Stellar explosions

02.24.14 -

by Trish Barker

Blue Waters enables UC Santa Cruz astrophysics team to simulate key supernova phase at unprecedented resolution.

The supernovae that Chris Malone studies, Type Ia, are “basically thermonuclear explosions of really compact stars,” the UC Santa Cruz postdoctoral researcher explains. Malone and the other members of the astrophysics research team led by UC Santa Cruz professor Stan Woosley use computational methods to follow the evolution of these massive explosions, and with the extreme scale of the Blue Waters supercomputer the team was able to complete a 3D simulation of a turbulent flame in a supernova at unprecedented resolution—135 m/zone, about eight times greater than typical simulations. These results were published in The Astrophysical Journal in February 2014.

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 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 movement of the star’s fluid. “It’s sort of like boiling water in a kettle,” Malone says.

The Woosley team previously had modeled this convection phase using the Maestro code developed in collaboration with Lawrence Berkeley National Lab. 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,” Malone says.

The team used these earlier calculations as the initial conditions for their high-resolution simulation of the explosion of the star 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. The power of Blue Waters (and the Titan system at Oak Ridge National Lab) 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 what we did, people triggered an explosion in the star but without a realistic convective flow pattern.

“That said, 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 simulation data derived from their ongoing simulations using 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.”

Citation

Malone, C. M., Nonaka, A., Woosley, S. E., Almgren, A. S., Bell, J. B., Dong, S., & Zingale, M. (2014). The Deflagration Stage of Chandrasekhar Mass Models for Type Ia Supernovae. I. Early Evolution. The Astrophysics Journal. 782(1). DOI: 10.1088/0004-637X/782/1/11

National Science Foundation

Blue Waters is supported by the National Science Foundation through awards ACI-0725070 and ACI-1238993.