Formation of Transition Alumina Dust around Asymptotic Giant Branch Stars: Condensation Experiments using Induction Thermal Plasma Systems

Mid-infrared spectroscopic observations of oxygen-rich asymptotic giant branch (AGB) stars show the common presence of dust species that have a broad feature at ∼11–12 μm. Chemically synthesized amorphous alumina (Al2O3) is widely accepted as the source of this emission, although it is not obvious that amorphous alumina can condense in circumstellar conditions. We performed condensation experiments of Al–Si–Mg–O and Mg–Al–O gases using induction thermal plasma systems, in which small particles condense from vapors with a steep temperature gradient. The condensates were analyzed using X-ray diffraction and Fourier transform infrared spectroscopy, and observed with a transmission electron microscope. The condensed nanoparticles from the Al and O gases were transition aluminas based on face-centered cubic (fcc) packed oxygen (δ- and λ-alumina, and an unknown phase). The fcc oxygen frameworks were maintained in the condensed alumina containing small amounts of Mg and Si. Condensates from the gases of Al:Mg = 99:1 and 95:5 had δ- and γ-alumina structures. Particles with λ- and γ-alumina structures formed from starting materials of Al:Si = 9:1 and Al:Si:Mg = 8:1:1, respectively. Amorphous silica-rich particles condensed from gases of Al/(Si+Al) < 0.75. The condensed transition alumina containing ∼10% Si showed similar spectral shapes to the observed dust emission from the alumina-rich AGB star T Cep. Based on the present results, it is reasonable that the source of 11–12 μm broad emission of alumina-rich stars is not amorphous alumina, but is transition alumina containing ∼10% Si.