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G. V. Brown (LLNL), P. Beiersdorfer (LLNL), K. R. Boyce (GSFC), H. Chen (LLNL), M. Chen (LLNL), K. C. Gendreau (GSFC), J. Gygax (GSFC), S. M. Kahn (Columbia), R. L. Kelley (GSFC), D. A. Liedahl (LLNL), F. S. Porter (GSFC), C. K. Stahle (GSFC), A. E. Szymkowiak (GSFC), K. Widmann (LLNL)
Owing to the high-resolution spectra provided by Chandra and XMM, it is now possible to use line-based diagnotics on non-terrestrial sources. Two of the stongest, most distinct lines observed in the spectra of stellar coronae by these missions, and by missions observing the solar corona, are the resonance and intercombination lines in Fe XVII located at 15.01 and 15.26 Å, respectively. Recent theoretical values are significantly higher than those observed. In the case of the Sun, this discrepancy has been attributed to the resonant scattering of the 15.01 Å\space line. The amount of scattering has been used to infer optical depth and column density. A large variation among theoretical values, however, has made the reliable use of this diagnostic impossible. To provide an accurate description of this line ratio in the optically thin limit we have measured it in the laboratory using the LLNL EBIT as a source and a spare Astro-E/XRS microcalorimeter system, and flat-c! rystal spectrometers for higher-resolution measurements. Our study includes conditions where different line formation mechanisms contribute to each line. Our ratios agree with those of Capella and of flaring active regions of the Sun demonstrating that there is little or no opacity effect in these sources. The ratio may become lower, however, if there is a substantial fraction of Fe XVI in the plasma. This is due to blending with Na-like innershell satellites. Thus, our work indicates that the ratio of the intensities of the 15.01 and 15.26 Å\space lines is more a diagnostic of electron temperature than of opacity.
Work by the UC-LLNL was performed under the auspices of DOE under contract W-7405-ENG-48 and supported by NASA Space Astrophysics Research and Analysis grants to LLNL, GSFC, and Columbia University.