UNRAVELING THE 10 MICRON SILICATE FEATURE OF PROTOSTARS - THE DETECTION OF FROZEN INTERSTELLAR AMMONIA

We present infrared spectra of four embedded protostars in the 750-123 0 cm(-1) (13.3-8.1 mu m) range. For NGC 7538 IRS 9, a new band is repo rted at 1110 cm(-1) (9.01 mu m), and several others may be present nea r 785, 820, 900, 1030, and 1075 cm(-1) (12.7, 12.2, 11.1, 9.71, and 9. 30 mu m). The band at 1110 cm(-1) is attributed to frozen NH3. Its pos ition and width imply that the NH3 is frozen in a polar, H2O-rich inte rstellar ice component. The NH3/H2O ice ratio inferred for NGC 7538 IR S 9 is 0.1, making NH3 as important a component as CH3OH and CO2 in th e polar ices along this line of sight. At these concentrations, hydrog en bonding between the NH3 and H2O can account for much of the enigmat ic low-frequency wing on the 3240 cm(-1) (3.09 mu m) H2O interstellar ice band. The strength of the implied NH3 deformation fundamental at 1 624 cm(-1) (6.158 mu m) can also account for the absorption at this po sition reported by ISO.