Searching for ammonia in grain mantles toward massive young stellar objects

We present a study of the mid-infrared spectral regions of 17 objects obser ved with the Short-Wavelength Spectrometer on the Infrared Space Observator y (ISO SWS) in a search for the presence of the elusive NH3 solid-state abs orption features at 2.96 and 9.0 mum. We compare the 3 mum profile with lab oratory spectra of pure water and water/ammonia mixtures from the Leiden Mo lecular Astrophysics database. We determine that the shape of the 3 mum wat er feature alone does not present conclusive evidence of the presence or la ck of NH3 ice on dust grain mantles because of the large number of unknown physical parameters that influence the 3 mum water profile: grain size dist ribution, grain shape, mantle composition, thermal history, and geometrical effects. In the 9 mum region, the presence of NH3 is characterized by a su dden change in slope at 8.5 mum and a peak at 9.0 mum within the silicate p rofile. Of the 16 silicate features studied, four show evidence of a 9 mum ammonia feature, including the previously reported detections in ground-bas ed observations of NGC 7538 IRS 9, and SWS observations of W33 A. Upper lim its are found for the remaining objects. Why NH3, which is an abundant gas- phase molecule in cold, dense cores, is not apparently present in significa nt amounts on most grain mantles, where it is expected to freeze out, is cu rrently a mystery.