IDENTIFICATION OF FEATURES DUE TO H-3+ IN THE INFRARED-SPECTRUM OF SUPERNOVA 1987A

THE molecular ion H-3(+) occupies a central position in theoretical models of interstellar chemistry 1,2. It forms readily in hydrogen-rich interstellar gas clouds when ionized H-2 reacts with neutral H-2. The H-3(+) ion can then donate a proton to oxygen, carbon and other heavy atoms. From this beginning nearly 100 different interstellar molecules are formed, such as the reactive hydroxyl radical OH, neutral CO, ethanol, the linear polyacetylenes HC(x)N and cyclic species such as cyclopropenylidene, C3H3. But in spite of a decade of searching 3,4, H-3(+) has eluded detection except in the hydrogen-rich atmosphere of Jupiter 5. Here we present evidence for the presence of H-3(+) in the envelope of supernova 1987 A, where it is produced by an unusual chemistry in which excited hydrogen atoms are the main source of molecule formation. Infrared spectra of SN1987A (ref. 6) in the first few hundred days after the explosion contain two previously unidentified peaks which we attribute to H-3(+). Chemical modelling produces quantities of H-3(+) consistent with the observed peak intensities, and also predicts significant amounts of HeH+, which may be responsible for some weaker features in the spectra.