On the onset of dust formation in AGB stars

Abstract

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.

The role of alumina in triggering stellar outflows

“…We present the results of the quantum-mechanical structure calculations of (Al 2 O 3 ) n clusters with n =1−10, including potential energies, rotational constants, charge distributions and structure-specific infrared spectra (vibrational frequencies and intensities). We find new global minima candidates for cluster sizes n =8, 9, and 10, that are partly reported in Gobrecht et al (2018). A homogeneous nucleation, where Al 2 O 3 monomers are successively added, is energetically viable in circumstellar conditions (p =10 −5 −10 Pa, T =500−6000 K).…”

Formation of AlO, AlOH, and Al(OH)3 in the Interstellar Medium and Circumstellar Envelopes of AGB Stars

“…42 Laboratory studies on meteorites show the presence of aluminum oxides in the innermost part of dust grains, 51 thus suggesting corundum as a viable seed for the nucleation of dust grains around late-stage stars. 52 Understanding the possible methods of formation of these small aluminum oxides will provide the foundation for understanding the formation of corundum dust grains. The reaction of aluminum hydride with water has recently provided a promising pathway to the formation of larger, hydrated aluminum oxide clusters, 53 but AlH 3 has yet to be conclusively observed toward any astronomical sources.…”