ASTROPHYSICS:
Simulating Supernovae

Answers to huge scientific questions may come from findings made in realistic simulations of exploding stars on the world's fastest supercomputers.

During the catastrophic death throes of massive stars, known as core-collapse supernovae, many elements were created, including those necessary for life on Earth. How and why these stars that were greater than 10 times the mass of our sun and that had evolved over millions of years died explosively in a few hours are mysteries that scientists cannot solve in laboratory experiments. However, simulations on supercomputers hold out hope of unraveling the secrets of supernovae.

The five-year TeraScale Supernova Initiative (TSI), an ORNL-centered, multidisciplinary computational initiative involving 10 universities, has already made a major discovery about core-collapse supernovae, thanks to funding from the Department of Energy's Scientific Discovery through Advanced Computing (SciDAC) program. The discovery was made by three researchers, including TSI leader Tony Mezzacappa, an ORNL computational astrophysicist. One of the supercomputers used was the IBM Power3 supercomputer at DOE's Center for Computational Sciences at ORNL.

Mezzacappa's team discovered that the shock wave—thought to stall while trying to propagate through the stellar core until reenergized by neutrino heating—can become unstable and change shape to look more like a cigar and less like a sphere than previously thought. If the shock wave generates the explosion and if this "supernova accretion shock instability" (SASI) occurs, the characteristics of a supernova explosion could be fundamentally altered. Observational evidence suggests that the light from a core-collapse supernova explosion is polarized. One explanation for the observed polarization could be that the explosion is shaped like a cigar. The SASI may also be the underlying mechanism whereby neutron stars born in supernovae are boosted to mean velocities of 450 kilometers per second.

TSI also discovered the importance of modeling both the microphysical and macrophysical aspects of core-collapse supernovae. Researchers found that the dynamics of an exploding star's gross characteristics depend on the nature of the atomic nuclei of iron and heavier elements that compose the stellar core. Theorists believe the star's explosion is caused by a shock wave that arises when the star's iron core collapses on itself, compressing its subatomic particles to the point where they repel each other and force the core to rebound. The rebounding force may be further energized by neutrino heating—the interaction of neutrinos in the star with its core. Neutrinos are nearly mass-less, radiation-like particles believed to be responsible for powering supernova explosions.

This TSI finding resulted from a merger of nuclear structure theory and astrophysics theory pertaining to core-collapse supernovae. Use of more sophisticated models of nuclei in the core to calculate the interactions of neutrinos with the stellar-core nuclei led to changes in the astrophysics models that will strongly impact scientists' understanding of the mechanisms responsible for supernova explosions and the synthesis of elements. To test new nuclear models that provide input to supernova models, neutrino-nucleus interaction experiments have been proposed for the ac-celerator-based Spallation Neutron Source being built at ORNL. The reason: neutrinos emerging from its target will have the same energy as supernova neutrinos.

ORNL researchers in TSI have found that one of their radiation transport codes that simulates neutrino heating and one of their hydrodynamics codes that simulates the core's turbulent fluid and rotation run 10 to 20 times faster on the CCS Cray X1 than on the IBM Power3 and Power4 supercomputers.

With leadership-class scientific computing, TSI will develop and run three-dimensional models that more realistically simulate the complex processes that drive a supernova and synthesize elements. As simulation needs escalate, huge leaps will be needed in data management, data analysis, networking capabilities, the development of software tools, and visualization. Simulation results will provide the theoretical foundation for understanding physics data from premier experimental and observational facilities, as well as insights into combustion, climate, fusion energy, and nuclear medicine. For scientists, simulating the stars will have benefits on Earth.

Web site provided by Oak Ridge National Laboratory's Communications and External Relations.
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