Investigation of 13C Isotopic Fractionation of CCH in Two Starless Cores: L1521B and L134N

We carried out observations of CCH and its two ¹³C isotopologues, ¹³CCH and C¹³CH, in the 84-88 GHz band toward two starless cores, L1521B and L134N (L183), using the Nobeyama 45 m radio telescope. We detected C¹³CH with a signal-to-noise ratio of 4, whereas no line of ¹³CCH was detected in either of the dark clouds. The column densities of the normal species were derived to be (1.66 ± 0.18) × 10¹⁴ cm^-2 and (7.3 ± 0.9) × 10¹³ cm^-2 (1σ) in L1521B and L134N, respectively. The column density ratios of N(C¹³CH)/N(¹³CCH) were calculated to be >1.1 and >1.4 in L1521B and L134N, respectively. The characteristic that ¹³CCH is less abundant than C¹³CH is likely common for dark clouds. Moreover, we find that the ¹²C/¹³C ratios of CCH are much higher than those of HC₃N in L1521B by more than a factor of 2, as well as in Taurus Molecular Cloud-1 (TMC-1). In L134N, the differences in the ¹²C/¹³C ratios between CCH and HC₃N seem to be smaller than those in L1521B and TMC-1. We discuss the origins of the ¹³C isotopic fractionation of CCH and investigate possible routes that cause the significantly high ¹²C/¹³C ratio of CCH especially in young dark clouds, with the help of chemical simulations. The high ¹²C/¹³C ratios of CCH seem to be caused by reactions between hydrocarbons (e.g., CCH, C₂H₂, l-C₃H and c-C₃H) and C⁺.