Determinations of metal abundances from ROSAT observations

Metallicities of thermal plasmas in the temperature range 1-30 x 10(6) K can formally be determined by spectral fitting of data from the Position Sensitive Proportional Counter (PSPC) on ROSAT. For the ROSAT band, iron is the dominant species in the metallicity determination. We examined the accuracy of PSPC abundance determinations by analyzing X-ray spectra of stars with known photospheric metallicities; coronal abundances are known for Capella from prior X-ray and extreme ultraviolet data. Metallicities are determined for 10 stars from PSPC spectra, with best-fit values equal to or less than 0.2 of the solar value, despite the fact that the known metallicities are typically near the solar value. This factor of 5 discrepancy between the known metallicities and the PSPC values is probably not due to systematically low chromospheric abundances or to instrumental miscalibration, although additional calibration tests may be warranted. Overestimation of the rate coefficients for electron-impact excitation of the Fe L lines could cause this metallicity discrepancy, so improved rate calculations would be valuable. In spectral fits to PSPC data, the derived values for the Galactic absorption column and the X-ray temperature depend upon the adopted metallicity. Lowering the metallicity from the solar value to 0.2 of the solar value can increase the absorption column and the X-ray temperature by as much as factors of 2 and 1.5, respectively.