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## The Treatment of Line Blocking

The approach which is best suited for our purposes is the *opacity sampling* technique (cf. Peytremann
1974; Sneden et al. 1976). This method allows us to account for the *line shift* in the wind, since
a rearrangement of opacities, as required for opacity distribution functions (cf. Hubeny XXX1997), does not
take place. In this way, the correct influence of line blocking on bound-bound transitions (cf. item (iii)),
which has to conserve the frequential position, can be treated.

For the *opacity sampling,* a set of frequency points is distributed in a logarithmic scale over the
relevant spectral range (160Å-1200Å
for O stars) and the transfer equation is solved for each point. Note that for the ionization calculations
it is important to extend the line-blocking calculations to the range shortward of the HeII edge
(cf. Pauldrach et al. 1994a).

In this way the exact solution is reached by increasing the number of frequency points. By investigating the
accuracy of typical photoionization integrals we found that convergence can be achieved with approximately 1000
to 2000 frequency points in the relevant range (cf. Sellmaier 1996; Pauldrach et al. 1997). The advantages
of the opacity sampling method are obvious: we can investigate the effects of line blocking on selected bound-bound
transitions if we spread additional points around the transition frequency of interest; furthermore, we can apply
the Doppler shift to the line absorption and emission coefficients.

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*1999-10-16*