Non‐LTE Treatment of Molecules in the Photospheres of Cool Stars

We present a technique to treat systems with very many levels, such as molecules, in non-LTE. This method is based on a superlevel formalism coupled with rate operator splitting. Superlevels consist of many individual levels that are assumed to be in LTE relative to each other. The usage of superlevels reduces the dimensionality of the rate equations dramatically and, thereby, makes the problem computationally more easily treatable. Our superlevel formalism retains maximum accuracy by using direct opacity sampling (dOS) when calculating the radiative transitions and the opacities. We developed this method in order to treat molecules in cool dwarf model calculations in non-LTE. Cool dwarfs have low electron densities and radiation fields that are far from blackbody radiation fields; both properties may disqualify them from the common LTE approximation. Therefore, the most important opacity sources, the molecules, need to be treated in non-LTE. As a case study we applied our method to carbon monoxide. We find that our method gives accurate results since the conditions for the superlevel method are very well met for molecules. Because of very high collisional cross sections with hydrogen and the high densities of H 2 , the population of CO itself shows no significant deviation from LTE.