We observed the $J=5-4$ rotational lines of the normal species and three $^{ 13}$C isotopologues of HC$_{ 3}$N 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 HC$_{ 3}$N in each core. The abundance ratios of the three $^{ 13}$C isotopologues in L1521B are derived to be [H$^{ 13}$CCCN]:[HC$^{ 13}$CCN]:[HCC$^{ 13}$CN] = 0.98 (+- 0.14) : 1.00 : 1.52 (+- 0.16) (1 sigma). 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 HC$_{ 3}$N is the neutral-neutral reaction between C$_{ 2}$H$_{ 2}$ and CN. On the other hand, their abundance ratios in L134N are found to be [H$^{ 13}$CCCN]:[HC$^{ 13}$CCN]:[HCC$^{ 13}$CN]= 1.5 (+- 0.2) : 1.0 : 2.1 (+- 0.4) (1 sigma), 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 HC$_{ 3}$N in L134N. We find out that the main formation pathways of the same molecule are not common even in the similar physical conditions. We discuss the possible factors to make a difference in fractionation pattern between L134N and L1521B/TMC-1.
Categories: astro-ph.GA astro-ph.SR