M NEW RESULTS FOR PHOTOIONIZATION AND RECOMBINATION OF ASTROPHYSICALLY ABUNDANT ATOMS AND IONS: THE CARBON SEQUENCE

Photoionization cross sections and electron-ion recombination rate coefficients are presented for carbon-like ions obtained using methods developed for the Opacity Project. General features of the new data are dis-cussed, and their astrophysical relevance is pointed out. The illustrative results for photoionization include extensive autoionization resonance structures and partial photoionization into specific states of the residual ion. It is shown that the excited state photoionization cross sections are not, in general, described by hydro-genic behavior and often contain strong resonances due to photoexcitation corresponding to dipole transitions in the core ion; these are referred to as photoexcitation-of-core resonances that attenuate the background cross section over much larger energy ranges than the Rydberg type of resonances. The effective photoioniza-tion cross sections are thus considerably enhanced, for excited bound states, relative to the hydrogenic form. A new method is described for the calculation of total recombination rate coefficients that accounts for both the radiative and the dielectronic recombination processes in an ab initio manner. The recombination calculations are carried out in the close coupling approximation using the Milne relation with detailed photoionization cross sections for large numbers of excited states of each atom or ion, and employing a precise theory of dielectronic recombination given by Bell & Seaton. The present recombination rate coefficients are compared with earlier works on radiative and dielectronic recombinations treated separately and significant differences are noted. Recombination rate coefficients are presented for C I, N n, 0 III, F IV, and Ne v at a wide range of temperatures for astrophysical applications. Subject headings: atomic data- atomic processe