in the Jan. 4 issue of the journal Nature. According to three-dimensional simulations they performed at the Leadership Computing Facility, located at ORNL, the spin of a pulsar is determined not by the spin of the original star, but by the shock wave created when the star's massive iron core collapses.

ORNL team discovers new way to spin up pulsars

This visualization shows the progression of spiral formation in a supernova, which eventually results in a pulsar's spin. The darkest portion of the accretion shock denotes the front edge of the wave as it rotates around the supernova's center. Three-dimensional computer models are the only models that show this effect. This 3D model of pulsar formation was performed at DOE's Leadership Computing Facility at Oak Ridge National Laboratory.

That shock wave is inherently unstable, a discovery the team made in 2002, and eventually becomes cigar-shaped instead of spherical. The instability creates two rotating flows—one in one direction directly below the shock wave and another, inner flow, that travels in the opposite direction and spins up the core.

"The stuff that's falling in toward the center, if it hits this shock wave that is not a sphere any more but a cigar-shaped surface, will be deflected," Mezzacappa said. "When you do this in 3-D, you find that you wind up with not only one flow, but two counterrotating flows."

The asymmetrical flows establish a "sloshing" motion that, in the complex 3-D models, accounts for the pulsars observed spin velocities from once every 15 to 300 milliseconds, which is much slower than previous models predicted.

Previously, astronomers did not have a workable explanation for how the pulsar gets its spin. The assumption to this point has been that the spin of the leftover collapsed core comes from the spin of the original star. Being much smaller, the pulsar would then spin much faster than the original star, just as a figure skater spins faster by pulling his or her arms in.