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Astron. Astrophys. 356, 654-664 (2000)


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X-ray irradiated model stellar atmospheres

J. Madej 1 and A. Róaska 2

1 Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland (jm@cirrus.astrouw.edu.pl)
2 Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland (agata@camk.edu.pl)

Received 22 November 1999 / Accepted 2 February 2000

Abstract

We present equations and details of the computer code for precise calculation of LTE model atmospheres which are illuminated by an external radiation field. Both radiative and hydrostatic equilibrium, and coherent Thomson electron scattering have been assumed. Radiation field at each frequency and depth level is computed with the method of variable Eddington factors. The code is fully suitable for the study of mutual illumination effects in close binaries.

We have subsequently computed few sets of model stellar atmospheres of [FORMULA] K, and [FORMULA] (B3 V type star), consisting of hydrogen and helium in solar proportion, and iron of number abundance [FORMULA]. We assume, that the atmosphere is illuminated by isotropic X-rays of diluted thermal spectral distribution with temperature [FORMULA] K. In case of iron-rich models external X-rays cause heating of the uppermost layers up to [FORMULA] K.

Such a choice of [FORMULA] and [FORMULA] differing by 4 orders of magnitude is relevant to the supposed conditions on some rings of the accretion disk in active galactic nuclei (AGN), illuminated by an X-ray source. Our code qualitatively reproduces the structure of an atmosphere and its spectrum for wavelengths ranging from infrared to X-rays. Outgoing spectra clearly exhibit all three components: spectrum of B star, external X-ray blackbody spectrum partly backscattered by the model atmosphere, and radiation from the uppermost layers, heated to very high temperatures. The latter shows He I and He II Lyman opacity jumps, and numerous b-f jumps of highly ionized iron, all of them in emission.

We demonstrate that in all atmospheres of our paper the external illumination by X-rays reduces the H I Lyman jump. This is in agreement with observations of AGNs spectra.

Key words: radiative transfer – stars: atmospheres – galaxies: active – book reviews

Send offprint requests to: A. Róaska

© European Southern Observatory (ESO) 2000

Online publication: April 10, 2000

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