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 interesting starting point.

Although these many-ringed molecules are abundant in the swirling gas in the interstellar medium, specific PAHs have historically been difficult to identify with infrared telescopes due to 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, DOÏ: 10.1126/science.adq6391). The molecule is the largest PAH detected with a radio telescope.

“We think a lot of the carbon is trapped in this particular PAH,” says chemist Ilsa Cooke from the University of British Columbia, one of the paper’s co-authors, because it “is particularly abundant due to its chemical structure.” Pyrene’s four fused aromatic rings provide what the team calls “an island of stability,” making it less susceptible to degradation by ultraviolet light.

Unfortunately, the symmetry of pyrene’s unsubstituted fused rings makes direct detection of the molecule with radio telescopes impossible. Molecules lacking a permanent dipole are invisible to the instrument, which collects rotational spectra. “But adding that the CN group essentially places a gigantic radio antenna on the molecule; this adds a very large permanent dipole moment,” explains the team leader Brett McGuirean astrochemist from the Massachusetts Institute of Technology. Estimating the concentration of pyrene based on readings of this derivative is quite simple.

The team needed a reference for 1-cyanopyrene before they could analyze 1,500 hours of data from the telescope. They therefore collected a rotation spectrum of the molecule in the gas phase in the laboratory, explains Gaby Wenzelpostdoctoral researcher in McGuire’s lab. It was not an easy task. The Earth-bound cyano-substituted PAHs had to be specially synthesized, she said, 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 have discovered by radio astronomy over the past 5 years. The discovery of 1-cyanopyrene in a cold molecular cloud is an exciting piece of the interstellar carbon cycle puzzle, according to Andrew Mattiodaa NASA research scientist not affiliated with the work. “It’s one thing to say, ‘It should be there,'” he adds. “It’s another to find it.”