On atomic line opacities for modelling astrophysical radiative transfer

ABSTRACT In astrophysics, atomic transition line opacity is a primary source of uncertainty, leading to orders of magnitude discrepancy in theoretical calculations of radiative transfer in the literature. Much of this uncertainty is dominated by the inability to resolve the lines in frequency, leading to the use of approximate frequency-averaged treatments, often employing the ‘line-expansion formalism’. In this short paper, we assess the usage of this formalism, specifically the prominent Eastman & Pinto formula (hereafter EP93). As a case study, we reproduce EP93 opacities from the commonly used stella simulations in order to highlight the orders of magnitude effect due to the choice of line treatment. We show that the widely used EP93 expansion opacity substantially underestimates photon emissivity and reprocessing rates, even when it correctly captures photon mean-free-paths. We also highlight the importance of introducing micro-plasma electron excitation level cut-offs in the equation of state for calculating opacity. Though a fully consistent coarse frequency solution does not currently exist for line modelling, we examine an alternative method for calculating emissivity based on a simple frequency-binned average of the lines, and introduce a physically motivated modification that leads to a minor reduction in opacity. Finally, we describe new features in our updated publicly available high-resolution frequency-dependent opacity table.