Using data from the Robert C. Byrd Green Bank Telescope, astronomers have identified emission from two isomers of a small polycyclic aromatic hydrocarbon (PAH) called cyanonapthalene in the starless cloud core TMC-1, which is part of the interstellar Taurus Molecular Cloud. The discovery, reported in a paper in the journal Science, suggests that PAHs 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.
PAH molecules overlaid on the Taurus Molecular Cloud, a large blue-white cloud made of gas and dust with large and small stars of varying luminosities interspersed throughout. Aromatic molecule structures shown in order from left to right: 1-cyanonaphthalene, 1-cyano-cyclopentadiene, HC11N, 2-cyanonaphthalene, vinylcyanoacetylene, 2-cyano-cyclopentadiene, benzonitrile, E-cyanovinylacetylene, HC4NC, propargylcyanide. Image credit: M. Weiss / Harvard & Smithsonian Center for Astrophysics.
PAHs are hydrocarbons composed of two or more fused aromatic rings of carbon and hydrogen atoms.
These primarily colorless, white, or pale yellow solid compounds are produced from both natural and anthropogenic sources with the latter constituting the significant sources.
Natural sources of PAHs include forest or bushfires and volcanic eruptions. Anthropogenic sources consist of various pyrolytic processes during industrial operations, incineration, vehicular emissions and power generation among others.
Astronomers have suspected the presence of PAHs in space since the 1980s but the new research provides the first definitive proof of their existence in molecular clouds.
“PAHs are thought to contain as much as 25% of the carbon in the Universe,” said Dr. Brett McGuire, an astronomer at the Harvard & Smithsonian Center for Astrophysics, MIT and National Radio Astronomy Observatory.
“Now, for the first time, we have a direct window into their chemistry that will let us study in detail how this massive reservoir of carbon reacts and evolves through the process of forming stars and planets.”
Dr. McGuire and his colleagues have been studying TMC-1, which is located about 440 light-years away from Earth in the constellation of Taurus, for several years because previous observations have revealed it to be rich in complex carbon molecules.
A few years ago, astronomers observed hints that the cloud core contains benzonitrile, a six-carbon ring attached to a nitrile group.
In the new study, Dr. McGuire’s team aimed to confirm the presence of benzonitrile in TMC-1.
In the data from the Green Bank Telescope, they also found signatures of two other molecules, 1- and 2-cyanonaphthalene, which consist of two benzene rings fused together, with a nitrile group attached to one ring.
“Detecting these molecules is a major leap forward in astrochemistry,” said Dr. Kelvin Lee, a postdoctoral researcher at the Harvard & Smithsonian Center for Astrophysics and MIT.
“We are beginning to connect the dots between small molecules — like benzonitrile — that have been known to exist in space, to monolithic PAHs that are so important in astrophysics.”
Finding these molecules in the cold, starless TMC-1 suggests that PAHs are not just the byproducts of dying stars, but may be assembled from smaller molecules.
“In the place where we found them, there is no star, so either they’re being built up in place or they are the leftovers of a dead star,” Dr. McGuire said.
“We think that it’s probably a combination of the two — the evidence suggests that it is neither one pathway nor the other exclusively. That’s new and interesting because there really hadn’t been any observational evidence for this bottom-up pathway before.”
“From decades of previous modeling, we believed that we had a fairly good understanding of the chemistry of molecular clouds,” said Dr. Michael McCarthy, an astrochemist and acting deputy director of the Harvard & Smithsonian Center for Astrophysics.
“What these new astronomical observations show is these molecules are not only present in molecular clouds, but at quantities which are orders of magnitude higher than standard models predict.”
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Brett A. McGuire et al. 2021. Detection of two interstellar polycyclic aromatic hydrocarbons via spectral matched filtering. Science 371 (6535): 1265-1269; doi: 10.1126/science.abb7535
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