7th International Conference on Matter and Radiation at Extremes (ICMRE2024)

"Opacities are coefficients characterizing the transport of radiative energy through matter” (M.J. Seaton 1996). From our local Sun to the most distant quasars, from the intergalactic medium to massive object mergers,  the radiation arises essentially from atomic processes involving the interaction of atoms, ions or molecules and electrons and/or photons. Among the most important processes, electron impact excitation, photoionization, recombination, photo-excitation and radiative decay have to be studied in order to interpret these observations and to model astrophysical plasmas. However the recent laboratory measurements at Sandia National Lab. of Fe opacities for conditions typical of the Solar plasma at the base of the convection zone (Bailey et al. Nature 2015) are questioning our ability to calculate such fundamental parameters. The difference between the measured Fe opacities and all the existing calculations reach a factor of two. This could be crucial for the long standing (18 years old) problem of the Solar composition. It also impacts all the different domains of research where radiative transfer is at play.

In the framework of the Opacity Project (OP), which aims at providing raw atomic data of high precision as well as opacities for astrophysical application since 1987, we approach the opacities from all aspects Theory - Experiment - Application. In order to understand the differences between the measured Fe opacities at the Sandia National Laboratory and all theoretical calculation, I will present recent efforts from OP in the experimental aspect with 2 experiments we are conducting  as well as our latest therotical calculation of Fe opacities. A detailed comparison between the 2 main methods to calculate opacities are being reviewed. The new set of Fe opacities is being tested on solar models and the effect on radiative accelerations will be presented. These new results pave the way in defining the best way for new calculations for the Opacity Project to prepare a new release of monochromatic opacities, Rosseland and Plank means as well as radiative accelerations.