First detection of HC515N in the interstellar medium

We report the first detection of HC₅¹⁵N with the J = 9-8 rotational line from the cyanopolyyne peak in Taurus Molecular Cloud-1 (TMC-1 CP) using the 45 m radio telescope of the Nobeyama Radio Observatory. The column density of HC₅¹⁵N is derived to be (1.9 ± 0.5) × 10¹¹ cm^-2 (1σ). We apply the double-isotope method to derive the ¹⁴N/¹⁵N ratios of HC₅N and HC₃N in TMC-1 CP. The ¹⁴N/¹⁵N ratios are calculated to be 344 ± 53 and 257 ± 54 for HC₅N and HC₃N, respectively. The ¹⁴N/¹⁵N ratio of HC₅N is lower than the elemental ratio in the local interstellar medium (∼440) and slightly higher than that of HC₃N in TMC-1 CP. Since HC₃N is formed via the neutral-neutral reaction between C₂H₂ and CN, the slightly higher ¹⁴N/¹⁵N ratio of HC₅N may support our previous suggestions that the main formation mechanism of HC₅N is the ion-molecule reactions between hydrocarbon ions (C₅H_n⁺) and nitrogen atoms.