Crystalline water ice in OH32.8-0.3

Abstract

The 2.2-200 μm spectrum of OH32.8-0.3 has been modelled. Mie theory and radiative transfer models of the 3-μm band of H₂O ice are consistent with a strongly crystalline structural phase. This is also confirmed by the presence of a 44/62-μm band complex analogous to that of laboratory crystalline H₂O ice analogues. The highly ordered phase may be the result of direct crystallization upon deposition as has been theorized by Kouchi et al. At the large total optical depths typical of the radiative transfer models for this object (τ_9.7~ 40), we find no significant difference between the Mie theory and radiative transfer models of the 3-μm band. On the other hand, large differences are found for the 9.7-μm silicate band. In contrast to Mie theory extinction profiles, those computed via radiative transfer modelling indicate that the 12-μm H₂O ice band (the so-called librational band) is substantially attenuated. This, in addition to the inherent broadness and weakness of the 12-μm ice band, may explain why this band has not been clearly identified in observational spectra of oxygen-rich evolved objects.