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

TL;DR

This paper reports the detection of cyanopyrene isomers in the interstellar medium TMC-1, providing evidence for their formation via direct CN addition to pyrene in hydrogen-rich, cold conditions, and constraining PAH chemistry.

Contribution

It is the first detection of interstellar cyanopyrene isomers and demonstrates their formation mechanism in space, advancing understanding of PAH chemistry in the universe.

Findings

Cyanopyrene isomers detected in TMC-1 with a 2:1:2 abundance ratio.

Evidence supports formation by direct CN addition to pyrene.

Constraints on H/CN ratio for PAHs in cold interstellar environments.

Abstract

Polycyclic aromatic hydrocarbons (PAHs) are among the most ubiquitous compounds in the universe, accounting for up to ~25% of all interstellar carbon. Since most unsubstituted PAHs do not possess permanent dipole moments, they are invisible to radio astronomy. Constraining their abundances relies on the detection of polar chemical proxies, such as aromatic nitriles. We report the detection of 2- 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 TMC-1.