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J. K. Lepson (Space Sciences Lab, UC Berkeley), P. Beiersdorfer (Lawrence Livermore National Laboratory), E. Behar (Physics Department, Technion), S. M. Kahn (Department of Physics, Columbia Univ.)
{\em Chandra} and {\em XMM-Newton} observations have found many unidentified relatively weak emission lines in stellar spectra in the 20 -- 70 Å\ region. Many of these may arise from the L-shell ions of argon, sulfur, silicon, and magnesium. Line lists for these elements are insufficient for identifying these lines. As part of a project to compile a comprehensive catalogue of astrophysically relevant ions in the EUV, we report here on spectra laboratory measurements from these ions as well as on calculations from the Hebrew University Lawrence Livermore Atomic Code. We used the LLNL electron beam ion traps to take spectra of argon and sulfur at densities similar to that of stellar atmospheres (\leq 1012 cm-3). By comparing spectra taken at different electron energies we compiled line lists for \ion{Ar}{9} -- \ion{Ar}{16} and \ion{S}{7} -- \ion{S}{14}. We identified 79 of the strongest transitions of argon and 127 transitions of sulfur. Initial results from measurements of \ion{Si}{5} -- \ion{Si}{12} will also be presented. The HULLAC calculations can have positional errors of > 100 mÅ, making identifications of weak transitions problematic. Comparing our data to {\em Chandra} observations of Procyon we confirm identifications of some of the smaller lines and correct others, for example, a line identified as \ion{S}{9} as \ion{Ar}{9}. In addition, we report a systematic discrepancy between measured and calculated ratios of 3d arrow 2p and 3s arrow 2p transitions similar to those found earlier for \ion{Fe}{17} and \ion{Fe}{18} (for example in {\em Chandra} observations of NGC4636). This work was supported by NASA SARA grant W-19,878 and performed under the auspices of the DOE by UC-LLNL under Contract W-7405-Eng-48.
Bulletin of the American Astronomical Society,
35#2
© 2003. The American Astronomical Soceity.