First astrophysical detection of the helium hydride ion (HeH$^+$)

During the dawn of chemistry when the temperature of the young Universe had fallen below $\sim$4000 K, the ions of the light elements produced in Big Bang nucleosynthesis recombined in reverse order of their ionization potential. With its higher ionization potentials, He$^{++}$ (54.5 eV) and He$^+$ (24.6 eV) combined first with free electrons to form the first neutral atom, prior to the recombination of hydrogen (13.6 eV). At that time, in this metal-free and low-density environment, neutral helium atoms formed the Universe's first molecular bond in the helium hydride ion HeH$^+$, by radiative association with protons (He + H$^+$ $\rightarrow$ HeH$^+$ + h$\nu$). As recombination progressed, the destruction of HeH$^+$ (HeH$^+$ + H $\rightarrow$ He + H$_2^+$) created a first path to the formation of molecular hydrogen, marking the beginning of the Molecular Age. Despite its unquestioned importance for the evolution of the early Universe, the HeH$^+$ molecule has so far escaped unequivocal detection in interstellar space. In the laboratory, the ion was discovered as long ago as 1925, but only in the late seventies was the possibility that HeH$^+$ might exist in local astrophysical plasmas discussed. In particular, the conditions in planetary nebulae were shown to be suitable for the production of potentially detectable HeH$^+$ column densities: the hard radiation field from the central hot white dwarf creates overlapping Str\"omgren spheres, where HeH$^+$ is predicted to form, primarily by radiative association of He$^+$ and H. With the GREAT spectrometer onboard SOFIA, the HeH$^+$ rotational ground-state transition at $\lambda$149.1 $\mu$m is now accessible. We report here its detection towards the planetary nebula NGC7027.