The largest polycyclic aromatic hydrocarbon (PAH) ever found in space, cyanocoronene, has been identified by a group of chemists and astronomers, marking a significant advancement in astrochemistry. This molecule was discovered in the cold, dark molecular cloud TMC-1, an area renowned for its complex chemistry and as a nursery for future stars. It is made up of seven interconnected benzene rings and a cyano group (C₂₄H₁₁CN).
An artist's impression of the newly identified, largest polycyclic aromatic hydrocarbon detected to date, cyanocoronene. This molecule, composed of seven interconnected benzene rings and a cyano group (C₂₄H₁₁CN), was found in the cold, dark molecular cloud TMC-1, a region known for its rich chemistry and as a cradle for new stars. Image Credit: NSF/AUI/NSF NRAO/P.Vosteen
A derivative of coronene, which is frequently referred to as the “prototypical” compact PAH because of its stability and distinct structure, is cycloocoronene. PAHs are important in the chemistry that creates stars and planets and are believed to lock away a significant fraction of the carbon in the cosmos. Only smaller PAHs have been found in space up until now. This new finding greatly expands the known size limit.
How did astronomers detect this enormous molecule? The researchers first produced cyanocoronene in a lab and then used sophisticated spectroscopy methods to measure its distinct microwave spectrum. Using this chemical fingerprint, scientists looked for cyanocoronene in data from the US National Science Foundation Green Bank Telescope (NSF GBT), the primary telescope used in the GOTHAM (GBT Observations of TMC-1: Hunting Aromatic Molecules) study.
The study team discovered many unique spectral lines of cyanocoronene, proving its presence with a statistical significance of 17.3 sigma—a significant finding by astronomical standards.
Cyanocoronene is presently the biggest individual PAH molecule discovered in interstellar space, with 24 carbon atoms in its core structure (excluding the cyano group). The amount of cyanocoronene discovered is comparable to that of smaller PAHs previously observed, refuting the notion that bigger molecules are uncommon in space.
This hints that even more complicated aromatic compounds may be found in the universe. The presence of such stable, massive PAHs lends credence to the notion that these molecules may serve as a huge carbon reservoir, possibly seeding new planetary systems with the building blocks for life.
The quantum chemical technique used in the paper demonstrates that cyanocoronene may be effectively formed in the freezing conditions of space via interactions between coronene and the CN radical, with highly buried energy barriers that do not hinder the process at low temperatures. This suggests that the chemistry that creates complex organics can occur even before stars form.
The discovery of cyanocoronene not only adds a new chapter to the saga of cosmic chemistry, but it also enhances the “PAH hypothesis”—the theory that these molecules are responsible for the unexplained infrared emission bands visible across the universe.
It also establishes a clear relationship between the chemistry of interstellar clouds, meteorites, and asteroids, implying that organic molecules in our solar system may have evolved in comparable settings long before the Sun emerged.
Scientists are currently looking for even bigger PAHs and their derivatives in space, as well as investigating how these compounds survive and evolve in the harsh environment between stars.
Each new detection brings us closer to understanding the origins of complex organic chemistry in the universe—and perhaps, the origins of the building blocks of life themselves.
Gabi Wenzel, Study Lead Author and Research Scientist, Department of Chemistry at Massachusetts Institute of Technology
During the 246th summer meeting of the American Astronomical Society, this science was presented during a press conference.