Seminar Details

3D Simulations of Hydrogen Ingestion Flashes in AGB Stars as Sites for i-process Nucleosynthesis

Paul Woodward, University of Minnesota
Monday, October 21, 12:30 PM - JINA Pizza Lunch
Biomedical & Physical Sciences Bldg., Rm. 1400

Analysis by Herwig and collaborators using 1-D stellar evolution codes of the well observed very late thermal pulse event in Sakurai's object indicate that hydrogen ingestion events in AGB stars can be as brief as a single day and produce neutron exposures that are intermediate between the s-process and r-process levels. The nucleosynthesis involved has a unique signature and has been termed i-process nucleosynthesis. The brevity of these events enables us to simulate them in 3D. At the same time, the speed of the nuclear reactions involved and the huge energy release they produce demands a 3D treatment. In collaborČation with Herwig, I have been building a new capability to perform such simulaČtions despite the need to simulate the entire convection zone for a great many eddy turn-over times and an enormous number of sound wave crossing times. To make this possible, we are using powerful new numerical algorithms as well as advanced, many-core computing devices. Over the last year, we have carried out a simulation of the event in Sakurai's object on a billion-cell grid following the star through 10 hours with 2 million time steps. This work is being done on the sustained petaflop/s Blue Waters computing system at NCSA. The same code has scaled to over 700,000 cores and 1.5 Pflop/s on this machine running problems involving only several sound crossing times on a trillion cell grid. The industry and government drive to exascale computing will make possible a whole range of new calculations of the details in 3D of convection-reaction processes in which the combustion and convective eddy turn-over time scales are comparable. I will explain what is new in the simulation code's numerical treatments, show evidence that we can obtain converged results for these problems at affordable grid resolution, discuss new light shed by our calculations on the details of the Sakurai object event, and mention a number of related problems that we feel we can attack in the years ahead with this new code running on new computing platforms. These related problems include hydrogen ingestion events in stars of very low or even zero metallicity, where we think the i-process is especially important.