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Astron. Astrophys. 323, 189-201 (1997)

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1. Introduction

erbig Ae/Be (HAeBe) stars are young objects with large infrared excesses, variable brightness, intrinsic polarization and complex profiles of spectral lines. Almost all of these features are due to the presence of circumstellar shells. Such shells may be the precursors of the protoplanetary disks discovered around [FORMULA] Pictoris, Vega and other ordinary A-type stars.

Sporadic deep minima when the star fades down to 3 stellar magnitudes are the attributes of about 25% of HAeBe stars (Thé et al. 1994). Wenzel (1969) and Wenzel et al. (1971) first proposed that such minima could be produced by opaque circumstellar dust clouds or clumps moving between the star and observer. The model of variable circumstellar extinction has been extended by Grinin (1988) and Voshchinnikov & Grinin (1992) to explain the behaviour of both colour excesses (the blueing effect) and linear polarization in minima. These effects were related with the light scattering by small grains distributed in the shells.

Extensive spectral observations of HAeBe stars have been made in the visual, near infrared and ultraviolet regions, but detailed data have been obtained only for a few brightest stars: AB Aur, HR 5999, etc. (Praderie et al. 1982; Catala et al. 1986a, 1986b, 1993; Tjin A Djie et al. 1989; Blondel et al. 1989). The main feature in their spectra is the strong [FORMULA] line with a variable profile. The profiles of other lines are usually variable as well. A sophisticated model of the stellar chromosphere and the wind formation region of AB Aur has been developed by Catala et al. (1984) and Catala & Kunasz (1987, hereafter CK). It was based on the observations of [FORMULA], C IV and Mg II lines originating in the inner layers of the circumstellar shells.

There are also observational data on the weak absorption lines which should originate in the outer layers of the shells (see, e.g., Tjin A Djie et al. 1989). As far as we know a simplified model of these layers has been created by Dyck & Milkey (1972) and only recently Sorelli et al. (1996) have studied the line formation in very small and dense clumps at the distances less than about [FORMULA]. The goal of the latter paper was to explain the redshifted absorption components of sodium lines.

In this paper, we consider the ionization degree of atoms in the outer layers of the shells and inside the clumps at the distances from the stars larger than the boundary of the H II region. Our model is based on the semi-empirical model of the wind formation region suggested by CK and the results of Voshchinnikov & Grinin (1992) obtained from the interpretation of the photometric and polarimetric observations of WW Vul in two minima. The aim of the work is to determinate whether the absorption lines originating in the clumps may be observed and what information could such observations provide. Observational data available are briefly discussed in Sect. 2. The model is described in Sect. 3. In Sect. 4, we present and discuss the results. The conclusions are summarized in Sect. 5. The expressions used in our calculations of the ionization and recombination rates, equivalent width of spectral lines, etc. are given in Appendices.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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