Searching for Ammonia in Grain Mantles toward Massive Young Stellar Objects

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Abstract
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.