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Astron. Astrophys. 357, L13-L16 (2000)


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Letter to the Editor

FeSi as a possible new circumstellar dust component

A. Ferrarotti 1, H.-P. Gail 1, L. Degiorgi 2 and H.R. Ott 2

1 Universität Heidelberg, Institut für Theoretische Astrophysik, Tiergartenstrasse 15, 69121 Heidelberg
2 ETH-Hönggerberg, Laboratorium für Festkörperphysik, 8093 Zürich

Received 25 February 2000 / Accepted 31 March 2000

Abstract

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[FORMULA] 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. 300 K FeSi shows two strong and comparatively narrow absorption bands located at [FORMULA]m for the pure substance and several weak bands. We propose that a strong narrow and presently not identified emission band around [FORMULA]m seen in AFGL 4106 (Molster et al. 1999) and possibly in some other highly evolved objects may be due to FeSi dust grains.

Key words: stars: circumstellar matter – stars: mass-loss – stars: AGB and post-AGB – stars: winds, outflows – ISM: dust, extinction

Send offprint requests to: gail@ita.uni-heidelberg.de

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000

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