Weak ice absorption features at 7.24 and 7.41 mu m in the spectrum of the obscured young stellar object W 33A

ISO-SWS observations of the highly obscured young stellar object W 33A reve al two broad absorption features centered at 7.24 and 7.41 mu m. Comparison to interstellar ice analogs shows that the 7.24 mu m band can be well matc hed by the CH deformation mode of formic acid(HCOOH), while the 7.41 mu m b and can be fitted both by the formate ion (HCOO-) and acetaldehyde (CH3HCO) The laboratory spectra reveal additional strong features of these molecule s which should make a more definite identification straightforward. While a n assignment of the 7.24 mu m band to the -CH3 deformation mode of aliphati c species maybe considered, the absence of a corresponding strong CH stretc hing mode argues against this possibility;These results and the earlier ten tative 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.