Carbon Chain Chemistry in Hot-core Regions around Three Massive Young Stellar Objects Associated with 6.7 GHz Methanol Masers

We have carried out observations of CCH (N = 1 - 0), CH₃CN (J = 5 - 4), and three ¹³C isotopologues of HC₃N (J = 10 - 9) toward three massive young stellar objects (MYSOs), G12.89+0.49, G16.86-2.16, and G28.28-0.36, with the Nobeyama 45 m radio telescope. Combined with previous results on HC₅N, the column density ratios of N(CCH)/N(HC₅N), hereafter the CCH/HC₅N ratios, in the MYSOs are derived to be ∼15. This value is lower than that in a low-mass warm carbon chain chemistry (WCCC) source by more than one order of magnitude. We compare the observed CCH/HC₅N ratios with hot-core model calculations. The observed ratios in the MYSOs can be best reproduced by models when the gas temperature is ∼85 K, which is higher than in L1527, a low-mass WCCC source (∼35 K). These results suggest that carbon-chain molecules detected around the MYSOs exist at least partially in higher temperature regions than those in low-mass WCCC sources. There is no significant difference in column density among the three ¹³C isotopologues of HC₃N in G12.89+0.49 and G16.86-2.16, while HCC¹³CN is more abundant than the others in G28.28-0.36. We discuss carbon-chain chemistry around the three MYSOs based on the CCH/HC₅N ratio and the ¹³C isotopic fractionation of HC₃N.