Aromatic Rings In The Central Molecular Zone: Benzonitrile

In recent years, several aromatic molecules (benzene-based rings) have been detected in the cold molecular cloud TMC-1, with its CN-derivative, benzonitrile (c-C₆H₅CN), also identified in other nearby cold sources.

However, observed abundances differ significantly from chemical model predictions, indicating an incomplete understanding of its chemistry and motivating searches in distinct environments.

We report new detections of benzonitrile in two warmer molecular clouds of the Central Molecular Zone (CMZ): G+0.693-0.027 and G+0.633-0.0604. Using Yebes 40m ultra-deep surveys in the 31–50 GHz range, we performed LTE and non-LTE analyses to derive the physical parameters of the emission.

We obtain column densities of N=(7.4±0.5)×10¹² and (2.60±0.13)×10¹² cm⁻², corresponding to abundances relative to H₂ of (6±1)×10⁻¹¹ and (4.3±0.9)×10⁻¹¹, consistent with values in cold Galactic clouds.

The HC₇N/benzonitrile ratio is lower (2.15-2.4) than in colder sources (4.5-30), suggesting environmental effects and a relative enhancement of aromatic chemistry in the CMZ. These results confirm that benzonitrile is widespread and can survive in harsher environments (e.g., high temperatures, shocks, enhanced cosmic-ray ionization) than those in Galactic cold clouds.

This suggests that aromatics are stable and abundant species that can significantly contribute to the total budget of interstellar carbon in molecular clouds. A top-down formation scenario, involving fragmentation of larger carbonaceous species, is consistent with the nearly constant abundances observed with molecular size.

V.M. Rivilla, D. San Andrés, M. Sanz-Novo, L. Colzi, I. Jiménez-Serra, A. López-Gallifa, A. Martínez-Henares, A. Megías, S. Martín, B. Tercero, S. Zeng, J. Loreau, M. Ben Khalifa, M. A. Requena-Torres, P. de Vicente

Comments: Accepted in Astronomy & Astrophysics
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Cite as: arXiv:2604.24510 [astro-ph.GA] (or arXiv:2604.24510v1 [astro-ph.GA] for this version)
https://doi.org/10.48550/arXiv.2604.24510
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Submission history
From: Victor Manuel Rivilla
[v1] Mon, 27 Apr 2026 14:12:08 UTC (13,156 KB)
https://arxiv.org/abs/2604.24510

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