Astrophysicists from Los Alamos
National Laboratory, New Mexico, have created the first 3-D computer
simulations of the spectacular explosion that marks the death of a
massive star. Presented to the American Astronomical Society meeting
in Albuquerque, N.M., today, the research by Michael Warren and Chris
Fryer eliminates some of the doubts about earlier 2-D modeling and
paves the way for rapid advances on other, more exotic questions
about supernovae.

“Modeling the collapse of a massive star represents one of the
greatest challenges in computational physics,” Warren said. “All
four fundamental forces of nature are in play, giving us a cosmic
laboratory with conditions unlike anywhere else in the Universe.
Only if we truly understand the fundamental physics involved and
do a perfect job of implementing the computational algorithms will
we be able to reproduce the ever-increasing quality of the
observational data.”

Based on results from one of the world’s fastest computers — the
IBM RS/6000 SP system at the National Energy Research Scientific
Computing Center, Oakland, Calif. — and a sophisticated suite of
simulation software, the work marks the latest milestone in a
scientific investigation spanning more than 35 years. It focuses
on the deaths of aged stars in supernova explosions, which are
among the most violent events in nature, unleashing power that
can briefly outshine a galaxy of 100 billinew tars. When a
supernova explodes, it blasts oxygen, carbon and other vital
chemical elements through space and creates heavier elements like
copper and nickel.

The first one-dimensional simulations of similarly powered
core-collapse supernova were announced in 1966 by Stirling Colgate
and Richard White of the Lawrence Radiation Laboratory, Livermore,
Calif. But later it was discovered that one-dimensional simulations
had a fatal flaw; they almost always failed to explode.

Some 30 years later, astronomy professor Willy Benz, then at the
University of Arizona, and researcher Marc Herant, Los Alamos
National Laboratory, along with Fryer and Colgate, showed that
two-dimensional simulations were qualitatively different from 1-D,
leading to a robust explosion without fine-tuning of the star’s
physical properties. They concluded that the explosion process
is critically dependent on convection, the mixing of the matter
surrounding the iron core of the collapsing star.

Since convection plays an integral part in the physics of the
explosion in 2-D, it was believed that the results could again
be changed radically by adding a third dimension. But the 3-D
simulations turned out to be similar to the 2-D results, Warren
said. The explosion energy, explosion timescale and remnant neutron
star mass does not differ by more than 10 percent between the 2-
and 3-D models.

Unlike Type I supernovae, which are powered by a thermonuclear
explosion of a white dwarf star, Type II supernovae, the more
frequently occurring type modeled by Warren and Fryer, are powered
by the massive star’s gravitational collapse. The star begins its
life burning hydrogen, then heavier elements as the hydrogen is
exhausted and the temperature rises. Eventually, the cot saf the
star consists entirely of iron, which can no longer provide the
energy to resist the enormous gravitational forces pushing down on

As the iron atoms are crushed together, the core temperature rises
to more than 10 billion degrees. The force of gravity overcomes
the repulsive force between the nuclei and, in a few tenths of a
second, the core of the star collapses from its original size of
about one-half the diameter of Earth to 100 kilometers. The core
heats the material surrounding it not with light, but by radiating
most of its energy in neutrinos, nearly massless sub-atomic
particles that can pass through tons of matter without being
affected. As the in-falling gas approaches the core, it is exposed
to a higher and higher flux of neutrinos. A tiny fraction of those
neutrinos are absorbed. They heat the gas, which expands and
becomes buoyant.

The heated gas floats upward in large bubbles carrying energy away
from the core and is replaced by colder gas that sinks toward the
core and in turn becomes heated. This heat transfer from the core
to the envelope of the star results in enough energy transfer to
create an explosion.

Stirling Colgate, now a senior fellow at Los Alamos, has been
following closely the progress in understanding supernovae.
“Intellectual honesty demands that we remove the approximations
in our models, so that we become confident that our simulations
duplicate reality,” he said. “Astrophysics is a realm where nature
arranges the experiments, so we have to pay the utmost attention
to our theoretical and computational assumptions.

“Our one-dimensional models didn’t allow for convection, so they
didn’t result in robust explosions,” he said. “The two-dimensional
models demonstrated that convection was important, but we knew
that turbulence is fundamentally different in two dimensions, so
there was still doubt.”

Colgate concluded, “With these three-dimensional results, we have
reached the final battleground and are ready to attack the more
exotic problems that involve rotation and non-symmetric accretion.”

The work of Warren and Fryer is part of a larger Supernova Science
Center effort, which includes scientists from the University of
Arizona, the University of California Santa Cruz and Lawrence
Livermore National Laboratory. The Supernova Science Center is
funded by the Department of Energy’s Office of Science, Scientific
Discovery Through Advanced Computing program. More information is
available at .

Other groups, including the Terascale Supernova Initiative headed
by Anthony Mezzacappa at Oak Ridge National Laboratory and the
group led by Thomas Janka at the Max Planck Institute for
Astrophysics in Germany, are also making rapid progress in the
area of core-collapse supernovae.

Los Alamos National Laboratory is operated by the University of
California for the National Nuclear Security Administration (NNSA)
of the U.S. Department of Energy and works in partnership with
NNSA’s Sandia and Lawrence Livermore national laboratories to
support NNSA in its mission.

Los Alamos enhances global security by ensuring the safety and
reliability of the U.S. nuclear weapons stockpile, developing
technical solutions to reduce the threat of weapons of mass
destruction and solving problems related to energy, environment,
infrastructure, health and national security concerns.

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