1999A&A...343..531M

In this paper we discuss new method for the atomic partition function (APF) computations, which is then applied to highly ionized iron, Fe V through Fe VII. Subsequently we have computed LTE model atmospheres of two hydrogen-rich white dwarf stars of the effective temperatures T_eff=8x10⁴K and 1.2x10⁵K, and logg=8.0 (cgs units), which included trace abundance of iron. Both models correspond to the hottest DA white dwarfs observed e.g. by ROSAT and EUVE missions in the extreme UV and X-rays, in which stars H-rich atmospheres contain small amount of metals driven outwards by radiation pressure. Computational results exhibit a significant impact of the new partition functions of Fe V through Fe VII on both the iron ionization stratification in analysed atmospheres, and their theoretical X-ray opacities and spectra. We conclude, that in both stars our improved APF predict increase of their EUV and X-ray luminosities by factors of 3 to 10, as compared with models based on previously published iron partition functions. We demonstrate also the importance of our APF on luminosity ratio s2/s1 in both EUV filters of the Wide Field Camera (ROSAT) in case of the hotter, Fe-rich white dwarf of T_eff=1.2x10⁵K.