Formation of benzene and its derivatives in dense molecular clouds

A number of aromatic molecules and small polycyclic aromatic hydrocarbons have recently been detected in dense molecular clouds, notably in TMC-1. Although these species are generally assumed to form through bottom-up chemical processes, current astrochemical models underestimate their abundances by several orders of magnitude. This work aims to identify and constrain the dominant chemical pathways leading to the formation of the first six-carbon aromatic compounds in dense molecular clouds. We revisited the chemistry of aromatic compounds in dense molecular clouds by systematically examining all neutral and ionic reactions that may lead to the formation of and chemically related species ( , , , , and ). Each reaction was analyzed according to its relevance to the formation of the first aromatic compounds. C6H6 C6H4 C6H5 C6H5+ C6H6+ C6H7+ We identified a limited number of key reactions that dominate the formation of the first aromatic ring. We found that ionic pathways involving l-C_3H_3^+ and l-C_3H_5^+ reacting with C_3H_4, together with the neutral reaction C + c-C_5H_6, are the main contributors to benzene formation. With the revised chemical network, the observed abundances of C_6H_4, C_6H_5CN, and C_6H_5C_2H in TMC-1 can be reproduced within a factor of 2. This result is notably better than what has been achieved with previous models. The revised bottom-up chemical scheme successfully reproduces the observed abundances of aromatic compounds, with significant uncertainties due to the lack of precise determinations for the branching ratios for many of the reactions involved. It also demonstrates the central role of neutral and ionic chemistry. The formation pathways of larger aromatic compounds remain to be explored. C6 C3