1998A&A...339..553D

We focus on the problem of the assignment of the ``XCN'' solid state feature observed in the spectra of numerous protostellar objects. This feature, located at 4.62µm has been tentatively identified with the molecular ion OCN^–. New laboratory experiments have been performed in order to confirm this identification. OCN^– is produced by UV photolysis of mixtures of CO:NH₃ (1:1) deposited on a cold substrate. Simultaneously, the counterion NH⁺₄ is also produced to balance the charge in the matrix. The use of electron donor and acceptor dopants incorporated in the mixtures allows us to reduce or enhance the production of the OCN^– and NH⁺₄ features, thus establishing their ionic character. The production of HNCO when an electron acceptor is used, and the formation of the OCN^– band during thermal annealing, constitute convincing arguments in favour of the formation of OCN^– and NH⁺₄ by acid-base reactions where the acid HNCO transfers a proton to the base NH₃. We consider the astrophysical implications for the interpretation of the ISO-SWS spectrum of the protostellar source RAFGL 7009S. In this source, the 4.62µm band is deep and large, although the derived abundance of this ion, relative to H₂O ice, is on the order of 4%. Identification of OCN^– is considered in several other sources. The contribution of NH⁺₄ to the unidentified 6.85µm feature is discussed.