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

¹ Physique des Interactions Ioniques et Moléculaires, Équipe Spectrométries et Dynamique Moléculaire, UMR 6633, Université de Provence, Centre de Saint Jérôme, Boîte 252, 13397 Marseille Cedex 20, France
² Institut d'Astrophysique Spatiale – CNRS, Bât 121, Université Paris-Sud, 91405 Orsay Cedex, France

Corresponding author: T. Chiavassa, thierry@piimsdm.univ-mrs.fr

Received: 12 September 2003
Accepted: 4 November 2003

Abstract

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 NHOCN^- (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.