Implication of formation mechanisms of HC₅N in TMC-1 as studied by ¹³C isotopic fractionation

We observed the J = 9 − 8 and 16 − 15 rotational transitions of the normal species and five ¹³C isotopologues of HC₅N 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 ¹³C isotopologues with high signal-to-noise ratios between 12 and 20, as well as the normal species. The abundance ratios of the five ¹³C isotopologues of HC₅N are found to be 1.00:0.97:1.03:1.05:1.16 (±0.19) (1σ) for [H¹³CCCCCN]:[HC¹³CCCCN]:[HCC¹³CCCN]:[HCCC¹³CCN]:[HCCCC¹³CN]. We do not find any significant differences among the five ${}^{13}C$ isotopologues. The averaged [HC₅N]/[¹³C isotopologues] abundance ratio is determined to be 94 ± 6 (1σ), which is slightly higher than the local interstellar elemental ¹²C/¹³C ratio of 60–70. Possible formation pathways are discussed on the basis of these results.