3-D Simulations of i-Process Nucleosynthesis in the Early Universe

University Of Minnesota-Twin Cities, Minneapolis MN

Investigators

Abstract

Astronomical observations are now probing the early stages of the universe, in which galaxies developed and merged, and the formation of the structures in the universe at the present era was set in motion. An important part of this story is the chemical evolution of galaxies. The formation of the elements in the first stars and their subsequent dispersal are powerful tracers of structure formation and evolution in the early universe. This project involves the simulation of processes deep inside stars of the early universe that experience hydrogen ingestion events caused by mixing of material across the boundaries of convection zones. In 1-D stellar evolution simulations, these give rise to rapid hydrogen burning, with enormous energy release and causing nucleosynthesis by neutron capture from neutron fluxes intermediate between the slow (s) and rapid (r) processes. This is the intermediate, or i-process nucleosynthesis that is targeted in this study. Accurate treatment of the convective boundary mixing that drives this i-process requires full 3-D simulation. The rapid hydrogen burning that ensues, together with the very large luminosities involved make 3-D simulation affordable on NSF?s powerful new supercomputing platform, Blue Waters, at NCSA. A new and powerful capability will be built in this project to simulate these events by coupling together 3-D simulations that cover brief time intervals with 1-D simulations spanning much longer times. In some cases that will be addressed, it is possible to simulate the entire event in 3-D. In others, 3-D simulations performed at intervals as part of a coupled 1-D and 3-D stellar evolution calculation will serve to recalibrate the coefficients in approximate 1-D models for dynamic mixing processes. Such coupled calculations will use these periodic model recalibrations to validate the use of the models as the calculation progresses. When unacceptably large revisions of model "constants" are indicated, a recomputation with 3-D recalibrations taken at shorter time intervals will result. This new process is called autocorrecting modeling. It involves periodic generation of large amounts of 3-D data from simulation codes. This data generation is followed by a detailed analysis of the data in terms of 1-D models. The results of this analysis are then fed back into the 1-D stellar evolution computation. Pioneering this new approach is a major challenge of the project; the database of condensed simulation results that will be built in the process will be a significant outcome of the project. This data will be made available first to collaborators and ultimately to the broad community. It will be possible to use this data to formulate, validate, and refine simplified models of the convective boundary mixing processes that are a major focus of this study. It will also be possible for others, potentially using quite different simulation software, to use this data to adopt an autocorrecting modeling approach in studies of related problems. In the first year of this project, a continuation of present work with the well observed case of Sakurai's object is planned along with application of the new techniques to simulations of low-mass, low-metallicity AGB stars.