2001ApJ...558..702G

We present a study of the mid-infrared spectral regions of 17 objects observed with the Short-Wavelength Spectrometer on the Infrared Space Observatory (ISO SWS) in a search for the presence of the elusive NH₃ solid-state absorption features at 2.96 and 9.0 µm. We compare the 3 µm profile with laboratory spectra of pure water and water/ammonia mixtures from the Leiden Molecular Astrophysics database. We determine that the shape of the 3 µm water feature alone does not present conclusive evidence of the presence or lack of NH₃ ice on dust grain mantles because of the large number of unknown physical parameters that influence the 3 µm water profile: grain size distribution, grain shape, mantle composition, thermal history, and geometrical effects. In the 9 µm region, the presence of NH₃ is characterized by a sudden change in slope at 8.5 µm and a peak at 9.0 µm within the silicate profile. Of the 16 silicate features studied, four show evidence of a 9 µm ammonia feature, including the previously reported detections in ground-based observations of NGC 7538 IRS 9, and SWS observations of W33 A. Upper limits are found for the remaining objects. Why NH₃, which is an abundant gas-phase molecule in cold, dense cores, is not apparently present in significant amounts on most grain mantles, where it is expected to freeze out, is currently a mystery.