Average-atom model calculations of dense-plasma opacities: Review and potential applications to white-dwarf stars

Methods using average-atom models in order to calculate dense-plasma opacities and conductivities are reviewed. Dense plasmas at moderate temperatures, of interest in white-dwarf modelling, are considered. Due to their relative simplicity of implementation, compared to more detailed models (detailed-level accounting, detailed configuration accounting, etc.), average-atom models are a privileged framework for the application of the most involved dense-plasma statistical modelling. Moreover, the average-atom models are well suited to the calculation of some thermodynamic properties, such as the equation of state. They can also be used in order to estimate broadband radiative properties of dense plasmas. After an introduction to the opacity issue in the modelling of white dwarfs, we make a short review of average-atom models. We then address the methods of calculating the opacity starting from the average-atom model, see some of their limitations, and briefly discuss some problems that remain open, such as the modelling of fluctuations, or the accounting for channel mixing and collective phenomena in the photoabsorption.