upGREAT is a development by the Max-Planck Institut (MPI) für Radioastronomie and the Kölner Observatorium für SubMillimeter Astronomie (KOSMA)/Universität zu Köln, in cooperation with the Deutsches Zentrum für Luft- und Raumfahrt (DLR; German Aerospace Center) Institut für Optische Sensorsysteme. The development of upGREAT is financed by the participating institutes, by the German Aerospace Center (DLR) under grants 50 OK 1102, 1103 and 1104, and within the Collaborative Research Centre 956, funded by the Deutsche Forschungsgemeinschaft (DFG). The work of D.N. was supported by grant 120364 from NASA’s Astrophysical Data Analysis Program (ADAP). SOFIA is jointly operated by the Universities Space Research Association (USRA), under NASA contract NAS2-97001, and the Deutsches SOFIA Institut (DSI), under DLR contracts 50 OK 0901 and 50 OK 1301 to the University of Stuttgart. We thank the SOFIA operations and engineering teams for their dedication and supportive responses to our requests, and E. Young and G. Sandell for making these observations possible. We are grateful to O. Novotny for recomputing the thermal rate coefficient for the dissociative recombination of HeH+, using published experimental merged-beam cross-section measurements in the literature27. We thank J. Loreau for providing published cross-section calculations for the radiative association reaction (reaction (1)) in tabular form, and for clarifying that these cross-sections apply specifically to collisions of H (1s) and He+ (1s) in the singlet state.

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Nature thanks Michael Barlow and Stephen Lepp for their contribution to the peer review of this work.