HEAD Division Meeting 1999, April 1999
Session 18. Stellar Coronae
Poster, Tuesday, April 13, 1999, 8:30am-6:02pm, Gold Room

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[18.02] Laboratory Calibration of the 3darrow2p Intensity Ratio for Opacity Diagnostics of High-Temperature Astrophysical Plasmas

G. V. Brown, P. Beiersdorfer, D. A. Liedahl, K. Reed, S. B. Utter (Lawrence Livermore National Laboratory), S. M. Kahn (Columbia Astrophysics Laboratory)

The n = 3arrow2 \space\, 1\!P1 and \, 3\!D1 resonance and intercombination lines are among the strongest spectral features found in L-shell X-ray spectra of neon-like ions, making them prime diagnostic candidates. Because the \, 1\!P1 line is a resonance line, it is preferentially scattered relative to the intercombination line and, therefore, the intensity ratio of these two lines in Fe XVII has been employed as an opacity diagnostic in solar observations. The line ratio is expected to play a similar role in CXO and XMM observations of hot plasmas. However, the reliability of this diagnostic depends heavily on an accurate intensity ratio for the optically thin case where no scattering occurs. Theoretical calculations of this ratio exhibit large variation. For example, in the case of Fe XVII, the calculated ratio varies between 2.7 and 4.7. This large variation prohibits the use of this ratio as an accurate diagnostic. In order to provide accurate values for use as optically thin benchmarks we have done a systematic laboratory study of this ratio for ions along the neon-like iso-electronic sequence between Cr XV and Kr XXXVII. Our measurements were done using the Electron Beam Ion Trap facility under conditions where the only line formation processes are electron impact excitation followed by radiative cascades. We have compared our measured ratios with those predicted by a distorted wave calculation. Our comparison shows a systematic difference between experiment and this particular theory of approximately 30%. Our laboratory calibration of this diagnostic ratio reduces the uncertainty in the optically thin value to 5% in the case of Fe XVII.

This work was performed under the auspices of the U.S.D.o.E. under Contract No. W-7405-Eng-48 and supported by NASA HEA X-ray Astronomy Research and Analysis grant NAG5-5123 and work order W-19127.

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