2000A&A...357L..13F

We performed chemical equilibrium calculations for a solid-gas mixture in order 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 considered in previous condensation calculations. It turned out that solid FeSi is the first condensate of the abundant refractory elements in S-stars with abundance ratios of C/O≃1 and also for the element mixture in LBV winds when stellar layers having burnt H via the CNO-cycle are exposed after serious mass-loss. The unusual material FeSi has recently attracted much attention in solid state physics and its optical properties have been carefully determined in that context. For temperatures below approx. 300K FeSi shows two strong and comparatively narrow absorption bands located at λλ=32, 50µm for the pure substance and several weak bands. We propose that a strong narrow and presently not identified emission band around λ=47.5 µm seen in AFGL 4106 (Molster et al., 1999A&A...350..163M) in some other highly evolved objects may be due to FeSi dust grains.