Letter to the Editor

New deuterated species in TMC-1: Detection of CH₂DC₄H with the QUIJOTE line survey^⋆

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: carlos.cabezas@csic.es, jose.cernicharo@csic.es
² LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
³ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain
⁴ Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain

Received: 2 December 2021
Accepted: 19 December 2021

Abstract

We report the first detection in space of the single deuterated isotopologue of methyldiacetylene, CH₂DC₄H. A total of 12 rotational transitions, with J = 8–12 and K_a = 0 and 1, were identified for this species in TMC-1 in the 31.0–50.4 GHz range using the Yebes 40m radio telescope. The observed frequencies allowed us to obtain, for the first time, the spectroscopic parameters of this deuterated isotopologue. We derived a column density of (5.5 ± 0.2) × 10¹¹ cm⁻². The abundance ratio between CH₃C₄H and CH₂DC₄H is 24 ± 2. This ratio is similar to that found for the CH₃C₃N/CH₂DC₃N analogue system, which is 22 ± 2. We did not detect the deuterated species CH₃C₄D, which has already been observed in laboratory experiments. The detection of deuterated CH₃C₄H allows us to extend the discussion on the chemical mechanisms of deuterium fractionation at work in TMC-1 using a new gas-phase chemical model with multiply deuterated molecules. Introducing a possible deuterium exchange reaction between CH₃CCH and atomic deuterium allows us to account for the CH₃C₄H/CH₂DC₄H abundance ratio.