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 $\sim 15$. 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 (Taniguchi et al. 2019). The observed ratios in the MYSOs can be best reproduced by models when the gas temperature is $\sim 85$ K, which is higher than in L1527, a low-mass WCCC source ($\sim 35$ 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.

Accepted by The Astrophysical Journal, 17 pages, 6 figures, 6 tables

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