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Next: The Treatment of Line Up: Realistic Models For Expanding Previous: Atomic Models

Non-LTE Line Blocking and Blanketing

>From the discussion in the previous sections it is evident that the construction of realistic models for expanding atmospheres requires a correct and consistent description of line blocking and blanketing. The method for this is well established for cold stars, where the atmospheres are hydrostatic and the assumption of LTE is justified (cf. Kurucz 1979, 1992); the method fails, however, for hot stars, where non-LTE effects are prominent and the atmospheres are expanding rapidly. In this case one has to account for:

The line shift caused by the Doppler effect due to the velocity field.

The influence of line blocking on the radiative photoionization rates.

The correct influence of the line-blocked radiation field on the radiative bound-bound rates.

The correct incident radiation for the radiative acceleration term.

The influence of line blocking opacities on the energy balance.

The first three items are connected to the fact that the radiation field is drastically reduced in the EUV due to line opacities and is redistributed to lower energies. The important effect of the velocity field hereby is that it increases the frequency range which can be blocked by a single line. The fourth item reflects what is usually meant by line overlap (see, for instance, Puls 1987), and the last item concerns the increase of the Rosseland optical depth and in consequence of the temperature due to line opacities; this refers to line blanketing.

So far, there are two other approaches to the theory which try to overcome the problem as described above. The method presented by Schaerer & Schmutz (1994; see also Schaerer & de Koter 1997) is at present restricted, since they do not solve the rate equations for the metals (most importantly, non-LTE calculations for Fe are missing) and hence they cannot account for items (ii) and (iii), whereas the method by Hillier & Miller (1997) does not appear to have basic restrictions. In the following we will give an overview of our approach.


next up previous
Next: The Treatment of Line Up: Realistic Models For Expanding Previous: Atomic Models