Formation of Dust Grains in Circumstellar Envelopes of Oxygen-Rich AGB Stars

We discuss dust formation in steady state dust driven winds around oxygen-rich AGB stars, including not only homogeneous Al₂O₃ and silicate grains but also heterogeneous grains consisting of an Al₂O₃ core and a silicate mantle. In the inner subsonic region, Al₂O₃ grains with radii of ～ 0.15 µm condense first, then condensation of silicate on Al₂O₃ starts slightly inside the sonic point, which accelerates the gas flow into the supersonic region. Also small silicate grains, whose radii are a few tens of Ångstroms form beyond the sonic point. The carrier of 13 µm feature observed towards oxygen-rich AGB stars is considered to be the core-mantle grains consisting of an α-Al₂O₃ core and a silicate mantle from the radiation transfer calculations based on the results of dust formation calculations.