Found in space: complex carbon-based molecules

Much of the carbon in space is believed to exist in the form of large molecules called polycyclic aromatic hydrocarbons (PAHs). Since the 1980s, circumstantial evidence indicates that these molecules are abundant in space, but that they have not been directly observed.

Now, a team of researchers led by MIT Assistant Professor Brett McGuire has identified two distinctive PAHs in a spatial area called the Taurus Molecular Cloud (TMC-1). PAHs were believed to only form effectively at high temperatures – on Earth, they occur as a byproduct of fossil fuel combustion, and are also found in charcoal marks on grilled foods. But the interstellar cloud where the research team observed them has yet to start forming stars, and the temperature is about 10 degrees above absolute zero.

The discovery suggests that these molecules can form at much lower temperatures than expected, and it could lead scientists to rethink their hypotheses about the role of PAH chemistry in the formation of stars and planets, the researchers say.

“What makes detection so important is that not only have we confirmed a hypothesis that has been in development for 30 years, but we can now look at all the other molecules from that one source and ask how they react to form PAHs. we see how the PAHs we see can react with other things to eventually form larger molecules, and what implications this may have for our understanding of the role of very large carbon molecules in the formation of planets and stars ”, says McGuire, who is one of the lead authors of the new study.

Michael McCarthy, associate director of the Harvard-Smithsonian Center for Astrophysics, is another lead author of the study, which appears today in Science. The research team also includes scientists from several other institutions, including the University of Virginia, the National Radio Astronomical Observatory, and NASA’s Goddard Space Flight Center.

Distinctive signs

Beginning in the 1980s, astronomers used telescopes to detect infrared signals suggesting the presence of aromatic molecules, which are molecules that typically include one or more carbon rings. About 10 to 25 percent of the carbon in space is thought to be found in PAHs, which contain at least two carbon rings, but infrared signals were not distinct enough to identify specific molecules.

“This means that we cannot dig into the detailed chemical mechanisms of their formation, their reaction with each other or with other molecules, their destruction and the entire carbon cycle throughout the star formation process and planets. and ultimately life, ”says McGuire.

Although radio astronomy has been a hobbyhorse of molecular discovery in space since the 1960s, radio telescopes powerful enough to detect these large molecules have only been around for a little over a decade. These telescopes can pick up the rotational spectra of molecules, which are distinctive patterns of light that molecules emit as they tumble through space. Researchers can then try to match patterns seen in space with patterns they’ve seen from those same molecules in laboratories on Earth.

“Once you have this pattern match, you know there is no other molecule that could emit this exact spectrum. And, the intensity of the lines and the relative strength of the different parts of the pattern tell you a lot about how much of the molecule there is, and how hot or cold the molecule is, ”McGuire says.

McGuire and his colleagues have been studying TMC-1 for several years because previous observations revealed that it was rich in complex carbon molecules. A few years ago, a member of the research team observed clues that the cloud contained benzonitrile – a six-carbon ring attached to a nitrile (carbon-nitrogen) group.

The researchers then used the Green Bank Telescope, the world’s largest steerable radio telescope, to confirm the presence of benzonitrile. In their data, they also found signatures of two other molecules – the PAHs reported in this study. These molecules, called 1-cyanonaphthalene and 2-cyanonaphthalene, consist of two benzene rings fused together, with a nitrile group attached to a ring.

“The detection of these molecules is a big step forward in astrochemistry. We’re starting to connect the dots between small molecules – like benzonitrile – that are known to exist in space, to the monolithic PAHs that are so important in astrophysics, ”says Kelvin Lee, an MIT postdoc. one of the study’s authors.

The discovery of these molecules in cold, starless TMC-1 suggests that PAHs are not only byproducts of dying stars, but can be assembled from smaller molecules.

“Where we found them there is no star, so they’re either being built in place or they’re the remains of a dead star,” McGuire says. “We think it’s probably a combination of the two – the evidence suggests it’s not one way or the other exclusively. It’s new and interesting because there really wasn’t any observational evidence for it. this upward path before. “

Carbon chemistry

Carbon plays a vital role in the formation of planets, so the suggestion that PAHs could be present even in starless and cold regions of space may prompt scientists to rethink their theories about the chemicals available during this period. of the formation of planets, McGuire says. When PAHs react with other molecules, they can start to form interstellar dust grains, which are the seeds of asteroids and planets.

“We have to completely rethink our models of how chemistry evolves, from these starless nuclei, to include the fact that they form these large aromatic molecules,” he says.

McGuire and his colleagues now plan to study in more detail how these PAHs formed and what types of reactions they may undergo in space. They also plan to continue scanning the TMC-1 with the powerful Green Bank telescope. Once they have these interstellar cloud observations, researchers can try to match the signatures they find with the data they generate on Earth by putting two molecules in a reactor and blasting them with kilovolts of electricity, breaking them into pieces and letting them recombine. This could result in hundreds of different molecules, many of which have never been seen on Earth.

“We have to continue to see what molecules are present in this interstellar source, because the more we know about the inventory, the more we can begin to try to connect the elements of this web of reaction,” says McGuire.