Solving the Nebular Abundances Anomaly: New Features in Photoionization and Recombination

解决星云丰度异常：光电离和复合的新特征

• 批准号: 1312441
• 负责人: Sultana Nahar
• 金额: $ 39.02万
• 依托单位: Ohio State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-08-15 至 2020-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1312441&HistoricalAwards=false
• 关键词: Solving; Nebular; Abundances; Anomaly; New

This project aims to resolve longstanding discrepancies in the abundances of common elements such as carbon, nitrogen, oxygen, and neon in photoionizaed diffuse nebulae, particularly planetary nebulae. The ionization structure in optically thin environments is governed by photoionization and recombination. The physical processes underlying spectral line formation are electron impact excitation (EIE) or electron-ion recombination (RC), and radiative decay rates. A perplexing discrepancy in abundances, of up to an order of magnitude of more, is found between those derived from emission lines excited due to EIE and those from electron-ion recombination lines of the same atomic species. This not only calls into question the basic physics of photoionization and recombination, but also the interpretation of the physical structure and distribution of elements within gaseous nebulae. This project will examine the former in order to address the latter. Specifically, calculations will be based on: i) the unified theory of electron-ion recombination that subsumes radiative and dielectronic recombination processes, and provides a self-consistent set of atomic data for photoionization, recombination and excitation processes, (ii) recent extensions in the state-of-the-art R-matrix method including relativistic effects not heretofore considered, and (iii) pilot calculations for photoionization and recombination demonstrating the role of low-energy resonances that may be crucial to resolving the abundance(s) anomaly problem. The project will involve large-scale and high-accuracy calculations of the critical atomic parameters for a number of elements. High precision is imperative since astrophysical models need to rule out atomic physics as any significant source of uncertainty. The focus will be on some of the most prominent elements in low ionization stages: carbon, nitrogen, oxygen, neon, magnesium, silicon and sulfur. The project will result in extensive and self-consistent sets of high-accuracy radiative and collisional atomic parameters.Through analysis of their optical and infrared spectra, astronomers attempt to understand the abundances of the elements in diffuse insterstellar clouds or "nebulae" in which hydrogen is ionized. They do this by measuring the strengths of the characteristic emission from ions of different elements and use these to determine the physical properties within the nebula including the abundances of the constituent elements. These emission lines can be excited via several processes and there is a long standing discrepancy between the abundances measured using spectral lines excited by collisions versus those due to electron capture. This project aims to remove one level of uncertainty by calculating the theoretical values for the atomic parameters that relate the excitation to the strengths of the emission. The results of this research will be applicable across a number of sub-fields within astrophysics ranging from studies of our own galaxy to abundances in the most distant quasars.

这个项目的目的是解决长期存在的共同元素，如碳，氮，氧和氖在光电离弥漫星云，特别是行星状星云的丰度差异。光学薄环境中的电离结构由光电离和复合控制。光谱线形成的物理过程是电子碰撞激发（EIE）或电子-离子复合（RC）和辐射衰减率。一个令人困惑的丰度差异，高达一个数量级以上，发现来自发射线激发由于EIE和那些从电子-离子复合线的相同的原子种类。这不仅对光致电离和复合的基本物理学提出了质疑，而且对气体星云内元素的物理结构和分布的解释也提出了质疑。本项目将审查前者，以解决后者。具体而言，计算将基于：i）电子-离子复合的统一理论，其包含辐射复合和双电子复合过程，并提供了光电离、复合和激发过程的自洽原子数据集，（ii）最新技术水平的R-矩阵方法的扩展，包括迄今未考虑的相对论效应，以及（iii）光致电离和复合的试验性计算，其展示了对于解决丰度异常问题可能至关重要的低能共振的作用。该项目将涉及对一些元素的临界原子参数进行大规模和高精度计算。高精度是必不可少的，因为天体物理模型需要排除原子物理作为任何重要的不确定性来源。重点将放在低电离阶段的一些最突出的元素上：碳、氮、氧、氖、镁、硅和硫。该项目将产生一套广泛和自洽的高精度辐射和碰撞原子参数，通过分析其光学和红外光谱，天文学家试图了解弥漫的星际云或氢电离的“星云”中元素的丰度。 他们通过测量不同元素离子的特征发射强度来做到这一点，并利用这些来确定星云内的物理性质，包括组成元素的丰度。 这些发射线可以通过几个过程来激发，并且使用碰撞激发的谱线与由于电子捕获而激发的谱线测量的丰度之间存在长期的差异。 该项目旨在通过计算将激发与发射强度相关联的原子参数的理论值来消除一个水平的不确定性。 这项研究的结果将适用于天体物理学中的许多子领域，从我们自己的星系到最遥远的类星体的丰度研究。