The helium hydride ion (or HeH+), the first molecule to form in the early universe, is still one of the most abundant molecules on Earth today yet it has so far eluded extraterrestrial observation. Our lab is searching for a possible answer to this mystery.
There is a broad consensus that the Big Bang theory is the best theory to understand the origin and evolution of the cosmos. This agreement is based mainly on three important observations: the expansion of the universe, the existence of the cosmic microwave background, and the abundance of light elements like hydrogen and helium.
According to the Big Bang theory, the universe is 13.7 billion years old and its evolution is roughly divided into three phases. The first phase, when particles had extremely high energies, happened during the first second of the universe and is still not well understood. When the temperature of the universe had massively dropped, the second phase took place: the protons, electrons and neutrons formed, followed by the nuclei (combining protons and neutrons), and finally, atoms (combining nuclei and electrons). The third phase is the formation of structures, such as stars and galaxies, as matter aggregated under the effect of the gravitation.
Our research is related to the formation of the first molecules, a step that took place between the second and the third phase, about 300,000 years after the Big Bang. At this time, the temperature of the universe had dropped to 4,000K and atoms of hydrogen (H), helium (He) and lithium (Li) formed. Before that, they were ionized, meaning that no electrons were bound to the nuclei. As the universe cooled down further, the electrons were captured by the ions, making them electrically neutral. The first species to become neutral was helium. It combined with the ionized hydrogen (H+) present to form the first molecule, HeH+. It was soon followed by He2+ and other small molecules.
The abundance of HeH+ and of the other atomic and molecular species composed of hydrogen, helium and lithium evolved with the age of the universe because they can be created or destroyed through various mechanisms. It is estimated that the abundances stopped evolving after about 100 million years. Therefore, it is crucial to know them at that point, as these molecules govern the formation of stars. Several models estimate the abundances of the various molecules and comparing estimates with observations allows us to choose the best model. These observations are usually made by space telescopes, which collect the electromagnetic radiation emitted by the molecules. Each molecule emits radiation at a characteristic frequency - much like a fingerprint through which it is possible to identify it - that can be calculated or measured in a laboratory on Earth, as is the case for HeH+.
All models predict HeH+ to be abundant in gaseous nebulae (giant clouds of gas, which are the birthplace of stars) and in planetary nebulae (layers of gas ejected from dying stars; see picture). But despite this prediction, none of the several attempts to observe its extraterrestrial presence have been conclusive, an intriguing fact since the other molecular species predicted by the models are observed.
The planetary nebula NGC 7027, in which HeH+ is supposed to be present (Hubble telescope, NASA).
The fact that HeH+ eludes observation means that either the mechanisms of creation of HeH+ are overestimated or that the mechanisms of its destruction are underestimated. As the production processes have already been extensively investigated, we focused on the destruction mechanisms, and in particular, on the photodissociation of the ion. This is when a photon (the particle composing the electromagnetic radiation) collides with the molecule, giving it a sufficient energy to break the link between the two atoms of the molecule.
The starting point of our work was to calculate the various possible energy states of the HeH+ ion. Indeed, in quantum mechanics (a physical theory used to describe matter at the microscopic level), the energy of the electrons in an atom or a molecule can only take certain values. Usually, a molecule is in the state with minimum energy, called “ground state”. We calculated the probability that a photon of a given energy will break the molecule in its ground state, confirming the results of the first experiment on the photodissociation of HeH+ that was made in 2007. However, the results obtained did not change the abundance estimates of HeH+ much.
A solution may come from the fact that HeH+ can exist in a state other than the ground state, called “metastable”, because it has a long lifetime. If HeH+ can be in this state for a long enough time, its abundance as well as its characteristic frequency would be altered. This possibility has never been considered. Thus, we are currently calculating the creation and destruction processes of the molecule in this metastable state as well as its lifetime.
The abundance of HeH+ mystery remains very much unsolved and it is hoped that our calculations, as well as observations from infrared telescopes such as the Herschel Space Telescope (launched by the ESA in 2009) and the Spitzer Space Telescope (launched by the NASA in 2003), can shed some new light on the issue. Indeed, observations of this metastable state could invalidate current models of the appearance of the first molecules, with consequences on our theories of the formation of stars and galaxies in the early universe.