Atomic data and spectral model for Fe III

We present new atomic data (radiative transitions rates and collision strengths) from large scale calculations and a non-LTE spectral model for Fe III. This model is in very good agreement with observed astronomical emission spectra, in contrast with previous models that yield large discrepancies with observations. The present atomic computations employ a combination of atomic physics methods, e.g. relativistic Hatree-Fock, the Thomas-Fermi-Dirac potential, and Dirac-Fock computation of A-values and R-matrix with intermediate coupling frame transformation and Dirac R-matrix. We study the advantages and shortcomings of each method. It is found that the Dirac R-matrix collision strengths yield excellent agreement with observations, much improved over previously available models. By contrast, the transformation of LS-coupling R-matrix fails to yield accurate effective collision strengths at around 10^4 K, despite using very large configuration expansions, due to the limited treatment of spin-orbit effects in the near threshold resonances of the collision strengths. The present work demonstrates that accurate atomic data for low ionization iron-peak species is now within reach.