ASTROPHYSICS

The Stellar Core-Collapse Group at the Max Planck Institute for Astrophysics (MPA) is able to conduct the presently most advanced 3D supernova simulations thanks to a suitably constructed description of the neutrino physics and a highly efficient, extremely well parallelized numerical implementation on petascale system SuperMUC. Because neither experiments nor direct observations can reveal the processes at the center of exploding stars, highly complex numerical simulations are indispensable to develop a deeper and quantitative understanding of this hypothetical “neutrino-driven explosion mechanism”, whose solid theoretical foundation is still missing.

Neutron stars are born as extremely hot and dense objects at the centers of massive stars exploding as supernovae. They cool by the intense emission of neutrinos, ghostly elementary particles that hardly interact with matter on earth but that are produced in huge numbers at the extreme temperatures and densities in nascent neutron stars. These neutrinos are also thought to trigger the violent disruption of the star in the supernova if only less than one percent of their huge total energy can be tapped to heat the stellar mantle that surrounds the neutron star.

Because neither experiments nor direct observations can reveal the processes at the center of exploding stars, highly complex numerical simulations are indispensable to develop a deeper and quantitative understanding of this hypothetical “neutrino-driven explosion mechanism”, whose solid theoretical foundation is still missing. The computational modeling must be done in three dimensions (3D), simulating the whole star, because turbulent flows as well as large-scale deformation play a crucial role in enhancing the neutrino-matter interactions. This requires the solution not only of the fluid dynamics problem in a strong-gravity environment including a description of the properties of neutron-star matter and of nuclear reactions. In particular the neutrino propagation and processes pose a grand computational challenge, because besides the three spatial dimensions there are additional three dimensions for neutrino energy and direction of motion. Not even the biggest existing supercomputers can solve such a six-dimensional, time-dependent transport problem in full generality.

With a suitably constructed description of the neutrino physics and a highly efficient, extremely well parallelized numerical implementation, the Stellar Core-Collapse Group at the Max Planck Institute for Astrophysics (MPA) is able to conduct the presently most advanced 3D supernova simulations. Nevertheless, despite approximations, one model run, following the supernova evolution for roughly half a second and using 16,000 processor cores in parallel mode on LRZ HPC system SuperMUC, takes about 4.5 months of uninterrupted computing.

On the way of producing the first-ever 3D explosion models with a highly sophisticated treatment of the neutrino physics, the MPA team made a stunning and unexpected discovery: The neutrino emission develops a strong dipolar asymmetry: Neutrinos and their anti-particles are not radiated equally in all directions but with largely different numbers on opposite hemispheres of the neutron star. If this novel neutrino-hydrodynamical instability happens in nature, it is a discovery truly based on the use of modern supercomputing possibilities and not anticipated by previous theoretical considerations. It will lead to a recoil acceleration of the neutron star and will have important consequences for the formation of chemical elements during the supernova explosion.