Polycyclic aromatic hydrocarbons in samples of Ryugu formed in the interstellar medium

Polycyclic aromatic hydrocarbons (PAHs) contain ≲20% of the carbon in the interstellar medium. They are potentially produced in circumstellar environments (at temperatures ≳1000 K), or by reactions within cold (~10 K) interstellar clouds, or by processing of carbon-rich dust grains. We report isotopic properties of PAHs extracted from samples of the asteroid Ryugu and meteorite Murchison. The doubly-13C substituted compositions (2×13C values) of the PAHs naphthalene, fluoranthene and pyrene are 9 to 51‰ higher than values expected for a stochastic distribution of isotopes. The 2×13C values are higher than expected if the PAHs formed in a circumstellar environment, but consistent with formation in the interstellar medium. In contrast, the PAHs phenanthrene and anthracene in Ryugu samples have 2×13C values consistent with formation by higher-temperature reactions.