Resonance-averaged Photoionization Cross Sections for Astrophysical Models

We present ground-state photoionization cross sections of atoms and ions averaged over resonance structures for photoionization modeling of astrophysical sources. The detailed cross sections calculated in the close-coupling approximation using the R-matrix method, with resonances delineated at thousands of energies, are taken from the Opacity Project database TOPbase and the Iron Project, including new data for the low ionization stages of iron Fe I-Fe V. The resonance-averaged cross sections are obtained by convolving the detailed cross sections with a Gaussian distribution over the autoionizing resonances. This procedure is expected to minimize errors in the derived ionization rates that could result from small uncertainties in computed positions of resonances while preserving the overall resonant contribution to the cross sections in the important near-threshold regions. The detailed photoionization cross sections at low photon energies are complemented by new relativistic distorted-wave calculations for Z ≤ 12 and from central-field calculations for Z > 12 at high energies, including inner shell ionization. The effective cross sections are then represented by a small number of points that can be readily interpolated linearly for practical applications; a FORTRAN subroutine and data are available. The present numerically averaged cross sections are compared with analytic fits that do not accurately represent the effective cross sections in regions dominated by resonances.