Opacity gives a measure of radiation transport in a medium such that higher or lower opacity indicates more or less attenuation of radiation. As the radiation propagates, opacity is caused by the absorption and emission of radiation by the constituent elements in the medium, such as astrophysical plasmas. It is also affected by photon scatterings. Hence opacity depends mainly on the intrinsic atomic processes, photo-excitation in a bound-bound transition, photoionization in a bound-free transition, and photon-electron scattering. Monochromatic opacity at a particular frequency, κ(ν), is obtained mainly from oscillator strengths (f) and photoionization cross sections (σPI). However, the total monochromatic opacity is obtained from summed contributions of all possible transitions from all ionization stages of all elements in the source. Calculation of accurate parameters for such a large number of transitions has been the main problem for obtaining accurate opacities. The overal mean opacity, such as Rosseland mean opacity (κR), depends also on the physical conditions, such as temperature and density, elemental abundances and equation of state such as local thermodynaic equilibrium (LTE) of the plasmas. For plasmas under HED (high energy density) conditions, fluid dynamics may be considered for shock waves such as in a supernova explosion. In this report, I will exemplify the necessity for high precision atomic calculations for the radiative processes of photoexcitation and photoionization in order to resolve some perplexing astrophysical problems relevant to elemental abundances and hence opacities. In particular I will present results on oscillator strengths of Fe XVIII and photoionization cross sections of Fe XVII which are abundant in high temperature plasmas, such as solar corona, and photoionization and recombination of O II which is abundant in low temperature plasmas, such as in a planetary nebula. Sophisticated atomic calculations under the Iron Project are revealing important and dominant features not included in the current opacities. Opacities with these new results are expected to resolve the longstanding problems on abundances in the sun, orion nebula etc.
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