Level-resolved photo-ionisation cross sections for modelling astrophysical plasma

Photo-ionised plasma is commonly found in a variety of settings in astrophysics, across a range of temperature and density scales. The photo-ionisation cross sections most often used to model emission and absorption in these settings are total cross sections calculated by the Opacity Project. While these high quality data are a standard in atomic physics, use of total cross sections in modelling plasma may incorrectly model level populations and ion fractions under certain conditions. Using cross sections resolved by both initial and final level are the best way to avoid such problems. New photo-ionisation calculations for S$^{+1-3}$ have been carried out using the atomic package Autostructure. Level-resolved cross sections from these and other calculations have been made available in one format for all the low charge states of C, N, O, Ne, Si and S. The IDL ionisation equilibrium routines from the latest version of CHIANTI have been modified so that this process can be included in ion fraction calculations. The sources of the photo-ionisation (PI) cross sections and the file formats are described in the original MNRAS article and given at the end of each data file. The modified CHIANTI routines are also available here and at CHIANTI-VIP hosted on Github. They allow the users to include PI into the CHIANTI advanced model routines. These will, amongst other things, allow the user to reproduce the models from Dufresne, Del Zanna and Badnell (2021) and Dufresne, Del Zanna and Storey (2021).