PRECISE DETERMINATION OF THE ISOTOPIC RATIOS OF HC3N_x000d_ IN THE MASSIVE STAR-FORMING REGION Sgr B2(M)

Abstract

Isotopic ratio is a critical parameter in understanding galactic chemical evolution. In addition, carbon isotopic ratio of an organic molecule reflects its formation mechanism. In the present study, we observed the simplest cyanopolyyne chem{HC_3N} and its isotopomers in the massive star-forming region Sgr B2(M) with Nobeyama 45 m radio telescope. The column density and the rotational temperature of chem{HC_3N} were determined to be 1.6$times$10$^{15}$ cm$^{-2}$ and 163 K, respectively. The ratios of the column densities for the $^{13}$C isotopomers were derived to be [chem{H^{13}CCCN}]:[chem{HC^{13}CCN}]:[chem{HCC^{13}CN}] = 1:1.03(4):0.99(3), where the rotational temperature was fixed to that of HC$_3$N. The ratios are almost the same, suggesting no isotopic fractionation for the specific carbon atoms in HC$_3$N. Therefore, it is considered that the $^{13}$C isotope exchange reactions do not contribute to make difference among the column densities of the three $^{13}$C isotopomers in the relatively warm region of Sgr B2(M). In contrast, the reported ratios in TMC-1 and L1527 are 1:1.0(1):1.4(2)footnote{S. Takano $et$ $al$., $Astron$. $Astrophys$. {bf 329}, 1156 (1998).} and 1:1.01(2):1.35(3),footnote{M. Araki $et$ $al$., $ApJ$ {bf 833}, 291 (2016).} respectively, where the ratios show higher abundance of HCC$^{13}$CN._x000d_ _x000d_ We also observed the transitions in the vibrational excited states of chem{HC_3N}. The rotational temperature of 362 K in the $nu_4$, $nu_5$, $nu_6$ and $nu_7$ excited states was obviously different from that of the vibrational ground state.