The helium hydride ion, to give HeH+ its full name, once posed something of a dilemma for science. Although its existence has been known from laboratory studies for almost 100 years, it had not been found in space, despite extensive searches. As a result, the chemical model calculations associated with it were called into question. But an international team of researchers led by Rolf Güsten of the Max Planck Institute for Radio Astronomy in Bonn has now succeeded in clearly detecting this ion in the direction of the planetary nebula NGC 7027. The proof was obtained using the German Receiver for Astronomy at Terahertz Frequencies (GREAT), a far-infrared spectrometer carried on board the Stratospheric Observatory for Infrared Astronomy (SOFIA). SOFIA is a joint project by the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) and NASA, the US space agency. The results were published in the 18 April 2019 issue of the scientific journal Nature.
"Over the last decade, people have had great hopes for space observatories such as Spitzer (NASA, launched 2003) and Herschel (ESA, launched 2009), but none of these telescopes were able to detect this ion. SOFIA has provided us with proof that this ion really can form in planetary nebulae. At present, there is no other telescope capable of observing at these wavelengths, so this observation platform will remain unique for many years to come," says Anke Pagels-Kerp, Head of the Space Science Department at the DLR Space Administration in Bonn.
In the late 1970s, astrochemical models suggested that a detectable quantity of HeH+ might be present within nebulae in the Milky Way. It was thought most likely to be found in what are known as planetary nebulae, which are shells of gas and dust that have been ejected from a Sun-like star in the last phase of their lifecycle. The high-energy radiation generated by the central star drives ionisation fronts into the envelope of ejected material. According to the model calculations, it is precisely here that the HeH+ ions are supposed to form. Yet despite its undisputed importance in the history of the early Universe, it had long proven impossible to find the HeH+ ion in interstellar space. Although it has been known to exist since 1925, specific searches for it in space have been unsuccessful over recent decades.
The molecule emits its strongest spectral line at a characteristic wavelength of 149.1 micrometres (corresponding to a frequency of 2.01 terahertz). Earth's atmosphere blocks all radiation in this wavelength range, preventing searches by ground-based observatories; therefore, the search must be conducted either from space or using high-flying observatories such as SOFIA. At an altitude of 13 to 14 kilometres, SOFIA operates above the absorbing layers of the lower atmosphere.
"SOFIA offers a unique opportunity to use the very latest technologies at any given time. The ongoing German-led development of the GREAT instrument has now made the detection of helium hydride possible. This underlines the importance of such instruments and the potential that their development holds for SOFIA in future," explains Heinz Hammes, SOFIA Project Manager at the DLR Space Administration.
After the Big Bang, chemistry began in the Universe
The HeH+ ion is very important by virtue of its role in the formation of the Universe; all chemistry began approximately 300,000 years after the Big Bang. Although the Universe was still in its early stages, the temperature had already fallen to under approximately 3700 degrees Celsius. The elements that formed in the Big Bang – such as hydrogen, helium, deuterium and traces of lithium – were ionised at first, due to the high temperatures. As the Universe cooled, they recombined with free electrons to create the first neutral atoms. This happened first with helium. At this point, hydrogen was still ionised and was present in the form of free protons, or hydrogen nuclei. These combined with the helium atoms to form the helium hydride ion HeH+, making it one of the very first molecular compounds in the Universe. As recombination advanced, HeH+ reacted with the newly-formed neutral hydrogen atoms, thus paving the way for the formation of molecular hydrogen and thus the chemical origins of the Universe.
"Thanks to recent advances in terahertz technology, it is now possible to perform high-resolution spectroscopy at the required far-infrared wavelengths," explains Rolf Güsten, Lead Author of the article. As a result of measurements performed using the GREAT spectrometer on board the SOFIA airborne observatory, the team can now announce the unambiguous detection of the HeH+ ion in the direction of the planetary nebula NGC 7027.
Rolf Güsten et al.: First detection of the helium hydride ion (HeH+) in space, published in the 18 April 2019 issue of Nature.