Formation of Al2O3 Grains and the 13µM Feature in Circumstellar Envelopes of Oxygen-Rich AGB Stars

The formation of dust grains in steady state dust driven winds around oxygen-rich AGB stars has been investigated to clarify the carrier of the observed 13µm feature. In the calculations not only homogeneous Al2O3 and silicate grains but also heterogeneous grains consisting of an Al2O3 core and a silicate mantle are included simultaneously. The radiation transfer calculations based on the results of condensation calculations demonstrate that the core-mantle grains consisting of an α-Al2O3 core and a silicate mantle formed in the vicinity of the sonic point can produce a distinctive emission feature similar to the observed 13µm feature when the mass loss rate is less than 2 × 10-5M·/yr.