Theory of Recombination Lines in Astrophysical Sources

精确测定元素丰度是天体物理学中的一个基本问题。但在个别和整个类别的资源中，丰度异常和不准确比比皆是。银河系和星系外的H II区是被观测到最广泛的天体之一；然而，从不同的方法中得到的丰度存在着几个因素甚至数量级的差异，例如从发射线研究中推断出的铁/镍比率的巨大变化。阿尼尔·普拉丹博士获得这一奖项的努力将使发射谱线的物理条件和元素丰度的精确测定成为可能，通过(A)对许多天体物理上丰富的离子的重组谱线强度的首次计算，以及(B)包括迄今为止被忽视的电子-离子重组对突出发射谱线的贡献。主要的理论问题（与观测的不确定性相反）是，对从丰富的原子离子中观测到的谱线的精确分析依赖于高度不确定的原子参数。两种主要机制是碰撞激发和电子-离子复合。天体物理学中一个长期存在的问题是，用这两种方法得出的丰度之间存在持续的差异。虽然在碰撞激发截面和速率系数的计算方面已经做了大量的理论工作，但除了类氢离子、类氦离子和少数其他轻离子外，对复合谱线理论的研究很少。这是由于在处理导致复合线形成的大量能级的原子物理学中存在固有的困难。丰度不均匀性也可能与结构和物理过程有关。非常详细的光学和近红外（O/NIR）光谱已经存在，并且随着新的8米级望远镜和高分辨率光谱仪的出现，将在质量和数量上得到巨大的提高。因此，原则上，光谱研究应该能够精确地确定元素的丰度。普拉丹博士的工作将有助于排除原子数据和被忽视的原子过程带来的不确定性。他将利用最先进的原子物理技术的最新扩展，这些技术是在不透明项目和正在进行的铁项目下发展起来的，使用强大的最先进的相对论r矩阵方法，以及一种计算总和特定能级复合速率系数的新方法，包括辐射和介电复合过程。对相对论精细结构和共振现象的详细处理对于准确性至关重要，并将以从头计算的方式纳入其中。主要的重点将放在一些最重要的元素C，N，O,Ne,Si，S， Fe和Ni在多种电离状态。物理学和天体物理学的几个领域将受益于拟议研究中的辐射过程和数据的研究。将计算非常精确和大的转移概率集，用于构建产生有效速率系数的重组级联矩阵。此外，Pradhan博士的理论方法是基于从头算量子力学方式对光电离和重组的自一致处理。设想了实验室和天体物理等离子体源的广泛应用。这项工作的结果应该广泛适用于了解恒星、星云和HII区域、星系、超新星等的元素组成，从而了解宇宙的化学历史。这个项目是专门为培养能够进行这些复杂计算的下一代天体物理学家而设计的。该项目在很大程度上依赖于高性能计算。这项工作所需的计算工具和资源已经通过俄亥俄州超级计算机中心的特殊分配在各种矢量和大规模并行平台上得到了保障，NSF无需支付任何费用

AST-0506889PradhanDetermination of accurate element abundances is a fundamental problem in astrophysics. But abundance anomalies and inaccuracies abound in individual and entire classes of sources. Galactic and extragalactic H II regions are among the most extensively observed objects; yet, abundances from different methods are discrepant by several factors or even orders of magnitude, such as the large variations in the iron/nickel ratio deduced from emission line studies. Dr. Anil Pradhan's effort with this award will enable accurate determination of physical conditions and element abundances derived from emission lines by (A) the first calculations of recombination line intensities for a number of astrophysically abundant ions, and (B) including the heretofore neglected contribution of electron-ion recombination to prominent emission lines. The main theoretical issue (as opposed to observational uncertainties) is that accurate analysis of observed lines from abundant atomic ions depend on atomic parameters that are highly uncertain. The two primary mechanismsare collisional excitation and electron-ion recombination. A longstanding problem in astrophysics is the continuing discrepancy between abundances derived using these two methods. Whereas considerable theoretical work has been done on the calculation of collisional excitation cross sections and rate coefficients, little effort has been devoted to the theory of recombination lines except for hydrogen-like, helium-like and a few other light ions. This is due to the inherent difficulty in the treatment of the atomic physics of a large number of levels that contribute to the formation of recombination lines. Abundance inhomogeneities may also be related to structure and physical processes. Very detailed optical and near-infrared (O/NIR) spectra already exist, and will undergo a huge increase in quality and quantity with the advent of new 8-m class telescopes and high-resolution spectrographs. In principle, therefore, spectral studies should be able to ascertain element abundances precisely. Dr. Pradhan's work will help rule out uncertainties due to atomic data and neglected atomic processes. He will utilize recent extensions of the most advanced atomic physics techniques, developed under the Opacity Project and the ongoing Iron Project, using the powerful and state-of-the-art relativistic R-matrix method, and a new method for calculating total and level-specific recombination rate coefficients subsuming both the radiative and the dielectronic recombination processes. Detailed treatments of relativistic fine structure and resonance phenomena are essential for accuracy, and would be incorporated in an ab initio manner. The primary emphasis will be on some of the most important elements C,N,O,Ne,Si,S, Fe, and Ni in multiple ionization states.Several areas of physics and astrophysics should benefit from the study of radiative processes and data in the proposed research. Very accurate and large sets of transition probabilities will be calculated for use in the construction of recombination-cascade matrices that yield the effective rate coefficients. In addition, Dr. Pradhan's theoretical approach is based on a self-consistent treatment of photoionization and recombination in an ab initio quantum mechanical manner. A wide range of applications to laboratory and astrophysical plasma sources are envisaged. The results of this work should be broadly applicable to the understanding of elemental composition of stars, nebulae and HII regions, galaxies, supernovae, etc., and hence the chemical history of the Universe. This project is especially designed for training of graduate students as the next generation of astrophysicists capable of these complex calculations. The project relies heavily on high-performance computing. The computational tools and resources needed for this undertaking have been secured on a variety of vector and massively parallel platforms through a Special Allocation at the Ohio Supercomputer Center at no cost to NSF.***