Pyrene molecules identified in the interstellar medium

Planetary systems, like the Solar System, are born in vast clouds of gas and dust. One of the big questions concerning this formation is that of the history of the molecules initially present in the clouds. Are they still present after the emergence of the star and its procession of planets and other small bodies? Can we use them as tracers of this evolution? The emphasis is particularly placed on molecules rich in carbon, from which all organic chemistry and life on Earth will arise. In this area, polycyclic aromatic hydrocarbons (PAHs) are particularly interesting due to their high stability. They are detected in asteroids and comets in the Solar System. It remained to observe them in the interstellar medium to complete their story. Indices corresponding to the simplest PAHs have been collected in recent years. Gabi Wenzel and Brett McGuire, from MIT (Massachusetts Institute of Technology), United States, and their colleagues have just taken a further step by announcing the detection of cyanopyrene in the cold molecular cloud TMC-1 located 430 light years away of the Earth. This species constitutes the largest PAH detected to date in the interstellar medium.

PAHs are large molecules produced during combustion phenomena, such as during a barbecue or in automobile exhausts. At least, that is the case on Earth. They are also suspected of being an important ingredient in star and planet formation regions. These molecules have the particularity of being very stable structures, formed of rings of several carbon atoms. Generally speaking, each carbon atom has four electrons that can participate in bonds with other elements, but when they are in cycles, two electrons bond with two other carbon atoms and one with a hydrogen atom . The last electron remains free and is shared on the cycle to which it belongs, we speak of “aromaticity” and this gives great stability to PAHs. In particular, PAHs are much more stable than standard non-cycling hydrocarbons, such as alkanes. Among the best-known PAHs are benzopyrene and anthracene, often associated with combustion phenomena on Earth and their toxicity.

In space, the presence of PAHs was proposed in the 1980s to account for the observation of certain characteristic infrared emission bands. The latter, called “infrared aromatic bands”, would testify to the omnipresence of PAHs in the interstellar medium, where they could contain between 10 and 20% of total carbon.

In 2018, PAH research in space took a decisive step forward with the use of radio telescopes. Thanks to that of Green Bank, in the United States, a team led by Brett McGuire identified cyanobenzene in TMC-1, an aromatic ring comprising a CN nitrile group. The technique used to measure the rotational spectrum subsequently made it possible to detect two isomers of cyanonaphthalene. “Benzene and naphthalene do not emit rotational signals strong enough to be detectable, but, when associated with a nitrile group, there is a charge asymmetry which, when rotating, emits a much stronger signal” , describes Christine Joblin, astrophysicist at the Institute for Research in Astrophysics and Planetology, in Toulouse.

Since these discoveries, molecular analyzes of samples from the asteroid Ryugu, brought back by the mission Hayabusa2highlighted the presence of a great diversity of PAHs, pyrene (C₁₆H₁₀) being one of the most abundant. Brett McGuire and his team therefore looked for this PAH within TMC-1. To do this, the researchers also had to use a modified version of the molecule: cyanopyrene (C₁₆H₉CN), a pyrene associated with a nitrile group.

Using once again the Green Bank radio telescope and after studying the rotational spectrum of cyanopyrene in the laboratory, the researchers spotted this molecule in large quantities within TMC-1: according to their estimates, the pyrene itself even would comprise up to 0.1% of the total carbon included in this molecular cloud. “This is an impressive figure given the diversity of carbon molecules present, because apart from carbon monoxide, all the other molecules which contain carbon and which we see by radio astronomy contain much less than 0.1% of the carbon” , specifies the researcher.

Molecular clouds like TMC-1 are the birthplaces of stars and planets. The discovery of pyrene in this type of region, as well as its presence in asteroids, reinforces the hypothesis according to which this molecule could play a key role in the chemistry of planetary systems. “For the moment we have only observed a few PAH molecules and pyrene is the largest of them, but there must be much larger ones,” points out Christine Joblin.

However, many questions remain unanswered: detections have so far only taken place within cold molecular clouds, poorly irradiated by ultraviolet radiation, otherwise these small molecules would have been destroyed. The population of molecules present in more irradiated regions must therefore be different. Before arriving at a more complete scenario of the fate of carbon in space, “we must first try to make the link between the PAHs present in these two types of environments, irradiated and less irradiated,” says the researcher. Thus, we will be able to use these aromatic molecules as tracers of all this evolution from molecular clouds, star formation, protoplanetary disks, planets and small bodies like asteroids and comets.”