Scientists have detected the oldest type of molecule in our universe in space for the first time.
The helium hydride ion (HeH +) was the first molecule that formed when, almost 14 billion years ago, the fall of temperatures in the young universe allowed the recombination of the light elements produced in the Big Bang.
At that time, ionized hydrogen and neutral helium atoms reacted to form HeH +, researchers at the Max Planck Institute for Radio Astronomy (MPIfR) in Germany said.
Despite its importance in the history of the early universe, HeH + has so far escaped detection in astrophysical nebulae – gas cloud and dust in outer space.
Operating the large distant infrared spectrometer aboard the flying observatory SOFIA, an international team reported the unequivocal detection of the molecule toward the planetary nebula NGC 7027.
During the dawn of chemistry, when the temperature in the young universe fell below 4000 Kelvin, the ions of the light elements (hydrogen, helium, deuterium and lithium traces) produced in the Big Bang nucleosynthesis recombined in inverse order of their ionization potential.
Helium first combined with the free electrons to form the first neutral atom, according to the study published in the journal Nature.
At that time, hydrogen was still ionized or present in the form of naked protons.
Helium atoms combined with these protons in the HeH + helium hydride ion, the first molecular link in the universe.
As the recombination progressed, HeH + reacted with the neutral hydrogen and created a first pathway for the formation of molecular hydrogen – marking the beginning of the modern universe.
Despite its unquestionable importance in the history of the early universe, the HeH + molecule has so far escaped detection in interstellar space, the researchers said.
Studied in the laboratory as early as 1925, dedicated searches over the last few decades have been unsuccessful, thus challenging our understanding of the underlying chemical networks, they said.
"The chemistry of the universe began with HeH +. The lack of definitive evidence of its existence in interstellar space has been a dilemma for astronomy for a long time," said Rolf Gusten of MPIfR.
In the late 1970s, astrochemical models suggested the possibility of HeH + being in detectable abundance in local astrophysical nebulae, and would be more easily observed in the so-called planetary nebula, ejected by Sun-like stars at the last stage of their life. .
The hard radiation field produced by the central white dwarf star with a temperature of over 100,000 degrees directs the ionization fronts to the ejected envelope, where HeH + formation is predicted.
The molecule will emit its strongest spectral line at a characteristic wavelength of 0.149 mm.
However, Earth's atmosphere is opaque at this wavelength for terrestrial observatories, requiring that such a quest be carried out from space or from a high-flying observatory like SOFIA sailing above the absorbing layers of the lower atmosphere.
"With the recent advances in terahertz technologies, it has become possible to perform high-resolution spectroscopy at far-infrared wavelengths," Gusten said.
Operating the large spectrometer aboard SOFIA, the team now reports unambiguous detection of HeH + toward the envelope of the planetary nebula NGC 7027.
"The discovery of HeH + is a dramatic and beautiful demonstration of the tendency of nature to form molecules," said David Neufeld of Johns Hopkins University.
"Although the unpromising ingredients available, a mixture of hydrogen with helium, non-reactive noble gas and a hostile environment at thousands of degrees Celsius, a fragile molecule forms," Neufeld said.
"Remarkably, this phenomenon can not only be observed by astronomers, but also understood using theoretical models that we develop," he said.
The detection of this special molecule brings a long search for a happy ending, and eliminates doubts that we may not understand the underlying formation and destruction, just as we thought, the researchers said.