Radiative Atomic Processes in Iron-Peak Elements For Non-LTE Astrophysical Models

Nahar博士和她的团队计算了前五个低电离阶段中铁的峰元素--铬、铁、钴、镍和锌的大规模、高精度跃迁和复合几率、振子强度和光致电离截面。在定量恒星光谱的非热力学平衡(NLTE)计算中，这些元素的改进参数对于谱线建模是必要的。在NLTE模型中，辐射场与微观材料性质的自洽积分是基于对源中耦合物理过程的显式处理。这需要为所有能级提供一套广泛的原子参数，以便以足够的精度形成谱线。NLTE计算得到的恒星光谱的总体精度取决于NLTE公式本身的近似。这项工作改进了NLTE恒星模型的物理框架，该模型可以使用足够大的光子频率网格，现在可以在超级计算平台上使用。计算采用相对论R-矩阵编码，结合Breit-Pauli方法，得到了铁族原子和离子的光致电离截面、辐射和双电子复合速率、电子碰撞激发速率、光激发和辐射衰减率。这些参数将改善对模型和观测的理解，如辐射驱动的质量损失、光谱“森林”或数百万条谱线的伪连续统，以及铁峰元素的相对比例的化学演化。该项目培训一名博士后，学习原子物理、等离子体物理和天体物理以及大规模并行平台上的高性能计算方面的先进理论和计算方法。

Dr. Nahar and her team compute large-scale, high precision transition and recombination probabilities, oscillator strengths, and photoionization cross-sections for the Fe-peak elements, Cr, Fe, Co, Ni and Zn in their first five low-ionization stages. Improved parameters for these elements are necessary for spectral line modeling in the non-thermodynamic equilibrium (NLTE) calculations used in quantitative stellar spectroscopy. The self-consistent integration of the radiation field with microscopic material properties in NLTE models is based on explicit treatment of coupled physical processes in the source. This requires an extensive set of atomic parameters for all levels contributing to spectral line formation with adequate precision. The overall accuracy of stellar spectra from NLTE calculations depends on approximations made in the NLTE formalism itself. This work improves the physical framework for NLTE stellar models that can use sufficiently large photon frequency grids now possible on supercomputing platforms. The computations use relativistic R-matrix codes in combination with a Breit-Pauli method and yield photoionization cross sections, radiative and dielectronic recombination rates, electron impact excitation rates, photoexcitation and radiative decay rates for the iron group atoms and ions. These parameters will improve understanding models and observations such as radiatively driven mass-loss, spectral "forests" or pseudo-continuum of millions of lines, and chemical evolution of the relative proportion of Fe-peak elements. This project trains a postdoctoral fellow in advanced theoretical and computational methods in atomic physics, plasma physics and astrophysics, and high-performance computing on massively parallel platforms.