MgO dust nucleation in M-Stars: Calculation of cluster properties and nucleation rates

M. Köhler, H.-P. Gail, and E. Sedlmayr

Astronomy & Astrophysics, 320, 553-567 (1997)

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Abstract:

The abundant metals Mg and Fe together with SiO form the iron-magnesium-silicate dust observed in the circum stellar shells of M-Stars. This dust component cannot nucleate directly from the gas phase but requires for its formation a dif ferent kind of seed nuclei. Further, there are some hints that besides the silicatic main dust component dust materials of a different composition may be present, perhaps oxides of iron and magnesium. We study in this paper the possibility that MgO particles are formed by nucleation from the gas phase and may form a dust component of their own or act as seed nuclei for the silicate dust.

For this purpose we calculated the structures and bond ener gies for small MgO clusters. The calculation is based on a modi fication of the classical 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 MgO. We calculated clus ter structures and bond energies with this empirical potential for cluster sizes N up to 16. Magic cluster sizes of enhanced stability are found for cluster sizes 2, 4, 6, 9, 12 and 15 for MgO. Their structure for N<=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 and the rate of dust particle formation by nucleation of MgO. The nucleation rate of MgO turns out to be too small to form directly from the gas phase as a separate dust component or to form the seed nuclei for silicate dust formation.

Keywords:

Molecular data - Molecular processes - Stars: AGB and post-AGB - Stars: late-type - circumstellar matter - mass-loss