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 C-13 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 C-13 isotopologues with high signal-to-noise ratios between 12 and 20, as well as the normal species. The abundance ratios of the five C-13 isotopologues of HC5N are found to be 1.00:0.97:1.03:1.05:1.16 (+/- 0.19) (1 sigma) for [(HCCCCCN)-C-13]:[(HCCCCCN)-C-13]: [(HCCCCCN)-C-13]:[(HCCCCCN)-C-13]:[(HCCCCCN)-C-13]. We do not find any significant differences among the five C-13 isotopologues. The averaged [HC5N]/[C-13 isotopologues] abundance ratio is determined to be 94 +/- 6 (1 sigma), which is slightly higher than the local interstellar elemental C-12/C-13 ratio of 60-70. Possible formation pathways are discussed on the basis of these results.