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

Abstract

We carried out observations of CCH and its two 13 C isotopologues, 13 CCH and C 13 CH, in the 84–88 GHz band toward two starless cores, L1521B and L134N (L183), using the Nobeyama 45 m radio telescope. We detected C 13 CH with a signal-to-noise ratio of 4, whereas no line of 13 CCH was detected in either of the dark clouds. The column densities of the normal species were derived to be (1.66 ± 0.18) × 10 14 cm −2 and (7.3 ± 0.9) × 10 13 cm −2 (1 σ ) in L1521B and L134N, respectively. The column density ratios of N (C 13 CH)/ N ( 13 CCH) were calculated to be >1.1 and >1.4 in L1521B and L134N, respectively. The characteristic that 13 CCH is less abundant than C 13 CH is likely common for dark clouds. Moreover, we find that the 12 C/ 13 C ratios of CCH are much higher than those of HC 3 N 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 12 C/ 13 C ratios between CCH and HC 3 N seem to be smaller than those in L1521B and TMC-1. We discuss the origins of the 13 C isotopic fractionation of CCH and investigate possible routes that cause the significantly high 12 C/ 13 C ratio of CCH especially in young dark clouds, with the help of chemical simulations. The high 12 C/ 13 C ratios of CCH seem to be caused by reactions between hydrocarbons (e.g., CCH, C 2 H 2 , l -C 3 H and c -C 3 H) and C + .