On the basis of radiative transfer models we have investigated the librational band of amorphous water-ice (H2O-ice), which might be expected to occur in the spectra of young stellar objects with dust shells, producing an absorption feature at about 13 μm. Models have been run with only bare silicate grains present, together with models including water-ice mantles. We find that the 13-μm feature may be substantially explained without the use of water-ice if laboratory silicates are employed and the 10-μm silicate feature is in self-absorption, a phenomenon which may arise for shells of intermediate optical depth (). The self-absorption process may lead to a variation in the shape of the local minimum between the 10- and 20-μm silicate features, the local minimum occurring at about 13 μm, close to the position of the water-ice librational band. This local minimum could easily be mistaken for the librational water-ice band itself. Nevertheless, the librational band undoubtedly does contribute to the finer details of the shape of the 13-μm absorption feature. We find that for shells of small optical depth the water-ice feature is not distinguishable from the local minimum between the 10- and 20-μm silicate features, while for shells of large optical depth it is washed out completely, so in neither of these two extreme cases should the feature be readily observable. The fact that relatively few objects exist in the intermediate state where the silicate feature is in self-absorption may account for the extremely small number of cases in which the librational band apparently appears as a distinct feature.
Radiative transfer models have been produced for the massive pre-main-sequence object AFGL 961E, with the aim of investigating whether the 13-μm feature in its spectrum is due to the librational band of water-ice. Clearly there is water-ice somewhere in the line of sight to AFGL 961, as evidenced by the strong 3.1-μm water-ice band, and the case for the 13-μm feature being due to the librational band is uniquely strong amongst all astronomical objects. However, we present evidence that the grains may have properties similar to laboratory silicates rather than astronomical silicates and that the feature may largely be a manifestation of the 10-μm silicate feature being in self-absorption, with amorphous water-ice being a minor contributor. Neither bare astronomical silicates nor water-ice mantled astronomical silicates will reproduce the 13-μm feature.