MGO DUST NUCLEATION IN M-STARS - CALCULATION OF CLUSTER PROPERTIES AND NUCLEATION RATES

The abundant metals Mg and Fe together with SiO form the iron-magnesiu m-silicate dust observed in the circumstellar shells of M-Stars. This dust component cannot nucleate directly from the gas phase but require s for its formation a different kind of seed nuclei. Further, there ar e some hints that besides the silicatic main dust component dust mater ials of a different composition may be present, perhaps oxides of iron and magnesium. We study in this paper the possibility that MgO partic les are formed by nucleation from the gas phase and may form a dust co mponent of their own or act as seed nuclei for the silicate dust. For this purpose we calculated the structures and bond energies for small MgO clusters. The calculation is based on a modification of the classi cal Rittner potential model for alkali halide molecules and clusters. An additional parametrized attractive term is introduced to simulate a large covalent contribution to the bonding in II-VI compounds like Mg O. We calculated cluster structures and bond energies with this empiri cal potential for cluster sizes N up to 16. Magic cluster sizes of enh anced stability are found for cluster sizes 2, 4, 6, 9, 12 and 15 for MgO. Their structure for N greater than or equal to 6 agrees well with experimental results and predictions from Hartree-Fock calculations. The results for the cluster properties are used to calculate particle densities of (MgO)(N)-clusters in stellar winds of late type giants an d the rate of dust particle formation by nucleation of MgO. The nuclea tion rate of MgO turns out to be too small to form directly from the g as phase as a separate dust component or to form the seed nuclei for s ilicate dust formation.