ISO-SWS Observations of Weak Bands of Trace Components of Ices Towards the Young Stellar Object W 33A

ISO-SWS observations of the highly obscured young stellar object W 33A reveal two broad absorption features centered at 7.24 and 7.40 μm. The position and width of these features indicates that they are due to the XH bending mode (X = C, N, or O) of molecules in the solid state. Comparison to interstellar ice analogs shows that the 7.24 μm band can be well matched by solid formic acid (HCOOH), while the 7.40 μm band can be fitted both by the formate ion (HCOO-) and acetaldehyde (CH3HCO). The laboratory spectra reveal additional strong features of these molecules which should make a more definite identification straightforward. This revised version was published online in August 2006 with corrections to the Cover Date.