Non-LTE models for the expanding atmospheres of Wolf-Rayet (WR) stars became available more than a decade ago in the form of two codes developed independently by Hillier (1987a,b) and by the Kiel/Potsdam group (Hamann & Schmutz 1987). While the first model generations were restricted to pure-helium atmospheres, the more complex atoms of C, N and O were accounted for in later model versions (e.g. Hillier 1989; Koesterke & Hamann 1995). However, line blanketing by iron group elements was still neglected because of the extreme complexity of these multi-electron systems with their overwhelming number of spectral lines.
Utilizing a Monte Carlo technique, Schmutz (1991, 1997) showed that line-blanketing affects WR atmospheres fundamentally. By a large number of spectral lines, radiation is blocked in the far UV but escapes at longer wavelengths. This strongly influences the emergent energy distribution, the ionizing radiation field throughout the atmosphere, and the dynamics of WR winds.
Hillier & Miller (1998) were the first to include non-LTE line-blanketing in their WR models. These models have already been applied by several authors (Hillier & Miller 1999; Crowther et al. 2000; Dessart et al. 2000; Herald et al. 2001), primarily for the analysis of WC stars, which show the most complex spectra among the WR population. In the present paper, we now describe the implementation of iron group line-blanketing in the Potsdam/Kiel code for expanding stellar atmospheres, and demonstrate the effects by comparison of a blanketed WC star model with its un-blanketed counterpart.
The principal difficulty in dealing with iron is the complexity of the model atom. The iron
group data of Kurucz (1991) comprise about
line transitions between several
thousands of energy levels. Therefore, a detailed non-LTE treatment, as being possible for a
few hundred levels and a few thousand lines of the CNO elements, is prohibitive for iron.
Instead we adopt the concept of super-levels, which was introduced by Anderson (1989, 1991)
and applied successfully by several authors (Dreizler & Werner 1993; Hubeny & Lanz 1995; Hillier & Miller 1998). In this approach,
the energy levels are represented by a much smaller number (of the order of 10 per ion) of
so-called "super-levels'' which then can be treated explicitly in non-LTE. When modeling
static atmospheres, line opacities may be re-arranged in frequency (cf. the technique of
opacity distribution functions, e.g. Carbon 1979). In expanding atmospheres, however,
neighboring frequencies are coupled by the Doppler effect. Therefore, in our radiation
transport all individual line transitions are calculated at their proper frequency.
It is highly debated whether the driving of WR winds is achieved by the radiative acceleration alone. By our explicit treatment of all line opacities, the complex line-line interaction which leads to multiple photon scattering is fully accounted for. Our models thus allow for a realistic calculation of the radiation pressure.
Un-blanketed model calculations are not perfectly reproducing the observed spectra, especially for the WC subtypes. A large number of iron line transitions (Fe IV, Fe V and Fe VI) form a pseudo continuum in the UV, which dominates the observable energy distribution in that spectral region. Another problem is to reproduce the observed wide range of ionization stages. We will show that the line-blanketing models lead to a better agreement and hence more reliable spectral analyses.
In Sect. 2 we give an overview over the main assumptions and methods used in the atmosphere code. On this basis we describe in Sect. 3 the implementation of the super-level concept. In Sect. 4, the code is applied to the early-type WC star WR 111, and the effects of iron group line-blanketing on the model atmospheres are discussed.
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