Oak Ridge National Laboratory team discovers new way to spin uppulsars (Forwarded)

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Jan. 5, 2007

ORNL team discovers new way to spin up pulsars
Written by Leo Williams

OAK RIDGE, Tenn. -- 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.

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.

The problem with that approach is that it would explain only the fastest
observed pulsars. The ORNL-NCSU team, on the other hand, predicts spin
periods that are in the observed range between 15 and 300 milliseconds.

The work was funded under the DOE Office of Science's Scientific Discovery
through Advanced Computing, or SciDAC, program.

"Our discovery came at a critical time," Mezzacappa noted. "It came at a
time when there was no description in the literature of how neutron stars
are spun up and, therefore, how pulsars are born, that are consistent with
observation. It was a crisis, if you will. Now our simulations come along
and provide a way around that conundrum."

The discovery is an outgrowth of the team's use of three-dimensional
simulations and the advances in high-performance computing that made the
simulations possible. The simulations performed for the Nature paper used
the Cray X1E system at ORNL, known as Phoenix. That system boasts a peak
performance of more than 18 teraflops and is currently the fastest vector
computer in the United States. Later simulations done by the team made use
of the center's Jaguar system, a Cray XT3 with a peak performance of more
than 50 teraflops.

The team used the VH1 code, developed by Blondin when he was a
postdoctoral research associate at the University of Virginia, and moving
the simulation data was problematic. Mezzacappa noted that researchers are
able now to perform visualizations remotely, without having to move the
data off site, but at the time of their early three-dimensional
simulations this capability was not in place.

He stressed also that the team is looking forward to further advances in
high-performance computing that will be coming to ORNL. For example, the
team's simulations have not incorporated the influence of nearly massless,
radiation-like particles known as neutrinos and the star's magnetic field.

The real prize, though, for his and other teams is a complete explanation
of how the collapse of a star's core leads to the explosion that ejects
most of its layers. So far, that explanation has proved elusive.

"In a nutshell, this rapid advance in supercomputing technology will give
us the tools to solve this problem and to make these important predictions
and to understand these events and their role in our universe. This is a
very, very exciting and very satisfying thing," Mezzacappa said.

Oak Ridge National Laboratory is managed by UT-Battelle for the Department
of Energy.

IMAGE CAPTION:
[http://www.ornl.gov/info/press_relea...s/pulsar72.jpg (12KB)]
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.