An Inventory of Interstellar Ices toward the Embedded Protostar W33A

This paper presents, for the first time, a complete 2.4-25 mu m spectrum of the dust-embedded young stellar object W33A. The spectrum was obtained with the Short Wavelength Spectrometer of the Infrared Space Observatory at a mean resolving power of similar to 750. The spectrum displays deep H(2)O ice and silicate absorptions centered at 3.0 and 9.7 mu m, respectively, together with absorption features identified with various other molecules in the solid phase. The 2.4-5.0 mu m region of the spectrum is used to investigate the long-standing problem of the H(2)O ice column density toward W33A, by means of the stretching and combination mode features at 3.0 and 4.5 mu m. Although no flux is seen at the center of the 3.0 mu m feature, its central depth may be constrained by fitting assumed profiles to the short- and long-wavelength wings in our spectrum. We deduce that a value of N(H(2)O) = (1.1 +/- 0.3) x 10(19) cm(-2) is consistent with these features, a factor of at least 3 less than predicted by the H(2)O bending mode at 6.0 mu m; the reason for this discrepancy is unclear. We report new results on the abundances of nitrogen-bearing species in the ices toward W33A. Solid NH, is detected for the first time in this line of sight, by means of the inversion-mode feature at 9.0 mu m. The column density is N(NH,)=(1.7 +/- 0.4) x 10(18) cm(-2) implying an abundance of similar to 15% relative to H(2)O, comparable to that recently reported toward the young star NGC 7538 IRS 9. However, we find no convincing evidence for absorptions associated with the C=N stretching mode of nitriles in the 4.4-4.6 mu m region of the spectrum. If nitriles are present in the ices along this line of sight, they must have column density no more than -10(17) cm(-2) or similar to 1% relative to H(2)O. This argues against identification of the deep 4.62 mu m 'XCN' feature with isonitriles, as an implausibly low nitrile to isonitrile abundance ratio (