The Iron Project: Photoionization and Photoexcitation of Fe XVII in Solar Opacity

Abstract

Opacity is a fundamental quantity for plasmas and gives a measure of radiation transport. It is caused by the absorption and emission of photons by the constituent elements of the plasma and hence depends mainly on the atomic processes of photoexcitation and photoionization. It is also affected by photon scatterings. Monochromatic opacity at a particular frequency, κ(ν), is obtained from oscillator strengths (f) for bound-bound transitions and photoionization cross sections (σ _PI). However, the total monochromatic opacity depends on the summed contributions of all possible transitions from all ionization stages of all elements in the plasma. Calculation of accurate atomic parameters for such a large number of transitions has been the main problem for obtaining accurate opacities. The overall mean opacity, such as Rosseland mean opacity (κ _R ), depends also on the physical conditions, such as temperature and density, elemental abundances and equation-of-state such as local thermodynamic equilibrium (LTE) of the plasmas.

In this report, I will illustrate the necessity for high-precision atomic calculations for the radiative processes of photoexcitation and photoionization in order to resolve some perplexing astrophysical problems, particularly solar abundances and opacities. Fe XVII is most abundant iron ion in the solar convection zone. I will present new results on oscillator strengths and new features in high energy photoionization cross sections of Fe XVII which give clear indication of the reason for discrepancy between measured and theoretically predicted abundances and on how the discrepancy is resolved or reduced.