An Inventory of Interstellar Ices toward the Embedded Protostar W33A

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Abstract

This paper presents, for the first time, a complete 2.4-25 μm 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 ~750. The spectrum displays deep H2O ice and silicate absorptions centered at 3.0 and 9.7 μm, respectively, together with absorption features identified with various other molecules in the solid phase. The 2.4-5.0 μm region of the spectrum is used to investigate the long-standing problem of the H2O ice column density toward W33A, by means of the stretching and combination mode features at 3.0 and 4.5 μm. Although no flux is seen at the center of the 3.0 μm 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(H2O) = (1.1 ± 0.3) × 1019 cm-2 is consistent with these features, a factor of at least 3 less than predicted by the H2O bending mode at 6.0 μm; the reason for this discrepancy is unclear. We report new results on the abundances of nitrogen-bearing species in the ices toward W33A. Solid NH3 is detected for the first time in this line of sight, by means of the inversion-mode feature at 9.0 μm. The column density is N(NH3) = (1.7 ± 0.4) × 1018 cm-2, implying an abundance of ~15% relative to H2O, 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 μm region of the spectrum. If nitriles are present in the ices along this line of sight, they must have column density no more than ~1017 cm-2 or ~1% relative to H2O. This argues against identification of the deep 4.62 μm `XCN' feature with isonitriles, 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 all known species detected in molecular ices toward W33A. A band strength appropriate 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 field 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 processing, respectively (where energetic processing may include either UV photolysis or energetic ion bombardment). Abundances for CH3OH vary from below 3% in unprocessed ices to ~20%-30% in W33A and GL 7009S, consistent with formation by energetic processing. In contrast, CH4 shows little evidence of variation. Abundance data for cometary ices indicate some general similarities with interstellar and protostellar ices. CO and CH3OH show comet-to-comet variations that may provide clues to their origins. The highest CH3OH concentrations in comets are comparable with average values for interstellar/protostellar ices.

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