A study of H2O ice in the 3 micron spectrum of OH 231.8+4.2 (OH 0739-14)

The authors present moderate-resolution infrared spectra (λ/Δλ ≡ 140 and 630) of the OH/IR source OH 231.8+4.2 in the 2.0 - 3.8 μm spectral region. Modeling of the absorption feature seen near 3.08 μm has demonstrated that a very good fit to the observed spectrum can be obtained using the optical properties of small spherical silicate grains coated with a layer of pure amorphous H2O ice. The large-scale features of the observed spectrum can only be modeled with an unusually large upper limit to the grain-size distribution of 0.6 - 0.7 μm. The new spectra revealed two new absorption features in the wings of the 3.08 μm feature, near 2.95 and 3.2 μm. Weak features near both these wavelengths appear in the laboratory spectra of crystalline H2O ice.