Carbon addition as the dominant pathway for aromatic growth in dense molecular clouds

Abstract

Recent radioastronomical detections of aromatic molecules and small polycyclic aromatic hydrocarbons (PAHs) in dense molecular clouds, particularly in TMC-1, have revealed unexpectedly high abundances that cannot be reproduced by standard astrochemical models. While ion–molecule and neutral–neutral chemistry can indeed account for the formation of benzene and cyclopentadiene, these mechanisms fail to explain the efficient production of larger aromatic species. We investigate whether atomic carbon addition provides a viable and dominant pathway for the growth of aromatic molecules and PAHs in cold, UV-shielded molecular clouds. We constructed a reaction network for aromatic growth based on barrierless atomic carbon addition followed by hydrogenation, supported by quantum chemical calculations. The network was implemented in the Nautilus gas–grain code. We identified a consistent growth sequence from small aromatics to larger PAHs through alternating C- and H-atom addition, with minimal carbon loss. The model successfully reproduces the observed abundances of key species such as , , , and . It predicts comparable abundances across successive PAH sizes. C6H5CN C6H5C2H C9H7 C10H7CN Our results suggest that sequential atomic carbon addition, followed by hydrogenation, can drive efficient bottom-up PAH growth in dense clouds, without requiring pre-existing large PAHs. This mechanism provides a natural explanation for the observed aromatic inventory in TMC-1 and could be broadly suitable in other cold interstellar environments.