Detections of interstellar aromatic nitriles 2-cyanopyrene and 4-cyanopyrene in TMC-1

Polycyclic aromatic hydrocarbons (PAHs) are among the most widespread compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent electric dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. Here we report the detection of 2-cyanopyrene and 4-cyanopyrene, isomers of the recently detected 1-cyanopyrene. We find that these isomers are present in an abundance ratio of ~2:1:2, which mirrors the number of equivalent sites available for CN addition. We conclude that there is evidence that the cyanopyrene isomers formed by direct CN addition to pyrene under kinetic control in hydrogen-rich gas at 10 K and discuss constraints on the H/CN ratio for PAHs in the Taurus molecular cloud (TMC-1). Our detections of the cyanopyrene isomers suggest that small PAHs like pyrene must be either formed in or transported to the cold interstellar medium, challenging assumptions about the origin and fate of PAHs in space. Wenzel et al. detect radio signatures of two forms of cyanopyrene, a small molecular sheet of carbon, which can be used as indicators of the abundance of pyrene. Their findings suggest that small polycyclic aromatic hydrocarbons must be formed in or transported to the cold interstellar medium.