What happened in our universe immediately after the Big Bang and how did life develop over the course of time? These are questions that people have been trying to solve for ages. Therefore, the first chemical compound since the Big Bang was the object of years of search. Researchers have known about its existence from laboratory studies for almost 100 years, yet, it had not been found in space, despite extensive searches. As a result, all chemical model calculations associated with it were called into question. Until now.
With the help of the aircraft observatory SOFIA (Stratospheric Observatory for Infrared Astronomy), an international team of researchers led by Rolf Güsten of the Max Planck Institute for Radio Astronomy (MPIfR) in Bonn has now succeeded in clearly detecting HeH+ in the direction of the planetary nebula NGC 7027. The results were published in the lates edition of Nature magazine.
“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,” says Anke Pagels-Kerp, Head of the Space Science Department at the DLR Space Administration in Bonn. “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.”
Decades of search successfully completed
HeH+ was one of the first ions to emerge around 300,000 years after the Big Bang when the extremely high temperatures in the young universe sank and the first chemical reactions began. “The ion tells us about the beginning of chemistry in our universe,” says Bernd Klein of the MPIfR. After the compound had already been detected in the laboratory in 1925, the search in space began in the 1970s. “With the detection in space, a decade-long search has now been successfully completed. With this discovery, we hope to better understand the early phase of the universe,” Klein continued.
Already in the late 1970s, astrochemical models suggested that a detectable quantity of HeH+ might be present within nebulae in the Milky Way. At the time, scientists assumed that it could best be found in planetary nebulae, which have been ejected from Sun-like stars in the last phase of their lifecycle. “The high-energy radiation generated by the central star drives ionization fronts into the envelope of ejected material,” DLR researchers explain. According to the model calculations, this the exact place where the HeH+ ions should form.
However, it could not be found until now. One reason for this is that the molecule emits its strongest spectral line at a characteristic wavelength of 149.1 micrometers (corresponding to a frequency of 2.01 terahertz), but Earth’s atmosphere blocks all radiation in this wavelength range to all ground-based observatories. Thanks to the flying observatory SOFIA, which operates at an altitude of 13 to 14 kilometers and thus above the absorbing layers of the lower atmosphere, scientists were able to trace the molecule.
“SOFIA offers a unique opportunity to use the very latest technologies at any given time,” explains Heinz Hammes, SOFIA Project Manager at the DLR Space Administration. ”
Beginning of chemistry in our universe
According to DLR, the outstanding importance of the HeH+ ion played a very important role in the formation of the universe: “All chemistry began approximately 300,000 years after the Big Bang,” explain the scientists. “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 ionized at first, due to the high temperatures. As the Universe cooled, they recombined with free electrons to create the first neutral atoms.”
At this point, hydrogen was still ionized and was present in the form of free protons or hydrogen nuclei. The combination with the helium atoms formed the helium hydride ion HeH+, making it one of the very molecular connections 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, the scientists explain.
“Thanks to recent advances in terahertz technology, it is now possible to perform high-resolution spectroscopy at the required far-infrared wavelengths,” explains project manager Rolf Güsten. As a result of measurements with the GREAT spectrometer on board the flying SOFIA observatory, the team can now announce the unambiguous detection of the HeH+ ion in the direction of the planetary nebula NGC 7027.
The Stratospheric Observatory for Infrared Astronomy is a joint project of the German Aerospace Center (DLR) and the National Aeronautics and Space Administration (NASA). The flying observatory for receiving terahertz radiation is a modified Boeing 747 with a telescope with a diameter of 2.7 meters that can be extended at an altitude of 12 to 13 kilometers.
“At this altitude, we avoid the disturbing influence of Earth’s atmosphere,” emphasizes Bernd Klein. Observing the “fingerprint” of the helium molecule in the far infrared range, its characteristic spectral line, is only possible in this environment. The decisive factor in the latest discovery was the technology of the GREAT receiver (German Receiver at Terahertz Frequencies) onboard SOFIA, which was developed in cooperation with several German research institutes.
In June 2019, SOFIA will start on another voyage with an improved GREAT receiver. “We are again hunting for important elements for a better understanding of the universe,” says Bernd Klein.
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