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A. Schweitzer, P. Hauschildt (Dept. of Physics & Astronomy, UGA)
We developed a technique to treat huge systems like molecules in non--LTE. This technique is based on the superlevel formalism. Superlevels consist of many individual levels that are assumed to be in LTE. The usage of superlevels reduces the size of the rate equations and the number of rates dramatically and, thereby, makes the problem computationally feasible. Our superlevel formalism retains maximum accuracy by using direct opacity sampling (dOS) when calculating the radiative transitions. We implemented this method in our current model atmospheres for cool dwarfs. Cool dwarfs have low electron densities and a radiation field that is far from a black body. Both properties invalidate the conditions for the common LTE approximation. Therefore, we need to treat the huge molecular systems in non--LTE. As a case study we applied our method on carbon monoxide. We find that our method gives accurate results since the conditions for the superlevel method are very well met for molecules. The test molecule CO shows significant deviations from LTE in the outer regions of cool photospheres.