Al 2 O 3 dust in OH/IR stars

Abstract

The presence of amorphous Al2O3 in the shells of five OH/IR stars has been investigated via radiative transfer modelling of their respective spectral energy distributions (SEDs). The results show that the inclusion of this species in the models leads to excellent fits to the respective 10-μm silicate bands as well as the region between the 10- and 20-μm silicate bands. The formation of Al2O3, or Al-O bonds within early condensates, is an accepted feature of theoretical models of dust condensation in O-rich dust shells, and while other metal oxides are not excluded, this study represents further evidence that this species may be present in the shells of high mass-loss asymptotic giant branch (AGB) and post-AGB stars. Significantly, it is found that the amorphous Al 2O3 grains are present as a distinct dust population. This may be the result of kinetic factors interrupting the dust condensation sequence at a stage which results in large amounts of Al2O 3 grains not being able to react with other species and thus being ejected in a pristine form.