Aromatic rings in the Central Molecular Zone: Benzonitrile

Context . In recent years, several aromatic molecules (i.e., rings based on benzene, c-C 6 H 6 ) have been reported toward the cold molecular cloud TMC-1; and benzonitrile (the CN-derivative of benzene, c-C 6 H 5 CN) has been identified in other nearby cold clouds with similar physical properties. The derived molecular abundances differ significantly from those predicted by current chemical models, indicating that its chemistry is still poorly understood. This motivates the search for these compounds in other physically distinct sources beyond the solar vicinity. Aims . In this work, we present new detections of c-C 6 H 5 CN in two warmer molecular clouds of the Central Molecular Zone (CMZ) of our Galaxy, G+0.693-0.027 and G+0.633-0.0604. Methods . We used Yebes 40m ultra-deep surveys toward both CMZ clouds covering the 31−50 GHz range, and applied local thermodynamic equilibrium (LTE) and non-LTE analysis to identify c-C 6 H 5 CN and derive the physical parameters of the emission. Results . We derived column densities of N =(7.4±0.5)×10 12 and (2.60±0.13)×10 12 cm −2 , which translate into very similar molecular abundances relative to H 2 of (6±1)×10 −11 and (4.3±0.9)×10 −11 , respectively, consistent with those found toward cold clouds in the Galactic disk. We found that the ratio of the unsaturated carbon chain HC 7 N to c-C 6 H 5 CN shows a dependence on the environmental conditions, being lower (2.15–2.4) in both CMZ clouds compared to colder sources (4.5–30). This points to local effects on the formation and destruction of these species, and a predominance of aromatic chemistry in the CMZ compared with linear carbon chains. Conclusions . This work confirms that c-C 6 H 5 CN is widespread in our Galaxy and can survive in harsher environments (high kinetic temperatures, strong shocks, and enhanced cosmic-ray ionization rates) than those in Galactic cold clouds, which suggests that aromatics are stable and abundant species that can significantly contribute to the total budget of interstellar carbon in molecular clouds in diverse environments. A top-down scenario, in which aromatic species are produced through the fragmentation and processing of carbonaceous solids or larger aromatics, agrees well with the strikingly nearly constant behavior of their molecular abundances with molecular size.