SILICON-CARBIDE IN CIRCUMSTELLAR ENVIRONMENT

The presence of small clusters of silicon carbide (SiC) in circumstell ar dust shells surrounding late-type stars is inferred from a broad em ission feature peaking at around 11 mu m in infrared spectra [1]. Thes e clusters are expected to condense from molecular arrangements compos ed of a few carbon and silicium atoms which are present in stellar win ds surrounding carbon-rich late-type stars. We have searched for the p ossible geometric structures of SiCn+ radicals (n less than or equal t o 5) on the basis of ah initio calculations. It is predicted that the linear structures are energetically favored compared to the other plan ar and 3D ones, except for SiC2+ where a doubt subsists and for SiC3where the rhombic structure is clearly more stable than the linear one . In the same way, a conspicuous even-odd alternation of the stability with the number of carbon atoms along the subset of linear species is shown. Vibrational frequencies of all the structures under considerat ion have then been determined. The possible destabilizing influence of a finite temperature effect on these structures has also been analyze d by using general considerations of thermal statistics.