Improving the atomic modelling for solar UV radiative transfer calculations

Abstract

Radiative transfer calculations have been produced over the years for many lines and continua in the UV wavelength range of solar and cool stellar atmospheres for a variety of conditions. Despite significant improvements in computing power and availability of atomic data over time, atomic models are often still limited in size and rely on approximations for data. There have also been inconsistencies in the way photo-ionisation and radiative recombination have been treated. Here, we incorporate into the Lightweaver radiative transfer code new data and updated modelling of atomic processes for the low charge states of C, Si and S. Data are taken from the CHIANTI database and other widely-available sources for the relevant elements. We show the significant impact this has on the UV continua in the 1100-1700{\AA} region, especially for Si. The results are in much better agreement with averaged, quiet Sun observations, and remove the need to invoke "missing opacity" to resolve discrepancies. The present treatment has important implications for radiative transfer calculations and the model atmospheres used as inputs.