Advancing the supernova mechanism study with computer modeling

Sean Couch , Assistant Professor
Department of Physics and Astronomy & CMSE , Michigan State University

We are star guts. The elements that we are made of were forged in massive stars billions of years ago, then spread throughout the young Milky Way galaxy by the explosive deaths of those same stars, known as supernovae. While supernovae are observed routinely to occur in galaxies near and far, the physical mechanism that drives these energetic explosions remains unclear.  Couch’s group at MSU is raising the bar in the study of the supernova mechanism with cutting-edge computational science.

3D Turbulence in Stellar Explosions

The most promising candidate for the supernova explosion mechanism is the so-called “delayed neutrino heating” mechanism.  Neutrinos carry away nearly all of the gravitational binding energy released via the collapse of the stellar core, about 100 times the energy necessary to drive robust supernova explosions.  The trouble is that neutrinos have an incredibly tiny cross section for interaction, making extracting much of this copious energy extremely difficult.  The most sophisticated 1D simulations have, for decades, shown that the neutrino mechanism fails in spherical symmetry.  The situation is somewhat more promising in 2D and 3D wherein a handful of self-consistent explosions have been obtained, but these explosions tend to be marginal.

A perennial question has been what phenomena in 2D and 3D aid explosion as compared to 1D?  While many hydrodynamic instabilities seem to help the neutrinos in driving supernova explosions, Couch and his collaborators recently showed that by far the most important difference is the presence of turbulence in 2D and 3D.  Turbulence behind the stalled supernova shock, driven by the neutrino heating, is extremely strong and violent.  This turbulence exerts an effective pressure on the stalled shock that can revival the background thermal pressure.  This is a huge effect that is completely missing from 1D calculations!  Couch showed that 2D and 3D calculations require much less neutrino heating to reach explosions precisely because of this turbulent pressure helping to push the shock out.  This realization represents a sea change in our thinking about what aids explosions in multidimensional simulations and points the way toward a robust model for successful supernova explosions. Couch's group is now using Laconia at HPCC to model the impact of turbulence on observable features of supernovae. 

3D Massive Stellar Evolution 

One critical aspect of the role of turbulence in the supernova mechanism is its connection to the presence of non-spherical structure in the progenitor stars. Couch and his colleagues have shown that the presence and strength of convection in the pre-collapse progenitor star directly impact the strength of the turbulence behind the stalled supernova shock: the stronger the progenitor convection, the stronger the resulting turbulence.  How strong such progenitor convection is in real massive stars was, however, completely unknown since the state-of-the-art in supernova progenitor calculations is still 1D models. Couch and his colleagues made the first steps forward in addressing these issues by carrying out the world's first 3D supernova progenitor simulation, directly calculating the final three minutes in the life of a massive star all the way to the point of gravitational core collapse.  Couch showed that the resulting aspherical progenitor structure was more favorable for a successful explosion than an otherwise identical 1D progenitor.  This work has a paradigm-shifting potential for the theoretical study of supernovae because it implies that the trouble all along may not have been with the neutrino heating mechanism, per se, but with the initial conditions we have been using. Together with collaborators around the country, Couch’s group is leading a cutting edge effort to produce the world's first and most realistic 3D supernova progenitor models.  Much of this work is being carried out on Laconia, as well.