IRON ABUNDANCE IN THE HOT DA WHITE-DWARFS FEIGE-24 AND G191-B2B

Model calculations of the far- and extreme-ultraviolet line spectra of highly ionized Fe species (Fe IV, Fe V, and Fe VI) for hot, high-gravity, H-rich stars are presented. A spectral analysis of a total 31 hr of exposure of the DA white dwarf Feige 24 with IUE in the echelle mode reveals the presence of Fe with an abundance relative to H by number n(Fe)/n(H) = (5-10) x 10(-6) with an uncertainty dominated by the determination of stellar parameters. An analysis of IUE data from the white dwarf G191 B2B results in a similar Fe abundance if this star shares similar atmospheric parameters (T(eff), g) with Feige 24. Fe is thus the second most abundant photospheric element in hot DA white dwarfs. Simulations of the extreme ultraviolet (EUV) spectral range, with Fe abundance determined from the present analysis, show a complex of hundreds of Fe IV, Fe V, and Fe VI resonance lines shielding a large portion of the spectrum and confirm the important role played by heavy elements in the EUV/soft X-ray flux deficiency observed in some hot DA white dwarfs. These relatively high Fe abundances exceed significantly the prediction of current selective radiation pressure calculations.