Electron-Ion Recombination Rate Coefficients, Photoionization Cross Sections, and Ionization Fractions for Astrophysically Abundant Elements. I. Carbon and Nitrogen

We present a comprehensive and self-consistent set of new atomic data for ionization balance in radiatively and collisionally ionized astrophysical plasmas. Complex resonant phenomena resulting in rapid energy variation in the cross sections for photoionization and recombination require accurate and large-scale calculations, as reported. Another new development is the consideration of the unified nature of the recombination process in an ab initio manner, via resonances embedded in the electron-ion continua, which have been heretofore considered as separate processes of radiative recombination and dielectronic recombination. A single set of total electron-ion recombination rate coefficients is thereby obtained as a function of electron temperature. The present calculations also meet the hitherto neglected, but theoretically essential, criterion of self-consistency between the rates for the inverse processes of photoionization and recombination, ensured by describing all atomic processes with an identical set of eigenfunction expansion within the close-coupling approximation using the R-matrix method. Photoionization cross sections and total electron-ion recombination rate coefficients for the carbon and nitrogen isonuclear sequences, C I-C VI and N I-N VII, are presented. Ionization fractions in coronal equilibrium are also computed. The present photoionization cross sections have been calculated using more extensive eigenfunction expansions than those in the Opacity Project. In addition to the total photoionization and recombination data, state-specific cross sections are also obtained for a large number of excited states for non-LTE models. Complete data sets are available electronically.