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

天文观测目前正在探索宇宙的早期阶段，即星系发展和合并的阶段，以及当前时代宇宙结构的形成。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和随后的扩散是早期宇宙结构形成和演化的有力踪迹。这些元素是在恒星内部合成的，远远超出了直接观测的范围。因此，理解它们的合成以及使之成为可能的环境显然是在理论领域。虽然恒星演化理论是计算科学中最古老的分支之一，但直到最近才有可能准确模拟恒星内部的关键事件和过程，这些事件和过程产生了重元素，这些元素被注入星际环境，而不是被坍塌的物体吞噬。这些事件在某些情况下可能非常短暂，在发生爆炸性现象的情况下只持续几年、几天甚至几秒钟。这些事件的简洁性使我们现在可以完整地模拟它们的3D细节。该项目旨在建立一种新的强大的能力来模拟这些事件，方法是将覆盖短时间间隔的三维模拟与跨越更长时间的一维模拟结合在一起。该项目将模拟早期宇宙中恒星内部的过程，涉及物质跨越对流区边界混合导致的氢摄取事件。在一维恒星演化模拟中，这些过程导致氢的快速燃烧，释放出巨大的能量，并通过在慢(S)和快(R)过程之间的中子流中俘获中子而引起核合成。这就是本研究的目标，即中间体或I-过程核合成。要准确处理驱动这一I过程的对流边界混合，需要进行全三维模拟。在某些情况下，这将得到解决，有可能以3D形式模拟整个事件。在其他方面，作为耦合的一维和三维恒星演化计算的一部分，每隔一段时间进行的三维模拟将用于重新校准动态混合过程的近似一维模型中的系数。随着计算的进行，这种耦合计算将使用这些周期性的模型重新校准来验证模型的使用。当指出对模型“常数”进行不可接受的大修改时，将导致以较短的时间间隔进行三维重新校准的重新计算。这种被称为自动校正建模的新过程涉及从模拟代码定期生成大量3-D数据。在此数据生成之后，根据1-D模型对数据进行详细分析。这一分析的结果然后被反馈到一维恒星演化计算中。在进行模拟的过程中，将生成一个大型数据库，其中包含恒星内部对流边界混合的三维模拟结果。这个数据库将被组织起来，以便模拟代码可以自动挖掘它，将其与混合过程的各种潜在有用的一维模型进行比较。这个数据库以及挖掘、分析和显示结果的工具将是独一无二的。这将是一项重大投资，不仅是在我们开发模拟代码和数据分析和可视化工具的时间上，而且也是在世界上最强大的计算系统之一的计算机时间上。因此，这个项目的广泛影响将是这个数据库和工具，使其在恒星演化研究中有用。该项目将为数据科学开发新的自动化技术，但除了这些技术之外，一个重要的成果将是数据本身，这些数据将以有用的格式在线提供给社区。流体力学模拟将能够澄清早期恒星I过程的核合成产额。PI将与NuGrid合作，确保结果被合并到数据集，供社区用于早期宇宙中星系和结构的化学演化模拟。国际和平研究所还与美国国家科学基金会S联合核天体物理研究所(JINA)的科学家合作。这项工作的另一个影响将是进行详细的模拟，以便评估这些新的速率测量对核合成的影响，以便可以将其与球状星团中的贫金属星的光谱和太阳前颗粒的同位素组成等可观测数据进行比较。

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. The elements are synthesized deep in the interiors of stars well beyond the reach of direct observation. For this reason, understanding their synthesis and the environment which enables it is clearly in the realm of theory. Although stellar evolution theory is one of the oldest branches of computational science, it has only recently become possible to accurately simulatesome of the key events and processes inside stars that give rise to the heavy elements that are injected back into the interstellar environment rather than being swallowed up in collapsed objects. These events can be quite brief in some cases, lasting only years, days, or even seconds in the case of explosive phenomena. The brevity of these events makes it possible for us now to simulate them in full 3-D detail. This project aims to build a new and powerful capability to simulate these events by coupling together 3-D simulations that cover brief time intervals with 1-D simulations spanning much longer times.This project will simulate processes deep inside stars of the early universe that involve 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. In some cases that will be addressed, it is possible to simulate the entire event in 3-D. In others, the 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 called autocorrecting modeling 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. In the process of carrying out the simulations a large database of results from 3-D simulations of convective boundary mixing in stellar interiors will be generated. This database will be organized so that the simulation codes can mine it automatically, comparing it to a variety of potentially useful 1-D models of the mixing process. This database, together with the tools that mine it, analyze it, and display results, will be unique. It will represent a significant investment, not only in our time developing the simulation codes and data analysis and visualization tools, but also in computer time on one of the most powerful computing systems in the world. A broad impact of this project will therefore be this database and tools that make it useful in stellar evolution research. The project will result in the development of new, automated techniques for data-enabled science but in addition to these techniques an important outcome will be the data itself which will be made available to the community on-line in a useful format. The hydrodynamics simulations will enable the clarification of the nucleosynthetic yields of the i-process in early generations of stars. The PI will work with the NuGrid collaboration to see that the results become incorporated into data sets that are available to the community for chemical evolution simulations of galaxies and structures in the early universe. The PI is also collaborating with the scientists at NSF?s Joint Institute for Nuclear Astrophysics (JINA). A further impact of this work will be to perform the detailed simulations that allow the assessment of the impact of these new rate measurements on nucleosynthesis, so that it can be compared with observables such as spectra of metal-poor stars in globular clusters and isotopic compositions of pre-solar grains.