13C Isotopic Fractionation of HC3N in Two Starless Cores: L1521B and L134N (L183)

Abstract

Abstract We observed the rotational lines of the normal species and three 13C isotopologues of HC3N at the 45 GHz band toward two low-mass starless cores, L1521B and L134N (L183), using the Nobeyama 45 m radio telescope in order to study the main formation pathways of HC3N in each core. The abundance ratios of the three 13C isotopologues in L1521B are derived to be [H13CCCN]:[HC13CCN]:[HCC13CN] = 0.98 (±0.14):1.00:1.52 (±0.16) ( ). The fractionation pattern is consistent with that at the cyanopolyyne peak in Taurus Molecular Cloud-1. This fractionation pattern suggests that the main formation pathway of HC3N is the neutral–neutral reaction between C2H2 and CN. On the other hand, their abundance ratios in L134N are found to be [H13CCCN]:[HC13CCN]:[HCC13CN] = 1.5 (±0.2):1.0:2.1 (±0.4) ( ), which are different from those in L1521B. From this fractionation pattern, we propose that the reaction between HNC and CCH is a possible main formation pathway of HC3N in L134N. We find out that the main formation pathways of the same molecule are not common even in similar physical conditions. We discuss the possible factors that could make a difference in the fractionation pattern between L134N and L1521B/TMC-1.