Dielectronic Recombination (via N = 2→ N ′ = 2 Core Excitations) and Radiative Recombination of Fe xx : Laboratory Measurements and Theoretical Calculations

We have measured the resonance strengths and energies for dielectronicrecombination (DR) of Fe XX forming Fe XIX via N=2 --> N'=2 (Delta_N=0) coreexcitations. We have also calculated the DR resonance strengths and energiesusing AUTOSTRUCTURE, HULLAC, MCDF, and R-matrix methods, four differentstate-of-the-art theoretical techniques. On average the theoretical resonancestrengths agree to within <~10% with experiment. However, the 1 sigma standarddeviation for the ratios of the theoretical-to-experimental resonance strengthsis >~30% which is significantly larger than the estimated relative experimentaluncertainty of <~10%. This suggests that similar errors exist in the calculatedlevel populations and line emission spectrum of the recombined ion. We confirmthat theoretical methods based on inverse-photoionization calculations (e.g.,undamped R-matrix methods) will severely overestimate the strength of the DRprocess unless they include the effects of radiation damping. We also find thatthe coupling between the DR and radiative recombination (RR) channels is small.We have used our experimental and theoretical results to produceMaxwellian-averaged rate coefficients for Delta_N=0 DR of Fe XX. For kT>~1 eV,which includes the predicted formation temperatures for Fe XX in an opticallythin, low-density photoionized plasma with cosmic abundances, our experimentaland theoretical results are in good agreement. We have also used our R-matrixresults, topped off using AUTOSTRUCTURE for RR into J>=25 levels, to calculatethe rate coefficient for RR of Fe XX. Our RR results are in good agreement withpreviously published calculations.Comment: To be published in ApJS. 65 pages with 4 tables and lots of figure