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 wit h the Short Wavelength Spectrometer of the Infrared Space Observatory at a mean resolving power of similar to 750. The spectrum displays deep H2O ice and silicate absorptions centered at 3.0 and 9.7 mu m, respectively, togeth er 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 H2O ice column density toward W33A, by me ans of the stretching and combination mode features at 3.0 and 4.5 mu m. Al though no flux is seen at the center of the 3.0 mu m feature, its central d epth may be constrained by fitting assumed profiles to the short- and long- wavelength wings in our spectrum. We deduce that a value of N(H2O) = (1.1 /- 0.3) x 10(19) cm(-2) is consistent with these features, a factor of at l east 3 less than predicted by the H2O 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 fo r the first time in this line of sight, by means of the inversion-mode feat ure 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 H2O, comparable to tha t recently reported toward the young star NGC 7538 IRS 9. However, we find no convincing evidence for absorptions associated with the C=N stretching m ode 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 densit y no more than -10(17) cm(-2) or similar to 1% relative to H2O. This argues against identification of the deep 4.62 mu m 'XCN' feature with isonitrile s, as an implausibly low nitrile to isonitrile abundance ratio (<0.1) would be implied. New and previously published results are combined to construct an inventory of column densities and abundances (normalized to H2O) for al l known species detected in molecular ices toward W33A. A band strength app ropriate to the cyanate ion is assumed to provide quantitative results for XCN. Results are compared with those for other well-studied lines of sight, including the Taurus held star Elias 16, the Galactic center source Sgr A* , and the young stellar objects NGC 7538 IRS 9, GL 2136, and GL 7009S. The CO and XCN abundances are used as indicators of thermal and energetic proce ssing, respectively (where energetic processing may include either UV photo lysis or energetic ion bombardment). Abundances for CH3OH vary from below 3 % in unprocessed ices to similar to 20%-30% in W33A and GL 7009S, consisten t with formation by energetic processing. In contrast, CH4 shows little evi dence of variation. Abundance data for cometary ices indicate some general similarities with interstellar and protostellar ices. CO and CH3OH show com et-to-comet variations that may provide clues to their origins. The highest CH3OH concentrations in comets are comparable with average values for inte rstellar/protostellar ices.