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.
