First detection of HC5 15N in the interstellar medium

We report the first detection of HC5 N-15 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 HC5 N-15 is derived to be (1.9 +/- 0.5) x 1011 cm(-2) (1 sigma). We apply the double-isotope method to derive the N-14/N-15 ratios of HC5N and HC3N in TMC-1 CP. The N-14/N-15 ratios are calculated to be 344 +/- 53 and 257 +/- 54 for HC5N and HC3N, respectively. The N-14/N-15 ratio of HC5N is lower than the elemental ratio in the local interstellarmedium (similar to 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 N-14/N-15 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.