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

Abstract

We have carried out observations of CCH ( N = 1 − 0), CH 3 CN ( J = 5 − 4), and three 13 C isotopologues of HC 3 N ( 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 HC 5 N, the column density ratios of N (CCH)/ N (HC 5 N), hereafter the CCH/HC 5 N 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/HC 5 N 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 13 C isotopologues of HC 3 N in G12.89+0.49 and G16.86-2.16, while HCC 13 CN is more abundant than the others in G28.28–0.36. We discuss carbon-chain chemistry around the three MYSOs based on the CCH/HC 5 N ratio and the 13 C isotopic fractionation of HC 3 N.