Formation of benzene and its derivatives in dense molecular clouds

¹ Université Bordeaux, CNRS, Bordeaux INP, ISM, UMR 5255, 33400 Talence, France
² Université Paris-Saclay, CNRS, Institut des Sciences Moléculaires d’Orsay, 91405 Orsay, France
³ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
⁴ Université Paris-Saclay, CNRS, Institut de Chimie Physique, UMR8000, 91405 Orsay, France
⁵ Synchrotron SOLEIL, L’Orme des Merisiers, 91192 Saint Aubin, Gif-sur-Yvette, France
⁶ Department of Physics, University of Trento, Via Sommarive 14, 38123 Trento, Italy
⁷ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. ; This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 26 February 2026
Accepted: 7 April 2026

Abstract

Context. 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.

Aims. 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.

Methods. 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 C₆H₆ and chemically related species (C₆H₄, C₆H₅, C₆H₅⁺, C₆ H₆⁺, and C₆H₇⁺). Each reaction was analyzed according to its relevance to the formation of the first aromatic compounds.

Results. We identified a limited number of key reactions that dominate the formation of the first aromatic ring. We found that ionic pathways involving 1-C₃H⁺₃ and 1-C₃H⁺₅ reacting with C₃H₄, together with the neutral reaction C + c-C₅H₆, are the main contributors to benzene formation. With the revised chemical network, the observed abundances of C₆H₄, C₆H₅CN, and C₆H₅C₂H in TMC-1 can be reproduced within a factor of 2. This result is notably better than what has been achieved with previous models.

Conclusions. The revised bottom-up chemical scheme successfully reproduces the observed abundances of C₆ 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 C₃ chemistry. The formation pathways of larger aromatic compounds remain to be explored.