Chemistry at the beginning: how molecular reactions influenced the formation of the first stars

Immediately after the Big Bang, which occurred around 13.8 billion years ago, the universe was dominated by unimaginably high temperatures and densities. However, after just a few seconds, it had cooled down enough for the first elements to form, primarily hydrogen and helium. These were still completely ionised at this point, as it took almost 380,000 years for the temperature in the universe to drop enough for neutral atoms to form through recombination with free electrons. This paved the way for the first chemical reactions.

The oldest molecule in existence is the helium hydride ion (HeH⁺), formed from a neutral helium atom and an ionised hydrogen nucleus. This marks the beginning of a chain reaction that leads to the formation of molecular hydrogen (H₂), which is by far the most common molecule in the universe.

Recombination was followed by the 'dark age' of cosmology: although the universe was now transparent due to the binding of free electrons, there were still no light-emitting objects, such as stars. Several hundred million years passed before the first stars formed.

During this early phase of the universe, however, simple molecules such as HeH⁺ and H₂ were essential to the formation of the first stars. In order for the contracting gas cloud of a protostar to collapse to the point where nuclear fusion can begin, heat must be dissipated. This occurs through collisions that excite atoms and molecules, which then emit this energy in the form of photons. Below approximately 10,000 degrees Celsius, however, this process becomes ineffective for the dominant hydrogen atoms. Further cooling can only take place via molecules that can emit additional energy through rotation and vibration. Due to its pronounced dipole moment, the HeH⁺ ion is particularly effective at these low temperatures and has long been considered a potentially important candidate for cooling in the formation of the first stars. Consequently, the concentration of helium hydride ions in the universe may significantly impact the effectiveness of early star formation.

During this period, collisions with free hydrogen atoms were a major degradation pathway for HeH⁺, forming a neutral helium atom and an H₂⁺ ion. These subsequently reacted with another H atom to form a neutral H₂ molecule and a proton, leading to the formation of molecular hydrogen.

Researchers at the Max-Planck-Institut für Kernphysik (MPIK) in Heidelberg have now successfully recreated this reaction under conditions similar to those in the early universe for the first time. They investigated the reaction of HeH⁺ with deuterium, an isotope of hydrogen containing an additional neutron in the atomic nucleus alongside a proton. When HeH⁺ reacts with deuterium, an HD⁺ ion is formed instead of H₂⁺, alongside the neutral helium atom.

The experiment was carried out at the Cryogenic Storage Ring (CSR) at the MPIK in Heidelberg — a globally unique instrument for investigating molecular and atomic reactions under space-like conditions. For this purpose, HeH⁺ ions were stored in the 35-metre-diameter ion storage ring for up to 60 seconds at a few kelvins (-267 °C), and were superimposed with a beam of neutral deuterium atoms. By adjusting the relative speeds of the two particle beams, the scientists were able to study how the collision rate varies with collision energy, which is directly related to temperature.

They found that, contrary to earlier predictions, the rate at which this reaction proceeds does not slow down with decreasing temperature, but remains almost constant. “Previous theories predicted a significant decrease in the reaction probability at low temperatures, but we were unable to verify this in either the experiment or new theoretical calculations by our colleagues,” explains Dr Holger Kreckel from the MPIK. ‘The reactions of HeH⁺ with neutral hydrogen and deuterium therefore appear to have been far more important for chemistry in the early universe than previously assumed,’ he continues. This observation is consistent with the findings of a group of theoretical physicists led by Yohann Scribano, who identified an error in the calculation of the potential surface used in all previous calculations for this reaction. The new calculations using the improved potential surface now align closely with the CSR experiment.

Since the concentrations of molecules such as HeH⁺ and molecular hydrogen (H₂ or HD) played an important role in the formation of the first stars, this result brings us closer to solving the mystery of their formation.