A quantitative analysis of OCN- formation in interstellar ice analogs

The 4.62 micron absorption band, observed along the line-of-sight towards various young stellar objects, is generally used as a qualitative indicator for energetic processing of interstellar ice mantles. This interpretation is based on the excellent fit with OCN-, which is readily formed by ultraviolet (UV) or ion-irradiation of ices containing H2O, CO and NH3. However, the assignment requires both qualitative and quantitative agreement in terms of the efficiency of formation as well as the formation of additional products. Here, we present the first quantitative results on the efficiency of laboratory formation of OCN- from ices composed of different combinations of H2O, CO, CH3OH, HNCO and NH3 by UV- and thermally-mediated solid state chemistry. Our results show large implications for the use of the 4.62 micron feature as a diagnostic for energetic ice-processing. UV-mediated formation of OCN- from H2O/CO/NH3 ice matrices falls short in reproducing the highest observed interstellar abundances. In this case, at most 2.7% OCN- is formed with respect to H2O under conditions that no longer apply to a molecular cloud environment. On the other hand, photoprocessing and in particular thermal processing of solid HNCO in the presence of NH3 are very efficient OCN- formation mechanisms, converting 60%--85% and ~100%, respectively of the original HNCO. We propose that OCN- is most likely formed thermally from HNCO given the ease and efficiency of this mechanism. Upper limits on solid HNCO and the inferred interstellar ice temperatures are in agreement with this scenario.