1999A&A...343..966S

ISO-SWS observations of the highly obscured young stellar object W 33A reveal two broad absorption features centered at 7.24 and 7.41µm. Comparison to interstellar ice analogs shows that the 7.24µm band can be well matched by the CH deformation mode of formic acid (HCOOH), while the 7.41µm 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. While an assignment of the 7.24µm band to the -CH₃ deformation mode of aliphatic species may be considered, the absence of a corresponding strong CH stretching mode argues against this possibility. These results and the earlier tentative detection of HCOOH towards NGC7538:IRS9 suggest that formic acid is a general component of the ices in the vicinity of embedded high-mass young stellar objects.