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

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Astronomical observations are now probing the early stages of the universe, in which galaxiesdeveloped and merged, and the formation of the structures in the universe at the present erawas set in motion. An important part of this story is the chemical evolution of galaxies. Theformation of the elements in the first stars and their subsequent dispersal are powerful tracersof structure formation and evolution in the early universe. This project involves the simulationof processes deep inside stars of the early universe that experience hydrogen ingestionevents caused by mixing of material across the boundaries of convection zones. In 1-D stellarevolution simulations, these give rise to rapid hydrogen burning, with enormous energy releaseand causing nucleosynthesis by neutron capture from neutron fluxes intermediate between theslow (s) and rapid (r) processes. This is the intermediate, or i-process nucleosynthesis thatis targeted in this study. Accurate treatment of the convective boundary mixing that drivesthis i-process requires full 3-D simulation. The rapid hydrogen burning that ensues, togetherwith the very large luminosities involved make 3-D simulation affordable on NSF?s powerfulnew supercomputing platform, Blue Waters, at NCSA.A new and powerful capability will be built in this project to simulate these events by couplingtogether 3-D simulations that cover brief time intervals with 1-D simulations spanning muchlonger times. In some cases that will be addressed, it is possible to simulate the entire eventin 3-D. In others, 3-D simulations performed at intervals as part of a coupled 1-D and 3-Dstellar evolution calculation will serve to recalibrate the coefficients in approximate 1-D modelsfor dynamic mixing processes. Such coupled calculations will use these periodic modelrecalibrations to validate the use of the models as the calculation progresses. When unacceptablylarge revisions of model "constants" are indicated, a recomputation with 3-D recalibrationstaken 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 datageneration is followed by a detailed analysis of the data in terms of 1-D models. The resultsof this analysis are then fed back into the 1-D stellar evolution computation. Pioneering thisnew approach is a major challenge of the project; the database of condensed simulation resultsthat will be built in the process will be a significant outcome of the project. This data will bemade available first to collaborators and ultimately to the broad community. It will be possibleto use this data to formulate, validate, and refine simplified models of the convective boundarymixing 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 autocorrectingmodeling approach in studies of related problems. In the first year of this project, a continuationof present work with the well observed case of Sakurai's object is planned along with applicationof the new techniques to simulations of low-mass, low-metallicity AGB stars.