FeSi as a possible new circumstellar dust component

We performed chemical equilibrium calculations for a solid-gas mixture in o rder to determine the possible condensates for the peculiar element mixture of S-stars and Luminous Blue Variables (LBV's). Some compounds of abundant elements where included in the calculation which seem to have been not con sidered in previous condensation calculations. It turned out that solid FeS i is the first condensate of the abundant refractory elements in S-stars wi th abundance ratios of C/O approximate to 1 and also for the element mixtur e in LBV winds when stellar layers having burnt H via the CNO-cycle are exp osed after serious mass-loss. The unusual material FeSi has recently attrac ted much attention in solid state physics and its optical properties have b een carefully determined in that context. For temperatures below approx. 30 0 K FeSi shows two strong and comparatively narrow absorption bands located at lambda lambda = 32, 50 mu m for the pure substance and several weak ban ds. We propose that a strong narrow and presently not identified emission b and around lambda = 47.5 mu m seen in AFGL 4106 (Molster et al. 1999) and p ossibly in some other highly evolved objects may be due to FeSi dust grains .