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