Structure and chemical composition of the silicate dust around OH/IR stars

Using a radiative transfer code we have studied the dust shells of the two OH/IR stars IRAS 17004-4119 (OH344.93) and IRAS 17411-3154 (OH357). The ISO-SWS spectra of both sources exhibit deep amorphous silicate absorption bands at 9.8 and 17.5 μm together with crystalline silicate emission bands at 33.6, 40.5 and 43 μm. In both sources the 9.8 μm silicate band shows a shoulder at 11.2 μm. The amorphous silicates are mainly composed of olivine. With the adopted set of optical constants, the amount of amorphous pyroxene-type silicates participating in the absorption cannot exceed ∼ 10% of the amorphous silicate mass. The crystalline silicates are identified with enstatite, forsterite and diopside. They represent ∼ 35% and ∼ 25% of the amorphous silicate mass for IRAS 17004-4119 and IRAS 17411-3154, respectively. In these sources, the 11.2 μm feature is attributed to the absorption of crystalline forsterite which is also observed in emission at 33.6 μm. Water ice is observed in both objects through two bands in the covered wavelength range, in absorption (at 3.09 μm) and in emission (at 43 μm). The presence of water ice and crystalline forsterite bands, observed in emission and absorption, emphasizes the necessity, in such objects, of modelling the radiative transfer in order to interpret the overall spectra. Our results, compatible with classical silicate formation theories, may help to put some constraints on dust formation models.