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 (0H344.93) and IRAS 17411-3154 (0H357). The ISO -SWS spectra of both sources exhibit deep amorphous silicate absorption ban ds at 9.8 and 17.5 mum together with crystalline silicate emission bands at 33.6, 40.5 and 43 mum. In both sources the 9.8 mum silicate band shows a s houlder at 11.2 mum. The amorphous silicates are mainly composed of olivine . With the adopted set of optical constants, the amount of amorphous pyroxe ne-type silicates participating in the absorption cannot exceed similar to 10% of the amorphous silicate mass. The crystalline silicates are identifie d with enstatite, forsterite and diopside. They represent similar to 35% an d similar to 25% of the amorphous silicate mass for IRAS 17004-4119 and IRA S 17411-3154, respectively. In these sources, the 11.2 mum feature is attri buted to the absorption of crystalline forsterite which is also observed in emission at 33.6 mum. Water ice is observed in both objects through two ba nds in the covered wavelength range, in absorption (at 3.09 mum) and in emi ssion (at 43 mum). The presence of water ice and crystalline forsterite ban ds, observed in emission and absorption, emphasizes the necessity, in such objects, of modelling the radiative transfer in order to interpret the over all spectra. Our results, compatible with classical silicate formation theo ries, may help to put some constraints on dust formation models.