Low D/H ratio for benzonitrile in TMC-1: Implication for the origin of polycyclic aromatic hydrocarbons in cold dark clouds^★

¹ Departamento de Química Física y Química Inorgánica, Facultad de Ciencias-I.U. CINQUIMA, Universidad de Valladolid, 47011 Valladolid, Spain
² Centro Singular de Investigación en Química Biolóxica e Materiais Moleculares (CiQUS) and Departamento de Química Orgánica, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain
³ Departamento de Astrofísica Molecular, Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
⁴ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain

^★★ Corresponding authors: amanda.steber@uva.es; marcelino.agundez@csic.es; dolores.perez@usc.es

Received: 16 May 2025
Accepted: 16 July 2025

Abstract

Radioastronomical observations have recently discovered polycyclic aromatic hydrocarbons (PAHs) of moderate size (up to 24 carbon atoms) in cold dark clouds, although it is currently unknown whether they are formed in situ through a bottom-up mechanism or from larger PAHs (20–100 carbon atoms) inherited from a previous diffuse stage in a top-down scenario. Infrared observations have recently shown that large PAHs present in UV-illuminated regions are strongly enriched in deuterium. In order to shed light on the origin of PAHs in cold clouds, we have searched for deuterated benzonitrile in the cold dark cloud TMC-1. To that purpose we have synthesized the three isomers (ortho, meta, and para) of monodeuterated benzonitrile, measured their rotational spectra across the 2–18 GHz and 75–110 GHz frequency ranges in the laboratory, and searched for them in TMC-1 using data from the QUIJOTE line survey. We did not detect any of the three species and have derived a 3σ upper limit on the column density of each of them of 3.0 × 10¹⁰ cm⁻², meaning a fractional abundance relative to H₂ of <3 × 10⁻¹². We derived a D/H ratio (which we define as the total number of D atoms with respect to the total number of H atoms present in benzonitrile) of <1.2%. This value is in line with the range of D/H ratios observed for other molecules in TMC-1 (0.06–3.3%), where deuterium enrichment is explained in terms of isotopic fractionation at low temperature. It is however below the range of D/H ratios derived for large unspecific PAHs from JWST observations of the galactic photodissociation regions (PDRs) Orion Bar and M 17 and the galaxies M51 and NGC 3256-S (between 1 and <17%). Although it is not straightforward to compare the deuteration of PAHs in dark and UV-irradiated clouds, our results suggest that the population of PAHs detected in cold dark clouds does not result from the fragmentation of larger PAHs inherited from the previous diffuse stage in a top-down scenario.