Letter to the Editor

Tentative detection of HC₅NH⁺ in TMC-1^⋆

¹ Grupo de Astrofísica Molecular, Instituto de Física Fundamental (IFF-CSIC), C/ Serrano 121, 28006 Madrid, Spain
e-mail: nuria.marcelino@csic.es
² Centro de Desarrollos Tecnológicos, Observatorio de Yebes (IGN), 19141 Yebes, Guadalajara, Spain
³ Observatorio Astronómico Nacional (IGN), C/ Alfonso XII, 3, 28014 Madrid, Spain

Received: 25 August 2020
Accepted: 4 September 2020

Abstract

Using the Yebes 40m radio telescope, we report the detection of a series of seven lines harmonically related with a rotational constant B₀ = 1295.81581 ± 0.00026MHz and a distortion constant D₀ = 27.3 ± 0.5Hz towards the cold dense cloud TMC-1. Ab initio calculations indicate that the best possible candidates are the cations HC₅NH⁺ and NC₄NH⁺. From a comparison between calculated and observed rotational constants and other arguments based on proton affinities and dipole moments, we conclude that the best candidate for a carrier of the observed lines is the protonated cyanodiacetylene cation, HC₅NH⁺. The HC₅N/HC₅NH⁺ ratio derived in TMC-1 is 240, which is very similar to the HC₃N/HC₃NH⁺ ratio. Results are discussed in the framework of a chemical model for protonated molecules in cold dense clouds.