Tentative identification of urea and formamide in ISO-SWS infrared spectra of interstellar ices

Laboratory experiments involving vacuum ultraviolet (VUV) irradiation of solid isocyanic acid (HNCO) at 10 K, followed by infrared spectroscopy (FTIR), are used to interpret the complex spectra associated with Interstellar Medium (ISM) dust grains, particularly the spectra associated with the icy phase observed toward dense molecular clouds. The comparison of the infrared spectra of the photolysis products with spectra recorded from the protostellar source NGC 7538 IRS9 shows that the unexplained 1700 cm(-1) feature can be attributed to the contribution of several species H2CO (formaldehyde), HCONH2 (formamide) and H2NCONH2 (urea) mixed with H2O as the main contributor. Urea, formaldehyde and NH4+OCN- (ammonium cyanate) may also contribute to a band at 1470 cm(-1), widely observed in many protostellar infrared sources and which remains up to now poorly explained in numerous ISO-SWS spectra. Isocyanic acid could be a precursor of formamide and urea in interstellar ices.