Tentative detection of HC₅NH⁺ in TMC-1

Using the Yebes 40m radio telescope, we report the detection of a series of seven lines harmonically related with a rotational constant B ₀=1295.81581 ± 0.00026 MHz and a distortion constant D ₀ = 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.