Paul Woodward - July 24, 2015

Simulating Hydrogen Ingestion Flashes in Stars in 3D

Over the last two years, we have been simulating brief events in stars that,
although violent, do not cause the stars to explode. These events are caused by
ingestion of combustible material into a convection zone, and subsequent
dragging of this fuel down to high temperature regions where it burns very
rapidly and releases an enormous amount of energy. Our collaborator, Falk Herwig,
at the University of Victoria, in Canada, has identified a number of
opportunities for such flash behavior, including very late thermal pulse (VLTP)
stars, AGB stars of very low metallicity, superAGB stars, and massive supernova
progenitors.  I will present our 3-D simulations of the hydrogen ingestion flash
in the VLTP star known as Sakurai's object and outline some of the other cases
that we are now beginning to study.  These simulations are carried out on the
25000-node Blue Waters computing system at NCSA.  They require simulation of the
entire stellar convection zone for many dynamical times, which demands millions
of time steps.  I will outline how we make the code scale to hundreds of
thousands of CPU cores for modest grids of only a few billion cells and thus
make these simulations practical.  We have also been adapting this code to run
on GPUs, and I will give some early results on how that is being done and the
performance benefits it produces.