Carbon sources spotted in cold interstellar clouds

To help understand the emergence of life on Earth, some scientists are trying to unravel the cosmic carbon cycle. The interstellar chemistry of polycyclic aromatic hydrocarbons (PAHs)—a large family of carbon-containing molecules found in space—is an enticing place to start.

Although these many-ringed molecules are abundant in the gas swirling in the interstellar medium, specific PAHs historically have been difficult to identify with infrared telescopes because of their broad, overlapping signal peaks. But now, using nearly 1,500 hours of data collected with the Green Bank Telescope, a group of researchers has identified 1-cyanopyrene in the Taurus molecular cloud (Science 2024, DOI: 10.1126/science.adq6391). The molecule is the largest PAH detected with a radio telescope.

“We think that a lot of carbon is locked up in this particular PAH,” says chemist Ilsa Cooke of the University of British Columbia, one of the paper’s coauthors, as it “is particularly abundant because of its chemical structure.” The four fused aromatic rings of pyrene offer what the team calls “an island of stability,” making it less sensitive to degradation with ultraviolet light.

Unfortunately, the symmetry of pyrene’s unsubstituted fused rings makes it impossible to detect the molecule directly with radio telescopes. Molecules lacking a permanent dipole are invisible to the instrument, which collects rotational spectra. “But adding that CN group essentially puts a gigantic radio antenna on the molecule; it adds a very large permanent dipole moment,” says the team leader Brett McGuire, an astrochemist from the Massachusetts Institute of Technology. Estimating the concentration of pyrene based on readings of this derivative is fairly straightforward.

The team needed a reference for 1-cyanopyrene before they could analyze 1,500 hours of telescope data. So they collected a rotational spectrum of the gas-phase molecule in the lab, says Gabi Wenzel, a postdoctoral researcher in McGuire’s lab. It was no easy task. The Earth-bound, cyano-substituted PAH had to be specially synthesized, she says, and didn’t “want to be in the gas phase here in our instruments.”

Ultimately, the team succeeded: using their spectrum, they found 1-cyanopyrene in the telescope data and added another molecule to the list of those they’ve discovered using radio astronomy in the last 5 years. The discovery of 1-cyanopyrene in a cold molecular cloud is an exciting piece of the interstellar carbon-cycle puzzle, says Andrew Mattioda, a NASA research scientist unaffiliated with the work. “It’s one thing to say, ‘It should be there,’ ” he adds, “It’s another thing to actually find it.”