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The abundances of some of the most common elements in the Sun has been called into question by recent analysis based on new non-LTE hydrodynamic models. The new abundances of C, N, O, Ne etc. are 30–40% lower than the ’standard’ solar abundances, which were heretofore well established and supported by precise helioseismological observations. A number of recent studies have addressed the issue from different perspectives. Given the inverse relation between opacities and abundances, another possible solution has been suggested: Upward revision of stellar opacities by about 10%. But the recalculation of astrophy sical opacities by The Opacity Project (OP) and the OPAL groups has led to remarkable convergence in the final results, in spite of some fundamental differences in atomic physics and plasma physics. Deviations beyond a few percent level in the Rosseland mean opacities have been ruled out. However, we discuss the accuracy of the input physics in opacities calculations, as well as detailed monochromatic (as opposed to mean) opacities, that do in fact yield large differences in radiative accelerations derived from OP and OPAL data. In addition, we note sources of “missing opacity” that might bear on the solar abundances issue. [ABSTRACT FROM AUTHOR]

Physical processes underlying X-ray spectral formation may be influenced considerably by particle energy distribution in the source. It is found that different electron distributions — Maxwellian or Gaussian — in the plasma yield significantly different X-ray line ratios. Owing to energetically uneven occurrence of dense resonances in electron impact excitation cross sections for transitions in Fe XVII, line ratios with Maxwellian and Gaussian averaged rates may differ by up to a factor of two, thereby explaining outstanding discrepancies between two independent laboratory measurements using Electron-Beam-Ion-Traps, as well as with astrophysical observations. The Close Coupling R-matrix theory which includes resonances via an ab initio wavefunction expansion, vs. several variants of the Distorted Wave theory which does not, is discussed. The R-matrix method also enables a unified and self-consistent theoretical treatment of photoionization and (e + ion) recombination (inverse) processes, that includes both the radiative and dielectronic recombination and yields total and level-specific rate coefficients valid at all temperatures. Finally, atomic databases from the Opacity Project and Iron Project at the Center de Donnees Astronomiques de Strasbourg and the Ohio Supercomputer Center are described, including a novel on-line database to compute stellar opacities for an arbitrary mixture of elements. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]

The R-Matrix method has long been employed to compute fundamental atomic parameters with high precision for large scale applications to astrophysical sources and magnetic and intertial fusion devices. Ongoing work is part of two projects: The Iron Project that focuses on Fe-peak elements, and the RmaX Project aimed at spectral diagnostics of laboratory and astrophysical X-ray plasmas. The primary atomic processes include electron impact excitation, photoionization, (e + ion) recombination, and spectral transitions. These data are employed in numerical simulations of high-temperature plasmas under stationary and transient conditions. The calculated paraemeters have been benchmarked against sophisticated experiments on electron-beam-ion-traps for excitation, synchrotron based light sources for photoionization, and heavy ion storage rings for (e + ion) recombination. Extensions of the two projects include a self-consistent and unified theoretical treatment of (e + ion) recombination that subsumes both the radiative and the dielectronic recombination processes. Cross sections are computed using an identical (e + ion) wavefunction expansion for both the photoionization and the recombination processes, thereby considering the resonant and non-resonant phenomena in an ab initio manner. Comparison with experiments ascertain an accuracy of 10–20%, within the uncertainties in experiment and theory, indicating that there are no significant shortcomings in the R-matrix method. We compare the close coupling R-matrix data with the Distorted Wave approximation and the effect on X-ray spectral analysis. Finally, we describe the electronic database TIPTOPbase, which will also make available new stellar opacities and radiative accelerations from the Opacity Project. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]