Scientists compute death throes of white dwarf star in 3D (Forwarded)

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March 22, 2007

Scientists compute death throes of white dwarf star in 3D

University of Chicago scientists will demonstrate how to incinerate a white
dwarf star in unprecedented detail at the "Paths to Exploding Stars"
conference on Thursday, March 22, in Santa Barbara, Calif.

White dwarf stars pack one and a half times the mass of the sun into an
object the size of Earth. When they burn out, the ensuing explosion produces
a type of supernova that astrophysicists believe manufactures most of the
iron in the universe. But these type Ia supernovas, as they are called, may
also help illuminate the mystery of dark energy, an unknown force that
dominates the universe.

"That will only be possible if we can gain a much better understanding of
the way in which these stars explode," said Don Lamb, Director of the
University of Chicago's Center for Astrophysical Thermonuclear Flashes. The
Flash Center has been simulating exploding stars since 1997 with funding
from the Office of Advanced Simulation and Computing of the National Nuclear
Security Administration.

Scientists for years have attempted to blow up a white dwarf star by writing
the laws of physics into computer software and then testing it in
simulations. At first the detonations would only occur if inserted manually
into the programs. Then the Flash team naturally detonated white dwarf stars
in simplified, two-dimensional tests, but "there were claims made that it
wouldn't work in 3D," Lamb said.

But in January, the Flash Center team for the first time naturally detonated
a white dwarf in a more realistic three-dimensional simulation. The
simulation confirmed what the team already suspected from previous tests:
that the stars detonate in a supersonic process resembling diesel-engine
combustion.

Unlike a gasoline engine, in which a spark ignites the fuel, compression
triggers ignition in a diesel engine. "You don't want supersonic burning in
a car engine, but the triggering is similar," said Dean Townsley, a Research
Associate at the Joint Institute for Nuclear Astrophysics at Chicago.

The temperatures attained by a detonating white dwarf star makes the
10,000-degree surface of the sun seem like a cold winter day in Chicago by
comparison. "In nuclear explosions, you deal with temperatures on the order
of a billion degrees," said Flash Center Research Associate Cal Jordan.

The new 3D white dwarf simulation shows the formation of a flame bubble near
the center of the star. The bubble, initially measuring approximately 10
miles in diameter, rises more than 1,200 miles to the surface of the star in
one second. In another second, the flame crashes into itself on the opposite
end of the star, triggering a detonation. "It seems that the dynamics of the
collision is what creates a localized compression region where the
detonation will manifest," Townsley said.

This process plays out in no more than three seconds, but the simulations
take considerably longer. The Flash Center team ran its massive simulation
on two powerful supercomputers at Lawrence Livermore and Lawrence Berkeley
national laboratories in California. Just one of the jobs ran for 75 hours
on 768 computer processors, for a total of 58,000 hours.

"I cannot say enough about the support we received from the high-performance
computing teams at Lawrence Livermore and Lawrence Berkeley national
laboratories," Lamb said. "Without their help, we would never have been able
to do the simulations."

The simulations are so demanding -- the Flash team calls it "extreme
computing" -- that they monopolize powerful computers of the national lab
during the allocated time. To ensure that these computers are used to their
maximum potential, the Flash team stands on alert to rapidly correct any
glitches that may arise.

"We have it set up so that if something goes wrong, text messages are sent
out instantaneously to everyone," said Flash Center Research Scientist
Robert Fisher. "It's like being a doctor on call 24/7."

But the scientific payoff for logging these long, stressful hours is
potentially huge. Astrophysicists value type Ia supernovas because they all
seem to explode with approximately the same intensity. Calibrating these
explosions according to their distance reveals how fast the universe has
been expanding at various times during its long history.

In the late 1990s, supernova measurements revealed that the expansion of the
universe is accelerating. Not knowing what force was working against gravity
to cause this expansion, scientists began calling it "dark energy." The
Flash Center simulations may help astrophysicists make better calibrations
to adjust for the minor variation that they believe occurs from one
supernova to the next.

"To make extremely precise statements about the nature of dark energy and
cosmological expansion, you have to be able to understand the nature of that
variation," Fisher said.

Telescopic images of the two supernovas closest to Earth seem match the
Flash team's findings. The images of both supernovas show a sphere with a
cap blown off the end.

"In our model, we have a rising bubble that pops out of the top. It's very
suggestive," Jordan said.

Support for the simulations to be presented at Santa Barbara was provided by
two separate DOE programs: the Advanced Simulation and Computing program of
the National Nuclear Security Administration, which has provided funding and
computer time to the Flash Center for nearly a decade, and INCITE
(Innovative and Novel Computation Impact on Theory and Experiment) of the
Office of Science, which has provided computer time.

To view the conference program, see
http://www.kitp.ucsb.edu/activities/...e/conf/?id=910

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