Implication of formation mechanisms of HC{sub 5}N in TMC-1 as studied by {sup 13}C isotopic fractionation

We observed the J = 9 − 8 and 16 − 15 rotational transitions of the normal species and five {sup 13}C isotopologues of HC{sub 5}N to study its formation mechanisms toward the cyanopolyyne peak in Taurus Molecular Cloud-1, with the 45-m radio telescope of the Nobeyama Radio Observatory. We detected the five {sup 13}C isotopologues with high signal-to-noise ratios between 12 and 20, as well as the normal species. The abundance ratios of the five {sup 13}C isotopologues of HC{sub 5}N are found to be 1.00:0.97:1.03:1.05:1.16 (±0.19) (1σ) for [H{sup 13}CCCCCN]:[HC{sup 13}CCCCN]:[HCC{sup 13}CCCN]:[HCCC{sup 13}CCN]:[HCCCC{sup 13}CN]. We do not find any significant differences among the five {sup 13}C isotopologues. The averaged [HC{sub 5}N]/[{sup 13}C isotopologues] abundance ratio is determined to be 94 ± 6 (1σ), which is slightly higher than the local interstellar elemental {sup 12}C/{sup 13}C ratio of 60–70. Possible formation pathways are discussed on the basis of these results.