On Atomic Line Opacities for Modeling Astrophysical Radiative Transfer

Abstract

In astrophysics, atomic transition line opacity is a primary source of uncertainty in theoretical calculations of radiative transfer. 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 in simulations, specifically the prominent Eastman \& Pinto 1993 formula (hereafter EP93). As a case study, we reproduce EP93 opacities from the commonly-used STELLA simulations. The latter previously yielded orders of magnitude discrepancy in observed emission relative to similar simulations from our group. The discrepancy is due to differences in line opacity 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 cutoffs in the equation of state (EOS) for calculating opacity. We propose a new method for calculating emissivity, based on a modification of the simple frequency-bin averaged opacity method, in a way that incorporates the effect of expansion on effective line strength. This formulation should reduce the overestimation of the opacity that may occur with the simple averaging method. To our knowledge, no fully-consistent coarse-frequency solution currently exists for line modeling in these systems. Finally, we describe new features in our updated publicly available high-resolution frequency-dependent opacity table.