RESONANCE-AVERAGED PHOTOIONIZATION CROSS-SECTIONS FOR ASTROPHYSICAL MODELS

We present ground-state photoionization cross sections of atoms and io ns averaged over resonance structures for photoionization modeling of astrophysical sources. The detailed cross sections calculated in the c lose-coupling approximation using the R-matrix method, with resonances delineated at thousands of energies, are taken from the Opacity Proje ct database TOPbase and the Iron Project, including new data for the l ow ionization stages of iron Fe I-Fe v. The resonance-averaged cross s ections 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 c ould result from small uncertainties in computed positions of resonanc es while preserving the overall resonant contribution to the cross sec tions in the important near-threshold regions. The detailed photoioniz ation cross sections at low photon energies are complemented by new re lativistic distorted-wave calculations for Z less than or equal to 12 and from central-held calculations for Z > 12 at high energies, includ ing inner shell ionization. The effective cross sections are then repr esented by a small number of points that can be readily interpolated l inearly for practical applications; a FORTRAN subroutine and data are available. The present numerically averaged cross sections are compare d with analytic fits that do not accurately represent the effective cr oss sections in regions dominated by resonances.