Supernova Discoveries

A partnership with high-performance computing helps astrophysicists understand how stars explode.

Armed with the computational power of ORNL's National Leadership Computing Facility, now capable of 40 trillion calculations per second, computational astrophysicists with the ORNL-led TeraScale Supernova Initiative have made new scientific discoveries regarding the explosions of massive stars.

John Blondin of North Carolina State University and Tony Mezzacappa, TSI's leader and a corporate fellow in ORNL's Physics Division, used Blondin's computer code to produce a three-dimensional (3D) simulation of a core-collapse supernova on 5000 processors of NLCF's Cray X1E supercomputer. Their discoveries relate to the shock wave generated when a star collapses and the core's inner part rebounds into matter falling inward. They discovered that, in 3D as well as 2D simulations, the shock wave becomes unstable, and this "standing accretion shock instability" induces rotation in what was previously a spherically symmetric configuration.

"The SASI induces counter-rotating flows of stellar matter on the inside of the star," Mezzacappa explains. "As matter spins and accretes on the central object of the simulated star, the process deposits angular momentum on the central object, spinning it up. We started with no spin and our simulation generated an object that spins at tens of milliseconds—just like a pulsar, which is a rotating neutron star."

Scientists have known for some time that supernovae are dying stars reborn as neutron stars. Pulsars are like lighthouse beacons, but their pulses are radio waves rather than visible light. The TSI simulation provides a plausible mechanism for explaining how a supernova can morph into a newly born, fast-spinning pulsar after forming and flinging into space elements responsible for life on Earth.

"We predict that SASI, along with neutrino transport and the magnetic field from within the star, will affect how the shock wave generates the explosion," says Mezzacappa.

Blondin and Mezzacappa also found that 3D simulations of supernovae are far more realistic than 2D and 1D simulations. In a 1D simulation the only dimension is the radius. In a 2D simulation, the radius and latitude are the dimensions. A 3D simulation adds longitude. "In our multidimensional simulations we also take into account neutrino direction, neutrino energy, and time," Mezzacappa adds.

The 3D models run thus far at ORNL simulate only fluid dynamics. Mezzacappa and his colleagues plan to add neutrino transport, magnetic fields, and more realistic particle and nuclear physics to the models.

ORNL's Raphael Hix, who was recently selected for the 2006 Young Investigator Award by Sigma Xi, The Scientific Research Society, has shown that understanding reactions involving neutrinos and nuclei in supernovae can help astrophysicists get a better grasp on both how shock waves disrupt the star and which chemical elements are formed in the process. During core collapse, electrons captured by nuclei slam protons, resulting in the emission of electrically neutral, almost massless neutrinos. These events significantly affect the location of the birth of the shock wave.

During the explosion, neutrinos are also involved in nucleosynthesis—the formation of new elements in varying abundances in supernovae, including many elements found in our bodies and our planet. Proper consideration of the neutrinos interacting with nuclei in the outer layers results in predicted nuclear compositions much more like those observed.

TSI is a five-year research program funded by the Department of Energy. The proposed, five-year Petascale Supernova Initiative will include at least 37 investigators from 15 universities and 4 DOE national laboratories, led by ORNL.

The initiative's proposal is to simulate supernovae even more realistically by developing codes that will run on NLCF's planned petascale supercomputer—a machine that would make a thousand trillion calculations per second—in about three years. What has not been promised is a real supernova in our Galaxy, which would supply observational data to neutrino and gravitational wave detectors on Earth that could be used to validate the astrophysicists' codes. To better understand stellar death and rebirth, researchers are wishing for a star.

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