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Astrophysics team simulates key supernova phase at unprecedented resolution

The supernovae that Chris Malone studies, Type Ia, are "basically thermonuclear explosions of really compact stars," says the University of California, Santa Cruz, US, postdoctoral researcher. Malone and the other members of the astrophysics team, led by professor Stan Woosley, use computational methods to follow the evolution of these massive explosions. Using the powerful Blue Waters supercomputer, the team simulated a turbulent flame in a supernova at unprecedented resolution - about eight times greater than typical simulations. The results appear in the February 2014 issue of The Astrophysical Journal.

The color map shows the magnitude of vorticity (the spinning motion of the fluid), with large regions of relatively strong turbulence shown in white/yellow. The burning flame initially has a shape similar to a torus or smoke ring. As the burning bubble makes its way toward the surface of the star, the ring shape breaks apart due to the turbulence, which pushes strong vortex tubes to the flame's surface. Unlike a smoke ring, however, this flame is continuously powered by thermonuclear reactions and does not dissipate within the star. Eventually, the vortex tubes penetrate the whole of the flame, and the bulk flow inside the flame becomes turbulent. This leads to an accelerated influx of fresh fuel and increased burning. Image courtesy National Center for Supercomputing Applications and University of California, Santa Cruz, US.

Type Ia supernovae occur in binary systems that pair a super-dense white dwarf with another star. While these supernovae remain a bit of a puzzle, one theory is that the white dwarf's gravitational pull attracts matter from its binary mate; the additional mass then tips the white dwarf's core into a fusion explosion. Some research indicates that this explosion isn't triggered immediately. Reactions in the white dwarf's core release energy, kickstarting the bubbling of the star's fluid. "It's sort of like boiling water in a kettle," Malone says.

The Woosley team has previously modeled this convection phase using the Maestro code developed in collaboration with Lawrence Berkeley National Lab in California, US. From these simulations they determined that ignition does not necessarily occur at the center of the white dwarf, as many prior simulations had assumed, but instead is slightly off center. "That has a big impact on how the explosion propagates through the star," says Malone.

The team used these earlier calculations as the initial conditions for their high-resolution simulation on Blue Waters, this time using the Castro hydrodynamics code. The flame's trip to the surface of the star is a quick one - the whole process takes only about a second. But simulating the full complexity of that second is computationally demanding. Blue Waters (and the Titan system at Oak Ridge National Lab in Tennessee, US) made it possible for the team to perform the first simulations to bridge the gap between ignition and flame propagation in the Chandrasekhar mass model of Type Ia supernovae.

"We wanted to know, does this background flow from convection affect the explosion as it moves through the star?" Malone says. "With such a large machine at our disposal, we triggered a 'flame' that propagates through the star. Prior to this, people triggered an explosion in the star but without a realistic convective flow pattern. We found that for a typical ignition location, the convective roiling doesn't really affect the flame as it makes its way to the surface."

The team continues to analyze the terabytes of data derived from their ongoing simulations on Blue Waters, and intends to explore other aspects of the supernovae explosion. For example, when the flame breaks through the surface of the star, it flings out material much like lava from a volcano. Some of the material escapes, but the flame itself continues to burn around the surface of the star.

"As this 'lava' of star material is moving very rapidly - almost at the speed of sound - across the surface of the star, there's a lot of shear and mixing going on between the hot material and the cooler material of the star," Malone says. "We're looking at this highly turbulent, highly shear-driven burning to see if that triggers another explosion."

- Trish Barker

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