Identification of features due to H₃⁺ in the infrared spectrum of supernova 1987A

Abstract

THE molecular ion H₃⁺ occupies a central position in theoretical models of interstellar chemistry^1,2. It forms readily in hydrogen-rich interstellar gas clouds when ionized H₂ reacts with neutral H₂. The H₃⁺ ion can then donate a proton to oxygen, carbon and other heavy atoms. From this beginning nearly 100 different inter-stellar molecules are formed, such as the reactive hydroxyl radical OH, neutral CO, ethanol, the linear polyacetylenes HC_xN and cyclic species such as cyclopropenylidene, C₃H₃. But in spite of a decade of searching^3,4, H₃⁺ has eluded detection except in the hydrogen-rich atmosphere of Jupiter⁵. Here we present evidence for the presence of H₃⁺ in the envelope of supernova 1987A, 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 H₃⁺. Chemical modelling produces quantities of H₃⁺ consistent with the observed peak intensities, and also predicts significant amounts of HeH⁺, which may be responsible for some weaker features in the spectra.