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Astron. Astrophys. 363, 1055-1064 (2000)

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1. Introduction

Effects of Compton scattering of X-rays on free electrons are very important in stellar atmospheres at various effective temperatures. In bursting neutron stars ([FORMULA] K), or even in the hottest white dwarf stars ([FORMULA] K), X-rays are created by thermal processes in the atmosphere. In case of cooler atmospheres, one can consider scattering of X-rays incoming from some external source. The latter is the subject of our paper, in which we formulate equations of model atmosphere and radiative transfer.

Our approach to the Compton scattering is valid for photons of arbitrarily large initial energies, which are subsequently scattered by electrons in fully relativistic thermal motion (Pomraning 1973). The equations are also suitable to the situation, when Compton scattering dominates other opacity sources. In each iteration step our code computes coefficient of scattering opacity from relativistic formulae, thus avoiding Klein-Nishina expression, the latter valid only for the scattering from electrons at rest. The equations were formulated and described in detail by Madej (1991, hereafter Paper I). However, algebraic decomposition of integro-differential equations and elimination scheme used here are taken after Madej & Rózanska (2000), hereafter Paper II, and are significantly changed as compared with Paper I. In the following sections most of equations are just reminded, while we do not present their detailed derivation.

In this paper we consider the structure and the spectrum of a hot main sequence B star ([FORMULA] K and [FORMULA], cgs units), illuminated from outside by very hot diluted thermal X-rays ([FORMULA] K). Two series of models were computed, the first one consisting of hydrogen and helium in solar proportions, and the second one including also iron with number abundance [FORMULA]. Transfer of radiation is solved with the method of variable Eddington factors. External illumination was included in the equation of transfer following the method by Rucinski (1970). All the models assume LTE equation of state.

This paper is a substantial extension of our previous research (Paper II), in which we have presented and solved set of equations defining a model atmosphere in radiative and hydrostatic equilibrium, exposed to the external radiation field. In Paper II we assumed that electron scattering is fully coherent (Thomson scattering). This restrictive assumption did not allow for realistic reproduction of the external illumination effects. In the present paper we append Compton scattering terms to the model atmosphere equations, leaving method of solution essentially unchanged.

Our code is a general purpose computer program and can be used for a wide variety of problems. Optionally, Compton scattering can be replaced by Thomson scattering without changing model equations. Also the external illumination can be simply set to zero for model atmosphere computations of single stars. The code can be used for the study of illumination effects in close binary stars, and the atmospheres of accretion disks in active galactic nuclei (AGN), and cataclysmic variables (CV). However, in case of an accretion disk the energy dissipation via viscosity should be taken into account (cf. Hubeny 1990a). We plan to study this problem in future work.

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© European Southern Observatory (ESO) 2000

Online publication: December 5, 2000
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