Astronomers have identified an aromatic molecule in space using a radio telescope for the first time, spotting benzonitrile in an interstellar dust cloud 430 light-years away. Brett A. McGuire, a Hubble fellow at the National Radio Astronomy Observatory, and colleagues plan to use the spectroscopic data they collected to figure out how this and other complex chemicals arise in outer space (Science 2018, DOI: 10.1126/science.aao4890).
Astrochemists have known that polycyclic aromatic hydrocarbons (PAHs) make up an estimated 10% of all interstellar carbon in the universe, but it has been challenging for them to distinguish one PAH from another. Bond-stretching motions in the molecules’ infrared spectra are too similar to parse, and many PAHs lack strong polarity, making signatures in their rotational spectra—typically collected with radio telescopes—difficult to detect.
Although benzonitrile isn’t strictly a PAH because of the nitrogen it contains, McGuire’s group set its sights on the molecule because of its strong dipole moment. Benzonitrile is thought to form from a reaction between benzene and cyanide, so measuring benzonitrile may be one way to estimate how much benzene, a PAH, exists in space, as well as other molecules.
Using the Robert C. Byrd Green Bank Telescope, in West Virginia, the researchers could make high-resolution measurements at specific radio frequencies. The team observed eight of benzonitrile’s nine predicted rotational transitions, the frequencies of which had been confirmed via lab experiments.
The discovery is good news for astronomers searching for molecules in space, according to Christine Joblin, an astrophysicist at the University of Toulouse. In an accompanying perspective article in Science, she and José Cernicharo of the Institute of Materials Science of Madrid explain that until now, most compounds in space have been detected by telescopes that scanned different wavelengths. “Since most of the environments are out of reach for a direct chemical analysis, we are gathering all the information we can from photons over the different frequency ranges,” Joblin says.