OBSERVATIONS OF C-13 ISOTOPOMERS OF HC3N AND HC5N IN TMC-1 - EVIDENCEFOR ISOTOPIC FRACTIONATION

The C-13 substitutions of HC3N ((HCCCN)-C-13, (HCCCN)-C-13, and (HCCCN )-C-13) were observed in TMC-1 using the J = 2 - 1, 4 - 3, and 5 - 4 r otational transitions at 18, 36, and 45 GHz, respectively. The spectra l lines of (HCCCN)-C-13 are stronger than those of (HCICCN)-C-13 in al l observed transitions, while the spectral lines of (HCCCN)-C-13 and ( HCCCN)-C-13 show similar intensity. These differences in the intensiti es are most probably due to C-13 isotopic fractionation in the formati on process of HC3N. The abundance ratios are 1.0:1.0:1.4 for [(HCCCN)- C-13] : [(HCCCN)-C-13] : [(HCCCN)-C-13] at the cyanopolyyne peak in TM C-1: the C-13 isotope is concentrated in a carbon atom adjacent to the nitrogen atom. Based on these observational results, the production m echanism of HC3N was discussed. As a result, the formation reactions b etween a hydrocarbon molecule with two carbon atoms (e.g. C2H2) and a molecule with a C-13 enriched CN group can explain C-13 isotopic fract ionation: a neutral-neutral reaction between C2H2 and CN is probably m ost important. The ratio of the contributions of two types of the HC3N formation reactions which can and cannot produce C-13 isotopic fracti onation is discussed. In addition, the C-13 isotopic species of HC5N ( (HCCCCCN)-C-13 and (HCCCCCN)-C-13) were also observed at the cyanopoly yne peak in TMC-1 using the J = 9 - 8 rotational transitions at 23.7 G Hz. The intensity of (HCCCCCN)-C-13 is marginally stronger than that o f HC13 CCCCN. Furthermore, the (J, K) = (2, 2) emission of NH3 was sim ultaneously observed with the C-13 isotopic species of HC5N. With the additional observation of the (J, K) = (1,1) emission of NH3, the rota tional temperature between the (1,1) and (2,2) levels, and the column density of NH3 are determined for the cyanopolyyne peak in TMC-1.