A team led by Michael Zingale of Stony Brook University is exploring the physics of Type Ia supernovas using the Titan supercomputer at the US Department of Energy’s (DOE’s) Oak Ridge National Laboratory. Titan is the flagship machine of the Oak Ridge Leadership Computing Facility (OLCF), a DOE Office of Science User Facility located at ORNL. The team’s latest research focuses on a specific class of Type Ia supernovas known as double-detonation supernovas, a process by which a single star explodes twice.
This year, the team completed a three-dimensional (3-D), high-resolution investigation of the thermonuclear burning a double-detonation white dwarf undergoes before explosion. The study expands upon the team’s initial 3-D simulation of this supernova scenario, which was carried out in 2013. A full article describing the work, written by Jonathan Hines, is posted on the ORNL Site. Excerpted portions are presented here.
“In 3-D simulations we can see the region of convective burning drill down deeper and deeper into the star under the right conditions,” said Adam Jacobs, a graduate student on Zingale’s team. “Higher mass and more burning force the convection to be more violent. These results will be useful in future studies that explore the subsequent explosion in three-dimensional detail.”
By capturing the genesis of a Type Ia supernova, Zingale’s team is laying the foundation for the first physically realistic start-to-finish, double-detonation supernova simulation. Beyond capturing the incredible physics of an exploding star, the creation of a robust end-to-end model would help astronomers understand stellar phenomena observed in our night sky and improve the accuracy of cosmological measurements.
To test this scenario, Zingale’s team simulated 18 different double-detonation models using the subsonic hydrodynamics code MAESTRO. The simulations were carried out under a 50-million core-hour allocation on Titan, a Cray XK7 with a peak performance of 27 petaflops (or 27 quadrillion calculations per second), awarded through the Innovative and Novel Computational Impact on Theory and Experiment, or INCITE, program. DOE’s Office of Nuclear Physics also supported the team’s work.
By varying the mass of the helium shell and carbon–oxygen core in each model, MAESTRO calculated a range of thermonuclear dynamics that potentially could lead to detonation. Additionally, the team experimented with “hot” and “cold” core temperatures—about 10 million and 1 million degrees Celsius, respectively.
In three-dimensional detail, the team was able to capture the formation of “hot spots” on the sub-Chandrasekhar star’s surface, regions where the star cannot shed the heat of burning helium fast enough. The simulations indicated that this buildup could lead to a runaway reaction if the conditions are right, Jacobs said.
Before translating its [latest] findings to the next step of double detonation, called the ignition-to-detonation phase, Zingale’s team is upgrading MAESTRO to calculate more realistic physics, an outcome that will enhance the fidelity of its simulations. On Titan, this means equipping the CPU-only code to leverage GPUs, which are highly parallel, highly efficient processors that can take on heavy calculation loads.
Working with the OLCF’s Oscar Hernandez, the team was able to offload one of MAESTRO’s most demanding tasks: tracking stars’ nucleus-merging, energy-releasing process called nucleosynthesis. For the double-detonation problem, MAESTRO calculates a network of three elements—helium, carbon, and oxygen. By leveraging the GPUs, Zingale’s team could increase that number to around 10. Early efforts to program the OpenACC compiler directives included in the PGI compiler indicated a speedup of around 400 percent was attainable for this part of the code.
“Right now our reaction network for x-ray bursts includes 11 nuclei. We want to go up to 40. That requires about a factor of 16 more computational power that only the GPUs can give us,” Zingale said.
Maximizing the power of current-generation supercomputers will position codes like MAESTRO to better take advantage of the next generation of machines. Summit, the OLCF’s next GPU-equipped leadership system, is expected to deliver at least five times the performance of Titan.
To read the full article: https://www.olcf.ornl.gov/2015/12/08/titan-helps-researchers-explore-explosive-star-scenarios/
Source: Oak Ridge National Laboratory
Image: Oak Ridge National Laboratory