On the onset of dust formation in AGB stars

A promising candidate to initiate dust formation in oxygen-rich AGB stars is alumina (Al₂O₃) showing an emission feature around ∼13μm attributed to Al-O stretching and bending modes (Posch+99,Sloan+03). The counterpart to alumina in carbon-rich AGB atmospheres is the highly refractory silicon carbide (SiC) showing a characteristic feature around 11.3μm (Treffers74). Alumina and SiC grains are thought to represent the first condensates to emerge in AGB stellar atmospheres. We follow a bottom-up approach, starting with the smallest stoichiometric clusters (i.e. Al₄O₆, Si₂C₂), successively building up larger-sized clusters. We present new results of quantum-mechanical structure calculations of (Al₂O₃)_n, n = 1-10 and (SiC)_n clusters with n = 1-16, including potential energies, rotational constants, and structure-specific vibrational spectra. We demonstrate the energetic viability of homogeneous nucleation scenarios where monomers (Al₂O₃ and SiC) or dimers (Al₄O₆ and Si₂C₂) are successively added. We find significant differences between our quantum theory based results and nanoparticle properties derived from (classical) nucleation theory.