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, indirect evidence has shown that these molecules are abundant in space, but they have not been directly observed.

Now, a team of researchers led by MIT assistant professor Brett McGuire has identified two distinct PAHs in a patch of space called the Taurus Molecular Cloud (TMC-1). PAHs were believed to be efficiently formed only at high temperatures – on Earth they occur as byproducts 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 not yet started to form stars, and its temperature is about 10 degrees above absolute zero.

This discovery suggests that these molecules can form at much lower temperatures than expected, and it may lead scientists to rethink their assumptions 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 30 years in the making, but that we can now look at all the other molecules in this one source and wonder how they respond to the PAHs. we see how the PAHs we see can react with other things to form potentially larger molecules, and what implications that may have for our understanding of the role of very large carbon molecules in the formation of planets and stars, ” says McGuire, who is a senior author on the new study.

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

Distinctive Signals

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

“ That means we can’t dig into the detailed chemical mechanisms for how these are formed, how they react with each other or with other molecules, how they are destroyed, and the entire carbon cycle in the process of star and planet formation. and ultimately life, ”says McGuire.

While radio astronomy has been a workhorse for molecular discoveries 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 light patterns that molecules give off as they tumble through space. Researchers can then try to match patterns observed in space with patterns they’ve seen from those same molecules in labs on Earth.

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

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

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

“Detecting these molecules is a giant leap forward in astrochemistry. We are starting to connect the dots between small molecules – such as benzonitrile – that are known to exist in space, and the monolithic PAHs that are so important in astrophysics” , says Kelvin Lee. , an MIT postdoc who is one of the authors of the study.

Finding these molecules in the cold, starless TMC-1 suggests that PAHs are not only the byproducts of dying stars, but can be composed of smaller molecules.

“In the place where we found them, there is no star, so either they build up in place or they are the remains of a dead star,” says McGuire. “We think it is probably a combination of the two. – the evidence suggests that it is not one path, nor the other exclusive. That’s new and interesting because there really wasn’t any observational evidence for this bottom-up path. ”

Carbon chemistry

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

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

McGuire and colleagues now plan to further investigate how these PAHs originated and what reactions they can undergo in space. They also plan to continue to scan TMC-1 with the powerful Green Bank Telescope. Once they have those observations of the interstellar cloud, the researchers can try to match the signatures they find with the data they generate on Earth by placing two molecules in a reactor and blowing them with kilovolts of electricity, breaking them into pieces and let them recombine. This can result in hundreds of different molecules, many of which have never been seen on Earth.

“We need to keep seeing which molecules are present in this interstellar source, because the more we know about the inventory, the more we can try to connect the pieces of this reaction web,” says McGuire.