Astrophysical detection of the helium hydride ion HeH+

Abstract

During the dawn of chemistry 1,2 when the temperature of the young Universe had fallen below ~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 + → HeH + + hν). As recombination progressed, the destruction of HeH + (HeH + + H → 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 3 , but only in the late seventies was the possibility that HeH + might exist in local astrophysical plasmas discussed 4,5,6,7 . 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 Strömgren spheres, where HeH + is predicted to form, primarily by radiative association of He + and H. With the GREAT spectrometer 8.9 on board SOFIA 10 the HeH + rotational ground-state transition at λ149.1 µm is now accessible. We report here its detection towards the planetary nebula NGC7027. The mere fact of its proven existence in nearby interstellar space constrains our understanding of the chemical networks controlling the formation of this very special molecular ion.To be published in Nature 568, pages 357-359 (2019) | KOSMA/Universität zu Köln, in cooperation with the DLR Institut für Optische Sensorsysteme.

Benchmark Rovibrational Linelists and Einstein A-coefficients for the Primordial Molecules and Isotopologues

“…HeH + is the smallest heteronuclear molecule and is supposed to be the first molecule formed in the primordial universe. Its recent discovery in space represents a spectacular progress for the field of molecular astrophysics (Güsten et al 2019). In Roberge & Dalgarno (1982) one can find a study of the formation and destruction of HeH + in astrophysical plasmas.…”

“…Outside the scope of astrophysics, accurate calculations for HD + are of interest for metrology, in connection with the development of molecular clocks, as well as for the physics of fundamental constants, namely the improvement of the electron/proton mass ratio (Patra et al 2018). Notably, HeH + has been detected very recently in space (Güsten et al 2019). But, in order to deepen the understanding of the cooling processes in the primordial universe and to study many other processes that became important in the later stages of the evolution of the universe, rovibrational linelists, as well as lists of Einstein A-coefficients, for these molecules, need to be determined with high accuracy.…”

Protonated and Cationic Helium Clusters

“…Experimentally, small protonated He clusters have, for example, been produced from He nanodroplets doped with molecular hydrogen for mass spectrometry and spectroscopic studies [35][36][37], and in low temperature drift tubes filled with He gas [38]. Consisting of the two most abundant elements in the universe, HeH + ions and complexes containing this ion as a chromophore are expected to be plentiful in astrophysical plasmas [39][40][41], but was only very recently discovered in spectra obtained by the SOFIA airborne observatory [42].…”

Databases for collisional and radiative processes in small molecules needed for spectroscopy use in astrophysics

“…Recently, the presence of the HeH + molecular ion was confirmed by Güsten et al (2019) in the planetary nebula NGC 7027. These observations were made possible due to advances in terahertz spectroscopy and high-altitude observatories (Heyminck et al, 2012).…”