Tentative detection of HC5NH+ in TMC-1

TL;DR

This paper reports the tentative detection of the protonated cyanodiacetylene cation (HC5NH+) in the TMC-1 cloud using radio telescope observations, supported by ab initio calculations and chemical modeling.

Contribution

It provides the first tentative identification of HC5NH+ in space, combining observational data with theoretical calculations and chemical models.

Findings

Detection of seven harmonically related lines with specific rotational constants.

Identification of HC5NH+ as the most likely carrier based on calculations.

HC5N/HC5NH+ ratio of 240 in TMC-1, similar to HC3N/HC3NH+ ratio.

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 +/- 0.00026 MHz and a distortion constant D0=27.3 +/- 0.5 Hz 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.