2010MNRAS.408.1956R

Radiative transfer models have been produced for stars surrounded by circumstellar dust shells in order to investigate the detectability of the 11.5-µm librational band due to water-ice (H₂O-ice). The dust grains were assumed to be composed of a core material of either amorphous silicate of olivine composition (MgFeSiO₄) or alumina (Al₂O₃), both grain species being coated with a water-ice mantle. The models may be divided into three classes: those with small, intermediate and large optical depths. It is found that in all three cases, even with only olivine present, the water-ice librational band feature is masked by radiative transfer effects and is therefore difficult to detect.

For the librational band to display its characteristic shape requires far larger amounts of water-ice than are present in any known circumstellar dust shell, as indicated by the depth of the much stronger 3.1-µm water-ice band. The best prospect for finding some evidence of the librational band is likely to be via using a small beam centred on the cooler outer regions of the dust shell; in this case the line of sight may encounter only water-ice coated grains, thereby reducing the contaminating effect of bare grains. If a grain species such as alumina is present, with a broad absorption band in the vicinity of 11.5µm, the identification of the librational band may be possible, perhaps surprisingly, as result of the sharpening of the 11.5-µm feature. We have fitted the spectrum of IRAS 22036+5306 and find that, although water-ice is clearly present in this object, as evidenced by the presence of the strong 3.1-µm band, the librational band has no obvious effect on its 10-µm spectrum.