2004MNRAS.349..665M

The phase of H₂O ice and the librational ice band detected toward the post-asymptotic giant branch (post-AGB) star OH231.8+4.2 have been examined using radiative transfer modelling. The results indicate that the ice is largely crystalline and not amorphous as previously reported in the literature. The earlier result is shown to be due to the assumption of unrealistically thick ice mantles. It has also been shown that, in comparison with radiative transfer modelling, Mie theory modelling of the 3-µm ice band detected towards moderately optically thick shells leads to an underestimate of the ice mantle thickness. The modelling results also suggest that the mid-infrared (MIR) feature previously identified with the librational band of H₂O ice could be the result of amorphous Al₂O₃. This dust component can also account for the observed enhanced opacity between the 10- and 18-µm silicate bands.