Nucleation of small silicon carbide dust clusters in AGB stars

摘要: Silicon carbide (SiC) grains are a major dust component in carbon-rich AGB stars. The formation pathways of these are, however, not fully understood.\ We calculate ground states and energetically low-lying structures (SiC)$_n$, $n=1,16$ clusters by means simulated annealing (SA) Monte Carlo simulations seed subsequent quantum-mechanical calculations on the density functional level theory. derive infrared (IR) spectra compare IR signatures to observational laboratory data.\ According energetic considerations, we evaluate viability SiC cluster growth at several densities temperatures, characterising various locations evolutionary circumstellar envelopes.\ discover new, for Si$_{4}$C$_{4}$, Si$_{5}$C$_{5}$, Si$_{15}$C$_{15}$ Si$_{16}$C$_{16}$, new Si$_{10}$C$_{10}$ Si$_{15}$C$_{15}$. with carbon-segregated substructures tend be more stable 4-9 eV than their bulk-like isomers alternating Si-C bonds. However, find cage ("bucky"-like) geometries Si$_{12}$C$_{12}$ Si$_{16}$C$_{16}$ low-lying, n $\ge$ 12. latter findings indicate thus regime sizes that differs from small as well large-scale crystals. Thus, owing stability geometry, may mark transition quantum-confined crystalline, solid bulk-material. The calculated vibrational ground-state shows significant emission. They include 10-13 $\mu$m wavelength range 11.3 feature inferred measurements observations, respectively, though overall intensities rather low.