During the dawn of chemistry1,2, when the temperature of the young Universe had fallen below some 4,000 kelvin, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With their higher ionization potentials, the helium ions He2+ and He+ were the first to combine with free electrons, forming the first neutral atoms; the recombination of hydrogen followed. In this metal-free and low-density environment, neutral helium atoms formed the Universe’s first molecular bond in the helium hydride ion HeH+ through radiative association with protons. As recombination progressed, the destruction of HeH+ created a path to the formation of molecular hydrogen. Despite its unquestioned importance in the evolution of the early Universe, the HeH+ ion has so far eluded unequivocal detection in interstellar space. In the laboratory the ion was discovered3 as long ago as 1925, but only in the late 1970s was the possibility that HeH+ might exist in local astrophysical plasmas discussed4,5,6,7. In particular, the conditions in planetary nebulae were shown to be suitable for producing potentially detectable column densities of HeH+. Here we report observations, based on advances in terahertz spectroscopy8,9 and a high-altitude observatory10, of the rotational ground-state transition of HeH+ at a wavelength of 149.1 micrometres in the planetary nebula NGC 7027. This confirmation of the existence of HeH+ in nearby interstellar space constrains our understanding of the chemical networks that control the formation of this molecular ion, in particular the rates of radiative association and dissociative recombination.
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The data presented here are available through the SOFIA data archive at https://dcs.arc.nasa.gov/ and can be retrieved by searching for the project identification code, 83_0405.
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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.
Nature thanks Michael Barlow and Stephen Lepp for their contribution to the peer review of this work.
Extended data figures and tables
Extended Data Fig. 1 Calibrated and baseline-corrected, but otherwise unprocessed, spectra observed in two different intermediate-frequency set-ups (νIF = 1.4 GHz and 1.2 GHz).
See text for details. The frequencies of the group of hyperfine transitions of the CH Λ-doublets are marked (blue, upper doublet from the signal band; purple, lower-doublet blending from the image band). A pattern fit (optically thin, with intensities of the hyperfine pattern as per Extended Data Table 1) is superposed with red lines. The bottom spectrum displays the co-added residuals of the two observations, after removal of the CH emission (shown is the residual after removal of the Gaussian fits). The atmospheric transmission is shown with a green line, for typical conditions encountered in May 2016 (precipitable water-vapour column 20 μm; opacity of dry atmospheric constituents scaled by a factor of 1.4 with respect to the reference model, for a sightline at 35° elevation).
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Nature Astronomy (2019)