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 ($N=1-0$), 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.
