Tentative detection of HC

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 HC(5)NH(+) and NC(4)NH(+). 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, HC(5)NH(+). The HC(5)N/HC(5)NH(+) ratio derived in TMC-1 is 240, which is very similar to the HC(3)N/HC(3)NH(+) ratio. Results are discussed in the framework of a chemical model for protonated molecules in cold dense clouds.