  We investigate optical properties of amorphous alumina (Al_2O_3) dust grains
in the envelopes around O-rich asymptotic giant branch (AGB) stars considering
the laboratory measured optical data. We derive the optical constants of
amorphous alumina in a wide wavelength range that satisfy the Kramers-Kronig
relation and reproduce the laboratory measured data. Using the amorphous
alumina and silicate dust, we compare the radiative transfer model results with
the observed spectral energy distributions. Comparing the theoretical models
with the observations on various IR two-color diagrams for a large sample
O-rich AGB stars, we find that the amorphous alumina dust (about 10-40 %) mixed
with amorphous silicate can reproduce much more observed points for the O-rich
AGB stars with thin dust envelopes.
