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Astron. Astrophys. 320, 553-567 (1997)

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6. Concluding remarks

We have discussed the process of nucleation of MgO in circumstellar dust shells. This discussion uses cluster properties which are calculated from a new semi-empirical potential model for II-VI compounds which is an extension of the standard semi-empirical potential (e.g. Martin 1983) used in the discussion of pure ionic clusters of I-VII compounds (alkali-halides). The new potential model accounts for the partially ionic, partially covalent bond character of alkaline-earth chalcogenides like MgO. It is shown that such a potential model can be applied to model simultaneously the properties of the molecule and that of the solid and can be applied to calculate the structure and properties of clusters of all intermediate sizes N.

The method applied in this paper to calculate nucleation rates is free from the arbitrary and unrealistic assumptions on which discussions of dust nucleation in circumstellar shells have been based up to now, especially when compared to classical homogeneous nucleation theory (cf. Sedlmayr 1994) used to calculate nucleation rates as is presently done. This has always been a subject of strong criticism (e.g. Nuth and Donn 1985).

This paper presents for the first time a method by which such calculations can be put on much safer basis. It can be extended to other compounds which are perhaps more likely to be responsible for nucleation in M-stars. This requires that similar semi-empirical potential models are constructed for compounds like [FORMULA], [FORMULA] or mixed oxide clusters of Si, Mg, Fe etc. which in principle is not too difficult. An extension of the calculations presented in this paper to the case of the nucleation of the MgS dust observed in C-Stars is possible and the results will be presented in a separate paper (Köhler et al. 1997).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998