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Astron. Astrophys. 343, 531-535 (1999)


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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

1 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, PL-00-478 Warszawa, Poland
2 Institute of Physics, Opole University, Oleska 48, PL-45-052 Opole, Poland

Received 19 August 1998 / Accepted 7 December 1998

Abstract

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 [FORMULA] K and [FORMULA] K, and [FORMULA] (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 [FORMULA] in both EUV filters of the Wide Field Camera (ROSAT) in case of the hotter, Fe-rich white dwarf of [FORMULA] K.

Key words: atomic data – stars: atmospheres – stars: white dwarfs – X-rays: stars

Send offprint requests to: J. Madej

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999

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