2002ApJ...566L.113G

Data from the Infrared Space Observatory Short Wavelength Spectrometer have allowed abundances of many species present in interstellar ices to be determined. However, that of H₂O, the commonest of all ices, has proved controversial because of inconsistencies between results from different vibrational modes: the 6.0 µm bending mode predicts column densities systematically higher than the stretching, combination, and libration modes in several lines of sight. We show that this discrepancy can be explained by excess absorption arising in a previously unrecognized blended feature, which we attribute to organic refractory matter (ORM). The strength of the excess absorption at 6.0 µm is correlated with that of the 4.62 µm ``XCN'' feature, the carrier of which is thought to form when interstellar ices undergo UV photolysis or ion bombardment. Our results are thus consistent with an origin for the ORM by energetic processing of simple ices.