Cyanopolyyne Chemistry around Massive Young Stellar Objects
  Recent radio astronomical observations have revealed that HC$_{5}$N, the
second shortest cyanopolyyne (HC$_{2n+1}$N), is abundant around some massive
young stellar objects (MYSOs), which is not predicted by classical carbon-chain
chemistry. For example, the observed HC$_{5}$N abundance toward the
G28.28$-$0.36 MYSO is higher than that in L1527, which is one of the warm
carbon chain chemistry (WCCC) sources, by more than one order of magnitude
(Taniguchi et al., 2017). In this paper, we present chemical simulations of
hot-core models with a warm-up period using the astrochemical code Nautilus. We
find that the cyanopolyynes are formed initially in the gas phase and accreted
onto the bulk and surface of granular ice mantles during the lukewarm phase,
which occurs at $25 < T < 100$ K. In slow warm-up period models, the peak
abundances occur as the cyanopolyynes desorb from dust grains after the
temperature rises above 100 K. The lower limits of the abundances of HC$_{5}$N,
CH$_{3}$CCH, and CH$_{3}$OH observed in the G28.28$-$0.36 MYSO can be
reproduced in our hot-core models, after their desorption from dust grains.
Moreover, previous observations suggested chemical diversity in envelopes
around different MYSOs. We discuss possible interpretations of relationships
between stages of the star-formation process and such chemical diversity, such
as the different warm-up timescales. This timescale depends not only on the
mass of central stars but also on the relationship between the size of warm
regions and their infall velocity.
