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

Abstract

Abstract 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+.