New Supernova Models Take On Third Dimension

"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 billion stars. 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 core of the star consists entirely of iron, which can no longer provide the energy to resist the enormous gravitational forces pushing down on it.

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 www.supersci.org.

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.

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