Asymptotic giant branch (AGB) stars are a major contributor to the global dust budget in galaxies (Höfner & Olofsson 2018). Owing to their refractory nature, alumina (stoichiometric formula Al2O3) is a promising candidate to represent the first dust condensate in oxygen-rich AGB stars. Related alumina clusters are thought to initiate dustformation in these environments and are often referred to as ‘seed particles’ (Gail & Sedlmayr 2013). However, the sizes and compositions of these aluminium oxide clusters are not well characterised. In this study, we investigate a range of Al:O stoichiometries in order to review these predictions and to construct realistic models of these initial dust seeds. The emergence of a specific condensate is predicted by its condensation temperature (Tielens 2005) and depends on the thermal stability of the solid, as well as on the gas density and its composition. Usually, the evaluation of the stability of the likely condensates is based on macroscopic bulk properties such as the vapour pressure, which is a measure of the volatility of a substance. Hence, the most refractory condensate is expected to have the lowest vapour pressure. Corundum (α-alumina), corresponding to the most stable crystalline bulk form of alumina, fulfils this condition (Gail et al. 2013). The growth and size distribution of dust grains is commonly described by classical nucleation theory (CNT). However, the applicabilityof CNT in an expanding circumstellar envelope has been questioned (Donn & Nuth 1985; Goumans & Bromley 2012; Bromley et al. 2016; Gobrecht et al. 2017). In particular, the concept of vapour pressures and the universal assumption of thermodynamic equilibrium (TE) are in contradiction with the synthesis and growth of dust grains in highly dynamical AGB atmospheres. Moreover, in CNT, the properties of small solids are derived from the (crystalline)bulk material. However, the properties of nano-sized clusters often differ significantly from those of bulk analogues. The constraints associated with extremely small sizes lead to clusters with non-crystalline structures, whose characteristics (e.g. energy, geometry, bond lengths and angles, atomic coordination) differ substantially from those of the bulk material (Bromley & Zwijnenburg 2016). In particular, the energetic stability of such nano-clusters is typically higher than that of clusters with structures directly obtained from ‘top-down’ cuts from the parent bulk crystalline material, which represent metastable, or even unstable, configurations (Lamiel-Garcia et al. 2017). In addition, the concept of surface free energy (or tension), which is fundamental in CNT, is not applicable to small clusters, where it is difficult to differentiate between surface and bulk. Surface energies can only be applied to clusters with fairly large sizes (e.g. facetted bulk cut clusters). We understand nucleation as the formation and growth of stable seed nuclei (i.e. clusters) from prevalent gas-phase molecules, whose abundance varies in time (i.e. is often not in equilibrium). Therefore, a cluster is intermediate in size between a molecule and a bulk solid.
Citation: Bromley et al. 2016
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