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Theoretical X-ray spectra of hot H-rich white dwarfs. Impact of new partition functions of iron, Fe V through Fe VII
J. Madej 1,
J. Halenka 2 and
B. Grabowski 2
Received 19 August 1998 / Accepted 7 December 1998
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 K and K, and (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 in both EUV filters of the Wide Field Camera (ROSAT) in case of the hotter, Fe-rich white dwarf of K.
Key words: atomic data stars: atmospheres stars: white dwarfs X-rays: stars
Send offprint requests to: J. Madej
Online publication: March 1, 1999