2022A&A...659L...8C

We report the detection of the protonated form of HC₇N in TMC-1. The discovery of the cation HC₇NH⁺ was carried out via the observation of nine harmonically related lines in the Q-band using the Yebes 40 m radiotelescope. The observed frequencies allowed us to obtain the rotational constants B₀ = 553.938802 (160) MHz and D₀ = 3.6292 (705) Hz. The identification of HC₇NH⁺ is further supported by accurate ab initio calculations. We derived a column density of (5.5 ± 0.7) x 10¹⁰ cm^–2, which constitutes another piece of evidence for the identification of the carrier. In addition, we revised the HC₇N column density and we derived a new value of (2.1 ± 0.2) x 10¹³ cm^–2. Hence, the abundance ratio HC₇N/HC₇NH⁺ is ∼380, while those for HC₃N/HC₃NH⁺ and HC₅N/HC₅NH⁺ are ∼230 and ∼240, respectively. Here, we discuss these results within the framework of a chemical model for protonated molecules in cold dense clouds.