From the golden years of a white dwarf to its ultimate fiery and explosive demise, Josh Martin of Stony Brook University’s Institute of Advanced Computational Science investigates the triggers of thermonuclear catastrophes. A white dwarf is the exposed stellar core of a formerly low-mass star e.g. our sun. Isolated, a white dwarf will quietly cool into darkness, but with a companion, a white dwarf can meet a far more destructive fate. When matter from a companion star flows onto a white dwarf, it can trigger a thermonuclear runaway resulting in one of the brightest explosions in the universe--a Type Ia supernova.
Due to their similarities in brightness, these supernovae are able to be used as standards of measurement for far-reaching cosmological distances. This utility led to the Nobel prize-winning (2011) discovery that our universe was expanding at an accelerating rate and thus led to the formulation of an energy which must be responsible for this acceleration—dark energy.
Type Ia supernovae are, however, not monolithic. Age and composition can affect the brightness and, critically, there are a host of different subgroups that don’t quite fit the “Type Ia” mold. The question becomes what are these other events and why do they even occur in the first place?
Josh Martin, a Ph.D. student at IACS, and one of SeaWulf’s top users over the past year is trying to help answer this question. He specifically investigates one of the largest subgroups of Type Ia supernovae called “Type Iax Supernovae” which are a dim class with characteristically lower explosion energies.
Martin investigates these Type Iax supernovae utilizing a massive 3D simulation powered by the FLASH code, a multi-scale, multi-physics, highly parallelized modular research code designed for compressible reactive flows. These events are inherently multi-dimensional and capturing the underlying physics, e.g. turbulent combustion, requires advanced platforms like SeaWulf for any hope of progress. At its peak, this grid-based simulation contains over 1,000,000 3D blocks, with each block containing 16 x 16 x 16 cells which each store 20 variables. This amounts to nearly 100 billion variables that need to be evolved each timestep.
Martin’s simulations are exclusively performed on the Seawulf HPC cluster which harnesses the cutting-edge Intel XeonMax Sapphire Rapids nodes with High Bandwidth Memory (HBM). Because FLASH is typically limited by memory-bandwidth, the HBM on the Sapphire Rapids nodes delivered a substantial performance advantage for these simulations. The project also benefited from the ability to scale efficiently across nearly forty 96-core nodes, enabling much larger and more detailed runs than would otherwise be possible.
Nearing completion is Martin’s paper that reveals the results of these Iax simulations—a near-central explosion mechanism which is consistent with the chemical and energetic yields detected in observational studies. Martin is hopeful that his work will illuminate how these particular explosions fit into the cosmic story we’re still learning to tell.
In modeling Type Iax supernovae, we care the most about the ejected quantities—the quantities we can observe. This is a plot of the density of the ejected matter where the gradient in colors represent an increase in density as we approach the center of the explosion.
This animation shows the initial velocity field that we place in our white dwarf prior to ignition. These realistic initial conditions have never before been added to a Type Iax supernova simulation. In this animation, blue represents outflowing velocity and red represents inflowing velocity.