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

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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.