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

天文观测正在探索宇宙的早期阶段，在这个阶段，星系发展和融合，宇宙结构的形成开始了。这个故事的一个重要部分是星系的化学演化。第一批恒星中元素的形成及其随后的扩散是早期宇宙结构形成和演化的有力示踪剂。该项目涉及模拟早期宇宙恒星内部的过程，这些过程经历了由对流区边界的混合物质引起的氢吸收事件。在1-D恒星演化模拟中，这些会引起快速的氢燃烧，释放出巨大的能量，并通过在慢(s)和快速(r)过程之间的中子通量捕获中子来引起核合成。这是中间过程，或者是本研究的目标核合成过程。精确处理驱动这一过程的对流边界混合需要完整的三维模拟。随之而来的快速氢燃烧，加上所涉及的非常大的亮度，使得NSF能够负担得起3d模拟。美国国家计算机研究中心强大的新超级计算平台“蓝水”。这个项目将建立一个新的强大的能力，通过将覆盖短时间间隔的3-D模拟与跨越更长时间的1-D模拟结合在一起，来模拟这些事件。在某些情况下，可以用3d模拟整个事件。在其他情况下，作为1-D和3-D耦合恒星演化计算的一部分，每隔一段时间进行3-D模拟，将有助于重新校准动态混合过程的近似1-D模型中的系数。随着计算的进行，这种耦合计算将使用这些周期性的模型再校准来验证模型的使用。当模型“常数”有不可接受的大修正时，将导致在较短的时间间隔内进行三维重新校准的重新计算。这个新过程被称为自动校正建模。它涉及到从模拟代码中周期性地生成大量的三维数据。数据生成之后是根据1-D模型对数据进行详细分析。这个分析的结果然后被反馈到一维恒星演化计算中。开拓这种新方法是该项目的主要挑战；将在此过程中建立的浓缩模拟结果数据库将是该项目的重要成果。这些数据将首先提供给合作者，最终提供给广大社区。将有可能使用这些数据来制定，验证和完善对流边界混合过程的简化模型，这是本研究的主要焦点。其他人也有可能使用完全不同的仿真软件，使用这些数据在相关问题的研究中采用自动校正建模方法。在这个项目的第一年，计划继续目前对樱井天体的观测，同时应用新技术模拟低质量、低金属丰度的AGB恒星。

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.